|
|
 |
|||||||||||||||||
|
|||||||||||||||||
 |
|||||||||||||||||
| JOURNAL HOME | HELP | CONTACT PUBLISHER | SUBSCRIBE | ARCHIVE | SEARCH | TABLE OF CONTENTS |
| ||||||||||||||||||||||||||||||||||||||||||||||||||||
1 Institute for Geophysical Research, Department of Physics, Mailstop #615, University of Alberta, Edmonton, Alberta T6G 2G7, Canada
2 Alberta Geological Survey, 4th Floor, Twin Atria Building, 4999-98 Ave. NW, Edmonton, Alberta T6B 2X3, Canada
Correspondence: E-mail: doug{at}phys.ualberta.ca or doug.schmitt{at}gmail.com
 |
ABSTRACT |
|---|
 TOP ABSTRACT INTRODUCTIONBACKGROUNDGEOLOGICAL BACKGROUNDMETHODS AND FIELD PROGRAMRESULTSDISCUSSIONCONCLUSIONSREFERENCES CITED |
|---|
·m than adjacent undisturbed Cretaceous-age rocks, which have VP >2200 m/s and r <10 ·m, respectively. Seismic-refraction velocities, seismic-reflection architecture, and electrical resistivity tomography images all indicate a deep (>300 m) valley. The reflection profile reveals a complex series of events consistent with heterogeneous deposition. One major reflection dips eastward across the valley. The reflection character changes abruptly on either side of this event, with dipping clinoform reflections beneath and nearly horizontal reflections above. These may be related to changes in deposition processes beneath a retreating ice sheet, as has been suggested by earlier workers. The buried valley's basal reflections are complex and possibly reflect glaciotectonic deformation or block plucking. The near-surface methane gas is within, but at the edge of, the valley. The methane gas–saturated zone displays anomalously high resistivities both in the well logs and in resistivity tomography. |
INTRODUCTION |
|---|
|
TOPABSTRACTINTRODUCTIONBACKGROUNDGEOLOGICAL BACKGROUNDMETHODS AND FIELD PROGRAMRESULTSDISCUSSIONCONCLUSIONSREFERENCES CITED |
|---|
Such valleys lie near the surface, but they often have no obvious geomorphologic surface expression, and they are difficult to both find and delineate. Indeed, many such features are only found by serendipitous water-well drilling. However, it is expensive to delineate buried valleys on the basis of drilling alone; geophysical investigations are of great utility in further outlining such structures.
This contribution provides the results from an integrated geological and geophysical profiling of one such buried valley in northern Alberta, Canada, from the initial discovery of the valley from geophysical well logs via the contrasts in physical properties, through refinement of the location of the valley with seismic-refraction profiling, and finally to delineation of the architecture of the buried valley with both seismic-reflection and electrical resistivity tomography (ERT) profiles (Ahmad, 2006). The structure of the valley, particularly as revealed in the seismic-reflection profile, may hold clues to the subglacial processes active beneath retreating ice sheets. Similar features are seen in studies of buried valleys in the North Sea and indicate that the same subglacial processes acted in both locales.
|
BACKGROUND |
|---|
|
TOPABSTRACTINTRODUCTIONBACKGROUNDGEOLOGICAL BACKGROUNDMETHODS AND FIELD PROGRAMRESULTSDISCUSSIONCONCLUSIONSREFERENCES CITED |
|---|
Tunnel Channels and Valleys
Advances and retreats of continental ice sheets dramatically alter Earth's surface. River valleys that predate glaciation and tunnel valleys contemporaneous with glaciation (O'Cofaigh, 1996; Shaw, 2002) are often filled by tills and glacial lake sediments (e.g., Ehlers and Linke, 1989; Huuse and Lykke-Andersen, 2000; Praeg, 2003; Jorgensen and Sandersen, 2006). Pleistocene buried valleys abound in the glaciated mid latitudes of North America and Europe, and much older fossil valleys have even been located at various locales such as Jordan, Australia, and Niger (e.g., Eyles and de Broekert, 2001; Denis et al., 2007). Hooke and Jennings (2006) summarized a number of characteristics that distinguish tunnel valleys, among which are their dimensions, which are typically from 5 to 20 km long, 150 to 500 m wide, and ~5 to 30 m deep (although, usually, the depth of the valley fill is unknown). However, larger dimensions up to many tens of kilometers, kilometer-scale widths, and hundreds of meters of depth are at least as common in the literature (e.g., Piotrowski, 1994; Huuse and Lykke-Andersen, 2000). For example, Kristensen et al. (2007) recently carried out, using three-dimensional (3-D) seismic volumes, a detailed analysis of the basal geometry of a series of buried channels in the North Sea off of Denmark. The valleys were typically 3–4 km wide, up to 350 m deep, and as long as 39 km. Their bottom topography undulates, creating subbasins and thresholds, and an adverse end slope is seen at their termination. This complex morphology suggests that the buried valleys were created by subglacial flow linked to the still poorly understood hydrological processes both beneath and outboard of continental glaciers (Brennand and Shaw, 1994; Huuse and Lykke-Andersen, 2000; Beaney, 2002), but likely including a combination of erosion by uniform flow, catastrophic discharge, and glacial ice abrasion and plucking (e.g., Praeg, 2003; Jorgensen and Sandersen, 2006; Lonergan et al., 2006).
Further, areal geophysical seismic and electromagnetic (EM) surveys reveal that tunnel valleys appear in three dimensions together in complex anastamosing and crosscutting patterns (Huuse and Lykke-Andersen, 2000; Kluiving et al., 2003; Praeg, 2003; Lonergan et al., 2006; Kristensen et al., 2007). Upon even closer examination, a given valley appears to be constructed of numerous generations of cut-and-fill structures (Jorgensen and Sandersen, 2006; Jørgensen and Sandersen, 2008), suggesting that the valleys had been eroded and filled many times.
A clear definition of the differences between channels and valleys is required, and here we use the terminology as reviewed recently by Fisher et al. (2005, p. 2377) (see also Clayton et al., 1999; Jorgensen and Sandersen, 2006). A tunnel channel is a depression formed by a "subglacial bankfull discharge, where the size of the channel matches the size of the valley." In contrast, a tunnel valley is excavated by horizontal and vertical migration of a tunnel channel scouring out the final valley.
The valley fill consists of complex packages of weakly consolidated glaciofluvial and glaciolacustrine sands, gravels, boulders, and clays as observed from well logs (e.g., Kluiving et al., 2003), cores (e.g., Russell et al., 2003), or directly in exposures (e.g., Ehlers, 2005; Fisher et al., 2005). Munro-Stasiuk (2003), for example, classified six different facies that depend to a large degree on whether sedimentation occurred during advance of, during (and hence beneath), or upon retreat of glaciation.
Glacially filled incised valleys are common both onshore and offshore in northern Europe. There are numerous studies published in the open literature, primarily because of the need for groundwater resources in those densely populated regions (e.g., Huuse et al., 2003). Buried valleys are also common in North America, which is represented by a large body of geomorphological and geological literature (e.g., Boyd et al., 1988; Sharpe et al., 1992; Brennand and Shaw, 1994; Clayton et al., 1999; Cutler et al., 2002; Russell et al., 2003; Hooke and Jennings, 2006; Ross et al., 2006), a complete review of which is beyond the scope of this contribution. In contrast to the European situation, however, in North American public domain geophysical surveys over such features are rare.
In Alberta and Saskatchewan, the existence of such buried valleys was known almost from the first geological explorations along the river valleys where exposures may be seen, and location of groundwater resources within them has been particularly important in the arid southern portions of these provinces. Stalker (1960) provided details of the location and character of the numerous buried valleys of central and southern Alberta and correlated the current and preglacial drainage patterns; this early work was followed by numerous later studies (Rains et al., 2002; Sjogren et al., 2002; Munro-Stasiuk, 2003). In northeastern Alberta, Andriashek and Atkinson (2007) used data from more than 30,000 wells to map buried valleys and glacial-drift aquifers in the extensive oil sands region.
Geophysical Investigations of Buried Channels and Valleys
While not focusing on buried valleys directly, there are also numerous geophysical studies of glacially derived sediments that are worth mentioning because they demonstrate that glacial deposits are not easily characterized (e.g., Roberts et al., 1992; Keiswetter et al., 1994; Buker et al., 2000; Nitsche et al., 2002; Eyles et al., 2003; Kilner et al., 2005; Ross et al., 2006; Smith and Sjogren, 2006). Their architecture can be highly heterogeneous and include a wide variety of sediment types from gravels to clays with different geophysical properties.
The overwhelming majority of geophysical studies of buried valleys come from northern Europe. A recent special issue of the Journal of Applied Geophysics provides an extensive overview of a number of geophysical investigations of buried valleys (Huuse et al., 2003), and the northern European BurVal Working Group recently released an extensive report (Kirsch et al., 2006) on locating groundwater resources in buried glacial valleys.
A wide variety of geophysical techniques have been employed, ranging from gravity (Gabriel et al., 2003; Gabriel, 2006; Møller et al., 2007), which searches for density contrasts, to ground-penetrating radar (Fisher et al., 2005), which images the detailed structure of the near-surface fill material. However, seismic, DC electrical, and electromagnetic (EM) methods are the most popular. High-resolution seismic images have been used for groundwater exploration (Buker et al., 1998; Wiederhold et al., 1998; Holzschuh, 2002; Jorgensen et al., 2003; Bergman et al., 2004) or to image glacial deposits and glacial basins (Pugin and Rossetti, 1992; Bradford et al., 1998; Musil et al., 2002). Both ground-based (Baines et al., 2002; Jorgensen et al., 2003; Thomsen et al., 2004; Kilner et al., 2005; Jorgensen and Sandersen, 2006; Bersezio et al., 2007) and airborne electromagnetic (Sørensen and Auken, 2004; Eberle and Siemon, 2006) techniques have also been extensively employed.
Arguably, however, marine seismic-reflection surveys provide some of the best information regarding both the regional configurations of buried valleys and their internal structure. Near Sable Island off the east coast of Nova Scotia, Boyd et al. (1988), some of the first workers to exploit commercial seismic data acquired for petroleum exploration, regionally mapped the pattern of tunnel valleys and showed the details of their interior structure. Huuse et al. (2001) interpreted over 6400 km of specially acquired high-resolution marine two-dimensional (2-D) profiles supplemented with over 18,000 km of conventional 2-D profiles to map Quaternary valleys offshore west of Jutland. Kluiving et al. (2003) interpreted a series of 2-D profiles in the North Sea off of the Netherlands. Eiriksson et al. (2006) also acquired 2-D profiles in the constricted southern Lillebælt marine area in eastern Denmark, and they were able to image and map buried valleys. Most recently, Halliday et al. (2008) interpreted marine seismic and multibeam sonar data to highlight the effects of methane within glacial sediments off the coast of British Columbia. Data from large 3-D marine surveys covering areas in excess of 1000 km2 and originally acquired for petroleum exploration in deeper prospective formations in the North Sea have also been profitably reprocessed and interpreted to highlight the overlapping geometry of successive episodes of tunnel valley formation during the Pleistocene (Praeg, 2003; Lonergan et al., 2006; Kristensen et al., 2007, 2008; Kuhlmann and Wong, 2008). Praeg (2003) also provided a comprehensive review of earlier literature. The topography of a series of valleys extracted from 3-D data by Kristensen et al. (2007) shows undulating bottoms and adverse end slopes that are generally characteristic of subglacial valleys.
In western Canada, geophysical investigations of buried valleys began early with gravity (Hall and Hajnal, 1962), electrical resistivity (Lennox and Carlson, 1967), and seismic methods (Hobson et al., 1970) all being employed. More recently, seismic methods have been used to characterize Quaternary sediments (Francese et al., 2002). Langenberg et al. (2002), as part of a study of the detailed structure of oil sands, provided a seismic profile that also coincidentally includes a buried Quaternary valley.
Shallow Gas
One additional factor that makes buried valleys interesting is that the complex geological structure may both contain and trap hydrocarbon gas, in some cases, in economic concentrations. Such "Quaternary" gas has been produced from different parts of China for many centuries (Kuhn, 2004). Indeed, wellbore drilling was developed there to exploit near-surface gas and associated brines that together were used in the manufacture of salt on a large scale. Major Quaternary biogenic gas fields are found in Qaidam Basin and the Yangtze River delta near Shanghai (Jianyi et al., 1999). Recently, several Quaternary shallow (30–55 m) gas pools have also been discovered and produced in China in the province of Zhejiang (Lin et al., 2004). These gas pools are generally located in deeply incised valleys filled with highly porous fluvial sediments.
Methane has been detected in the marine seismic surveys in northern Europe. Such gas can be problematic for seismic imaging, first because of its low velocity and associated distorting "pull-down" effects on deeper reflections, and second due to the loss of signal beneath such gas-filled zones, referred to as "acoustic turbidity" or "blanking" (Eiriksson et al., 2006; Halliday et al., 2008). The gas-saturated zones will also produce strong "bright spot" reflections (Kuhlmann and Wong, 2008).
Glacial deposits are also known to host modest economic gas reserves in North America, such as in Pointe-du-Lac, Québec, and Bouttineau County, North Dakota (Anderson et al., 2006). Commercial amounts of gas have been produced from glacial deposits of the Rainbow and the Sousa fields in northern Alberta (Pawlowicz et al., 2004), and local "permanent" burning gas seeps occur near the current study area (Letourneau et al., 2000). These fields lie to the west between High Level and Rainbow Lake, Alberta (Fig. 1). In these fields, the gas lies at depths of <100 m; this creates a substantial hazard during drilling of water or deeper hydro carbon wells, and disastrous blowouts have occurred as recently as 2005.
|
|
GEOLOGICAL BACKGROUND |
|---|
|
TOPABSTRACTINTRODUCTIONBACKGROUNDGEOLOGICAL BACKGROUNDMETHODS AND FIELD PROGRAMRESULTSDISCUSSIONCONCLUSIONSREFERENCES CITED |
|---|
|
Physical Properties from Geophysical Logs
Some of the properties of the glacial and sedi mentary lithologies both within and outside of the buried valley were obtained from geophysical logging. Details of each logging method are beyond the scope of this paper; for the physics of such logging tools, readers are referred to Hearst et al. (2000) or Ellis (1987), and to Darling (2005) for interpretation and geological applications of such logs. The geophysical logs shown (Fig. 3) were obtained from the publicly available petroleum well-log databases for western Canada. The data were obtained by various geophysical contractors at the time the wells were drilled.
|
Both the Quaternary and pre-Quaternary sediments are also easily differentiated on the gamma-ray (GR) log (Fig. 3A). The gamma-ray value of the clay-rich siliciclastic Cretaceous bedrock outside the valley is high (>200 American Petroleum Institute units), and it is generally substantially lower (~50 API) in the valley-fill glacial sediments. In both wells, the GR data are unreliable in the Paleozoic carbonates below 355 m depth because these boreholes do not extend into this formation. The sonic log (Fig. 3B) also shows discernible differences between the three distinct units. The Paleozoic carbonates are clearly evident in the out-of-valley (OC) well by their high velocity (>4500 m/s). Unfortunately, there are no sonic log values above 230 m depth for the in-valley (IC) well. However, the velocities of the Quaternary sediments (~1500 m/s to 2000 m/s) can be easily distinguished from the Cretaceous bedrock (>2200 m/s). The Quaternary sediments are generally substantially more electrically resistive (>10 ·m) than the older Cretaceous bedrock (<10 ·m) (Fig. 3C). This difference is likely due to the higher conductivity of the clay-rich, predominantly marine Cretaceous rock. The Quaternary deposits, in contrast, contain greater proportions of clean sands and gravels that are filled with freshwater and maybe even insulating free gas. The lack of clays within the Paleozoic carbonates below ~355 m depth may be a reason for their high resistance (~50 ·m). Consequently, the Quaternary, the Cretaceous, and the Paleozoic units are all clearly distinguished on the basis of their resistivity for the OC well. The absence of the Cretaceous unit is evident by the high resistivity throughout the entire depth of the IC well. The bulk densities range between 2.0 g/cm3 and 2.5 g/cm3 (Fig. 3D). A discontinuous jump in the density is seen across the sub-Cretaceous unconformity on the OC log, but it is not clearly seen on the IC log. Differences are more subtle in this log. As expected, the densities of the upper Quaternary sections in both logs are similar. Beneath the pre-Quaternary discontinuity in the OC log, the density of the Cretaceous rocks generally exceeds that of the fill material in the valley. The differences, however, are small, and interpretation of the valley versus the nonvalley materials on the basis of the density log is difficult.
The geophysical logs of Figure 3 illustrate the physical property differences between the water-saturated valley fill and the older Cretaceous and Paleozoic bedrock. It is interesting, however, to also compare the rare log acquired through a shallow gas-saturated zone (Fig. 4), since the differences here will also show up as a geophysical anomaly in the surface surveys carried out in this study. The producing gas-saturated zone, highlighted by gray overlay from 64 m to 72 m depth (Pawlowicz et al., 2004), is clearly apparent only in the resistivity log, which shows a reading generally in excess of 30 ·m, with some values exceeding 100 ·m. Such high values are anomalous and indicate the presence of insulating pore fluids. A small highly resistive anomaly (100 ·m) at 100 m depth in the IC log may also be indicative of free gas. The resistive zone is not clearly visible in the other logs in Figure 4, although there appears to be a drop in the sonic velocity and a discontinuity in the natural gamma-ray logs below 63 m depth. The sonic velocity through the zone (~1600 m/s to 1700 m/s) of Figure 4 also appears lower than in the nearby IC log (~1800 m/s to 1900 m/s) in Figure 3. There is insufficient information to know to what degree gas may saturate the formation below the producing interval, but the resistivity remains high (>30 ·m) and the sonic velocity remains low below 72 m; this may be suggestive of additional partial-gas saturation in the fill material.
|
|
METHODS AND FIELD PROGRAM |
|---|
|
TOPABSTRACTINTRODUCTIONBACKGROUNDGEOLOGICAL BACKGROUNDMETHODS AND FIELD PROGRAMRESULTSDISCUSSIONCONCLUSIONSREFERENCES CITED |
|---|
Seismic Measurements
The field area was selected on the basis of analysis of existing well logs that suggested the existence of a deep valley. The availability of earlier ERT soundings and preexisting seismic clearings cut through the forest were also taken into consideration when designing the program. The goal of the survey was to image the structure above the sub-Cretaceous unconformity that lies at depths of ~350 m to 360 m, and as such, a nonstandard acquisition program employing both close spatial group spacing (4 m) and high frequencies was employed. High-frequency (40 Hz) geophones were selected to attenuate both ground roll and potential noise from the nearby highway. Based on our field experiences, the loss of the signal's frequency content below 40 Hz is more than compensated by the boost in the relative strength of the near-surface reflections, primarily because of the attenuation of the surface waves. These acquisition parameters are similar to some land-based studies (Wiederhold et al., 1998; Jorgensen et al., 2003) but generally are of smaller lateral spatial resolution than most marine 3-D studies, which are able to take advantage of existing commercial seismic data acquired in petroleum exploration.
The ground roll was further reduced by using an 8 s linear vibrator sweep beginning at 20 Hz through to 250 Hz produced with a high-frequency MinivibTM unit (Industrial Vehicles International of Tulsa, Oklahoma). A 240-channel semidistributed GeodeTM seismograph (Geometrics Ltd., San Jose, California) acquired the raw seismic traces. High spatial resolution was maintained by using geophone singles at 4 m station spacing. The vibration point spacing of 24 m (six stations) was a compromise between the maximum number of shots (up to 150) that could be carried out during the shorter daylight hours and the desire to maximize midpoint fold. An average fold of 40 was obtained for the survey. Upon completion of the survey, the data were archived in a SEGY format with geometry in UTM coordinates.
The raw seismic-reflection data were processed using the standard common midpoint (CMP) method (e.g., Yilmaz, 2001) with a commercial seismic data-processing package (Vista® by GEDCO, Calgary). The procedures for processing of near-surface seismic data differ in many respects from usually much deeper conventional petroleum exploration. For example, the final common midpoint trace spacing in this study was only 4 m, and only geophone singles were used in the acquisition (but with 4-m-long CMP bins averaging the response of two geophones). Hence, the optimum-offset processing concepts developed by Hunter et al. (1984) were adopted. The processing details can be found in Ahmad (2006); briefly, some steps worth noting include removal of the surface wave noise cone (Baker et al., 1998; Schmitt, 1999) and the refractions by a combination of top-mutes (e.g., Schmelzbach et al., 2005) and normal moveout (NMO) stretch mutes (e.g., Miller, 1992), and the use of optimum offsets. The muting unfortunately makes portions of the reflection image above ~80 ms two-way travel-time (TWT) suspect (Praeg, 2003). The length of the CMP bins was also doubled to 4 m from the nominal 2 m spacing; this doubling of the trace spacing, and hence the fold, resulted in a marked improvement in the quality of the profile. Conventional velocity analysis was carried out interactively every 50 m along the profile using the semblance method (Yilmaz, 2001).
One final, and as yet not completely satisfactorily ameliorated, problem in this study deals with lateral variations in the surface materials between mineral soil and low-velocity muskeg (sphagnum moss-filled bog). These variations result in "static" shifts to the traveltimes of reflections (see Yilmaz, 2001). In the current study, although the raw CMP gathers are of very good quality, the stacked seismic data quality deteriorated due to severe vertical and lateral velocity variations in the surface material. This makes proper normal moveout (NMO) correction difficult. Severe velocity gradients due to the presence of the near-surface water table effectively degraded the quality of the CMP (Miller and Xia, 1998). These static variations were attenuated by using a suite of methods, beginning with refraction statics based on Hagedoorn's plus-minus method (e.g., van Overmeeren, 2001). The remaining minor static errors were improved by iteratively applying surface-consistent residual statics. Regardless of these corrections, the static time shifts remain problematic and lower the quality of the profile relative to that expected from the raw shot gathers.
Resistivity Profiling
The ERT data were acquired commercially in January 2005. Cold winter conditions (<–35 °C) with deep hard snow provided good coupling of the 1 m steel electrode to the ground, making for excellent data quality. The Wenner array was used for the ERT data acquisition, using 15 m electrode spacing that would sense to depths of ~200 m. In Wenner profiling, the distance between the current and the potential electrodes remains the same, and a large number of measurements are taken in a sequence to make a "pseudosection." One potential advantage of the Wenner array is that the effect of any shallow conductive body is reduced as measurements are taken at different positions by moving the complete array along the profile.
The data were inverted using a commercially available software package Resis2dinv® (Geotomo Software, Malaysia) (Loke, 2004), which uses a nonlinear smoothness-constrained least square inversion technique (Loke and Barker, 1996) with 14 layers. A finite-element subroutine was used to calculate the forward modeling operator, a Jacobian matrix, which then was used to calculate modeled, true resistivity. As this inversion procedure is smoothness constrained, some kind of smoothing is required as an input parameter for the inversion algorithm. Moderate horizontal flatness and minimum vertical flatness were allowed during inversion.
|
RESULTS |
|---|
|
TOPABSTRACTINTRODUCTIONBACKGROUNDGEOLOGICAL BACKGROUNDMETHODS AND FIELD PROGRAMRESULTSDISCUSSIONCONCLUSIONSREFERENCES CITED |
|---|
|
The shot gather obtained at an intermediate position along the profile (Fig. 5B) is consistent with a two-layer model. However, while the "direct" wave (yellow highlight) is similar to that in Figure 5A and suggests propagation through Quaternary fill, the observed head wave (gray highlight) indicates both a lower-velocity and a shallower top depth that is more consistent with the Cretaceous rocks. The strong Paleozoic top reflection (magenta highlight) is still clear, but there are insufficient source-receiver offsets to observe the head wave from this interface. This shot gather still suggests a relatively thick and low-velocity Quaternary fill, but one that now immediately overlies Cretaceous siliciclastic rocks of more intermediate velocity.
The last shot gather toward the west end of the profile (Fig. 5C) is more complicated; it displays three linear arrivals that correspond to the direct wave through the Quaternary fill (yellow highlight), the head wave from the top of the Cretaceous bedrock tK (gray highlight), and the head wave from the top of the Paleozoic bedrock (cyan highlight). The strong Paleozoic reflection is still clearly seen (magenta highlight). This series of arrivals can be interpreted as a thin, low-velocity Quaternary layer that overlies a relatively thick section of moderate-velocity Cretaceous bedrock that in turn lies upon the high-velocity Paleozoic bedrocks.
Taken together, these three shot gathers illustrate the evolution of the characters of the seismic arrivals along the profile from east to west. A more quantitative, but still simple two- or three-horizontal-layer analysis was carried out using most of the shot gathers obtained in the survey in order to better understand the changes in the velocity structure along the profile. Material velocities were determined by simple linear fits to the observed traveltimes, and thicknesses were measured using the offsets and times of the crossovers (i.e., those points in the shot gathers at which different direct and head wave curves intersect). This conventional refraction interpretation gives a general idea about the subsurface structure in terms of the number of major layers, their velocities, and thicknesses. The velocity structure so determined (Fig. 6B) displays the valley as the thick, low-velocity fill immediately on top of the Paleozoic rocks to the east and the three layers of fill, Cretaceous rock, and Paleozoic rock structure outside of the valley to the west.
|
|
|
The final seismic-reflection profile is shown in both grayscale (Fig. 6D) and in color (Fig. 6E) without and with interpretation guides, respectively. It is important to point out a number of features in the profile.
There are attenuated "washout" zones denoted by "w" in Figure 6E that extend vertically throughout the profile and in which little or no coherent reflections are observed. Such wash-outs are often observed in areas, such as shallow seafloor or lake floor sediments, with high free-gas saturation (e.g., Garcia-Gil et al., 2002).
The sub-Cretaceous unconformity at the top of the Paleozoic bedrock (pK) is the most conspicuous reflection in the profile. It is continuous across the entire line aside from the masking in washout zones. To the west of station 1400, the reflection character of pK is uniform and parallel to the weaker events above and below. Although there appears to be some minor topography on this western surface, this is more likely due to static time-shift problems. To the east of station 1400, there is a large change in the character of the pK event, which apparently begins to dip to the east. The event remains strong but is highly chaotic with much apparent topography over the range of stations from 1050 to 560 (see expanded view in Fig. 7). To the east of station 560, pK again becomes more uniform but is more reverberatory. At this location, the event also appears at greater traveltimes (>400 ms) than in the west (~375 ms).
|
There are a series of weaker events (K) clearly visible to the west of station 1400 that parallel pK. No evidence for these events is seen anywhere to the east of this point. These continuous events originate from the still-uneroded Cretaceous marine stratigraphy. Their disappearance to the east of station 1500, together with the traveltime pull-down effects, possibly indicates a rather steep-sided valley in agreement with the coarser refraction profile of Figure 6B. The wall of the valley, however, is not imaged. This could be due to washout of the image at this location or to difficulties encountered in imaging a steeply dipping feature in surface reflection profiles.
Q1 is an east-dipping ramp that is clearly continuous between stations 250 and 1040 and appears to represent a boundary within the Quaternary fill that delimits two seismic facies. Above Q1, Q2 marks a relatively complex package of primarily flat-lying and strong reflections above 250 ms that overlies Q1 and ranges between stations 200 and 800. This package indicates strong contrasts in the differing materials. Below Q1, Q3 highlights a series of dipping reflections between stations 1200 and1350 (Figs. 6E and 8). These are located at the transition between the valley fill and the undisturbed sediments. It is possible that the uppermost of the reflections marked Q3 could in fact be the western extension of Q1. A potential concern is that the lower events could be multiple reverberations, as examination of the full profile (Fig. 6D) in this range might suggest. However, the more detailed view (Fig. 8) shows that the two Q3 events denoted have quite different character, implying that the lower of the two is a primary reflection and not a multiple. Q4 is a complex pattern in the character of pK between CMP stations 700 and 1050. The events appear to be doubled in some parts of this zone.
|
The penetration depth depends on the conductivity of the near-surface layers. The western part of section shows low resistivity, ~<15 ·m, which is due to the conductive Cretaceous shales. In the middle of the section, there is a highly resistive anomaly at ~150 ·m. This anomaly is at the western edge of Quaternary strata. The eastern part of the ERT shows low resistivity, ~15 to ~25 ·m, which could be silty clay. In the easternmost end of the profile, one higher-resistive anomaly with resistivity values of ~100 ·m is also present.
|
DISCUSSION |
|---|
|
TOPABSTRACTINTRODUCTIONBACKGROUNDGEOLOGICAL BACKGROUNDMETHODS AND FIELD PROGRAMRESULTSDISCUSSIONCONCLUSIONSREFERENCES CITED |
|---|
Seismic-Reflection Features
The seismic-reflection profile (Figs. 6D and 6E) reveals some of the finer structure of this buried valley that may provide insight into its erosion and filling. This is discussed in terms of the various features noted here and comparisons to earlier seismic studies from Europe.
The Q4 events (Fig. 7) at the base of the Quaternary fill are complicated, and to our knowledge, similar features have not been described in previous geophysical assessments. The rough topography to pK between stations 700 and 1050 has a number of possible interpretations. First, the double reflections could be produced by 3-D structure on pK, as might be associated with motion of blocks due to glacial thrusting (e.g., Fennel et al., 2001; Rafaelsen et al., 2007) or by quarrying (e.g., Glasser and Bennett, 2004). Such deformation would likely require ice to be in direct contact with the bedrock, and it suggests that glacial abrasion was active in deepening the valley. While movement of boulders seen in glacial outwash fans by catastrophic flooding has been suggested (e.g., Brennand and Shaw, 1994; Piotrowski, 1994), the kilometer-scale continuity of the Q4 reflections argues against such a mechanism here. The two small depressions centered at common midpoint numbers 980 and 780 and bottoming near 450 ms TWT could be more local basal channels produced by subglacial or fluvial flows. However, the lack of core and transverse seismic-reflection profiles does not allow for a more informed interpretation.
Within the valley fill, the gently (<3°) eastward-dipping Q1 feature is the most continuous of the in-valley reflections, and Q1 likely represents either erosion or a change in deposition at the top of an earlier fill that has since been covered by younger materials. The dip of this event differs from the reflections both beneath and particularly above it, and this further reinforces the interpretation of a boundary, but the nature of this boundary is not known. One of Praeg's (2003) seismic images (his Fig. 4C) also shows a strong reflection dipping to the NW that is not unlike the east-dipping Q1 in Figure 7. In a similar but more controlled study, Boyce et al. (1995) showed that many of the reflections seen in their high-resolution seismic profiles correlated with erosional surfaces that separated distinct tills. Other workers (Huuse and Lykke-Andersen, 2000; Kluiving et al., 2003; Praeg, 2003) have attempted to interpret various reflections within glacial sediments on the basis of sufficiently detailed well-log information, but, to our knowledge, no study has yet been able to link a given valley-fill reflection to an actual lithological variation. One possibility is that it represents an erosional unconformity produced by glacial abrasion, and, hence, it is indicative of grounded ice and consolidation of the underlying materials. Erosion of the sediments within an already existing valley either by subglacial or surficial fluvial action is another possibility. A third possibility is rapid change in the style of deposition upon an existing surface, as evidenced by the differences in the seismic facies above and below Q1, although this change in deposition style occurs regardless of whether or not Q1 represents an erosional surface. Of these, the length (at least 3 km) and uniformity of the event suggest that the surface was produced by a uniform consolidation beneath ice.
Within the reflection profile, the eastward-dipping Q3 features are of interest (Fig. 8), and comparisons with earlier studies from the North Sea are warranted. In particular, Praeg (2003) carried out an extensive analysis of buried tunnel valleys from 2-D and 3-D seismic data over a portion of the North Sea Basin immediately to the west of the United Kingdom. Despite substantial differences in the scale and frequency content of his marine and the current land-based survey, the seismic images obtained in both are largely consistent. He noticed clinoform features within the lower portions of the valley that may be similar to the present Q3. Similar clinoform features can be seen in other marine seismic and some land seismic data sets from northern Europe (e.g., Wiederhold et al., 1998; Huuse and Lykke-Andersen, 2000; Kluiving et al., 2003), Ontario (Pugin et al., 1996), and Nova Scotia (Boyd et al., 1988). The images here (Fig. 6D) are reminiscent of these studies in that they show steep-sided valley walls, clinoform events toward the edges of the valleys, unconformities, and series of flat-lying sedimentary deposits.
The origin of such features remains controversial. Praeg (2003) argued for a time-transgressive formation of the buried valleys, which are first excavated during and backfilled during headward retreat of the margin of the ice sheet. He argued that the deeper clinoform structures are evidence of backfilling. Lonergan et al. (2006) observed series of wedge-shaped features that characterize their seismic facies unit 2. They interpreted the wedges to have been deposited subglacially and for the sediments to have come from upstream within the ice sheet, where the material consists of both eroded and reworked materials. They suggest that the on-lapping character of the reflections indicates uphill flow that would require a substantial pressure head. Kristensen et al. (2008) recently showed similar, but larger, clinoform features in marine seismic images from the North Sea. They suggested that such depositional features arise either from subglacial deposition of sediments trapped within by debris-filled ice near the toe of the glacier or from successive deltas deposited in the open valley during glacial retreat. Supercooling of the water may play a role in the development of ice and sediment trapping. The Q3 reflections highlighted in Figure 8 appear to be similar to these earlier observations, but there is insufficient supporting information to make further interpretation as to their provenance in this case.
Q2 is the package of nearly flat-lying reflections highlighted in Figure 7. The series of reflections are parallel and nearly horizontal, and this may be suggestive of glaciolacustrine deposition (e.g., Munro-Stasiuk, 2003). Such features, too, have been seen by many workers in North Sea seismic images. Lonergan et al. (2006) suggested that the subhorizontal draping package that defined their seismic facies unit 3 was deposited in quiet-water, proglacial lacustrine and latter marine settings and likely consisted of fine-grained muddy sediments. Praeg (2003), assisted in the interpretation of his "subhorizontal" seismic unit by the availability of cuttings and geophysical logs, suggested that this unit was primarily composed of lacustrine and marine muds. Earlier workers (Boyd et al., 1988; Huuse and Lykke-Andersen, 2000) suggested that these upper sections of their profiles, too, were fine-grained lacustrine sediments.
The seismic-reflection imaging highlights the complex architecture within the valley fill. Although detailed interpretations are hampered by the paucity of sufficient detail from nearby boreholes and the complete lack of any outcrop information, the reflection profile displays a number of features that are compatible with various processes related to erosion, deposition, and defor mation during multiple glacial advances and retreats, and subglacial hydrological processes. The interpretation of the gross structure of the valley is also consistent with the broad seismic interpretations of earlier workers (Boyd et al., 1988; Huuse and Lykke-Andersen, 2000; Kluiving et al., 2003; Praeg, 2003; Jørgensen and Sandersen, 2006; Lonergan et al., 2006; Kristensen et al., 2008), although these authors may disagree on finer points of valley excavation and subsequent sediment-filling processes. In other words, there does not appear to be any unique differences between the northern European structures and the one studied here, and the same processes appear to have been active at both locations.
In particular, the complexity of the structure (where the two major seismic facies are distinguished by the Q2 and Q3 seismic facies) suggests, similar to the study of Lonergan et al. (2006), that the valley here is the result of a number of processes over time. Given the depth (~350 m) of this feature and its deep bowl-like shape as mapped from well logs (Fig. 1C), it is likely that pressurized subglacial meltwater was responsible for eroding the valley. Generally, the reflections above the interpreted valley are flat-lying, suggesting that the valley-fill material was not deposited during rapid discharge events. The strong reflector that dips across the image suggests an unconformity and hence possibly indicates the existence of at least one additional erosional event. However, on the basis of the existing geophysical data alone, it is impossible to know whether this erosion occurred subglacially. The events above this unconformity are all flat-lying and suggestive of lacustrine deposition during the retreat of the glaciers.
Methane Gas
The central resistive (>100 ·m) anomaly marked "B" in Figure 6F is of particular interest because it crosses through an area where a number of wells produce economic methane from depths of only 50 m to 55 m (Fig. 1). The depth of resistivity anomaly B at 50 m closely matches with the producing horizon of those shallow wells, and, consequently, it is likely that anomaly B indicates a dry gas saturated sand.
An expanded portion of the seismic-reflection profile, converted to depth, is compared directly to the ERT data in Figure 8. The seismic image is not reliable above ~100 m depth, although close examination shows there may be some amplitude anomalies (Fig. 8A). On the ERT profile (Fig. 8B), the gas anomaly appears slightly larger than its true in situ dimensions because of the smearing effects associated with the resistivity inversion.
A detail in this study is the existence of substantial methane gas-saturated zones at shallow depths just below 30 m deep, and that this shallow gas existed in commercial quantities. The gas is clearly evident by high resistivities in the nearby electrical logs and in the ERT images. In any event, the geophysical profiles show that the gas-saturated zone lies within the glacial deposits immediately to the east of the valley edge. This is consistent with the knowledge that the gas is geochemically identical to that produced from the Cretaceous Bluesky sandstone, which was exposed by erosion of the glacial valley. It is interesting to note further that the producing gas wells (Fig. 1C) fall within an area roughly 4 km by 4 km. This suggests that there is continuity of both the porous materials and the less permeable seal material over these dimensions. However, whether these reservoirs and seals are distinct stratigraphic units over these dimensions within the valley fill is not known.
Having only a 2-D profile here, interpretations are limited, but more advanced seismic tomography algorithms could be applied to increase the lateral and vertical detail in the velocity model. This might be able to further discriminate differences in the materials above and below reflection event Q1, which is believed to originate at an erosional surface within the fill. Fully 3-D geophysical information, especially a high-resolution seismic-reflection volume, would provide additional insight into the valley detail, as shown by the numerous studies in the North Sea areas that were able to take advantage of the existence of extensive petroleum industry surveys.
As a final comment, it is worth noting that the details of the in-valley structure observed in the seismic-reflection profile cannot at present be fully appreciated because there is scant supporting geological information. It would be useful if future drilling projects were able to obtain more detailed information on the detailed stratigraphy and possibly ages of the buried valley sediments.
|
CONCLUSIONS |
|---|
|
TOPABSTRACTINTRODUCTIONBACKGROUNDGEOLOGICAL BACKGROUNDMETHODS AND FIELD PROGRAMRESULTSDISCUSSIONCONCLUSIONSREFERENCES CITED |
|---|
This study highlights the utility of seismic and electrical geophysical investigations in delineating buried valleys. Large contrasts in compressional wave velocities and electrical resistivity between valley fill and the bounding uneroded Cretaceous and Paleozoic rocks led both the seismic and the resistivity measurements to similar interpretations of the gross valley structure. In particular, the seismic-refraction information, normally ignored in commercial surveys, provided easily obtained information that was useful to delineate the edges of the buried valley. The valley-fill structures seen in the seismic profile are similar to many of those observed in the North Sea, suggesting that they formed via similar processes.
|
ACKNOWLEDGMENTS |
|---|
|
REFERENCES CITED |
|---|
|
TOPABSTRACTINTRODUCTIONBACKGROUNDGEOLOGICAL BACKGROUNDMETHODS AND FIELD PROGRAMRESULTSDISCUSSIONCONCLUSIONSREFERENCES CITED |
|---|
Ahmad, J. 2006, High-Resolution Seismic and Electrical Resistivity Tomography Techniques Applied to Image and Characterize a Buried Channel [M.Sc. thesis]: Edmonton, University of Alberta, 201 p.
Anderson, F.J., Shurr, G.W., and Fischer, D.W. 2006, History, geology, and potential hydrogeochemical indicators of natural gas occurrence and production from Quaternary glacial drift and upper-Cretaceous sedimentary bedrock in north-central North Dakota: North Dakota Geological Survey Geological Investigations 33, 25 p.
Andriashek, L.D., and Atkinson, N. 2007, Buried channels and glacial-drift aquifers in the Fort McMurray region, NE Alberta: Alberta Energy and Utilities Board, Alberta Geological Survey, Energy and Utilities Board/Alberta Geological Survey Earth Sciences Report 2007–01, 170 p.
Baines, D., Smith, D.G., Froese, D.G., Bauman, P., and Nimeck, G. 2002, Electrical resistivity ground imaging (ERGI): A new tool for mapping the lithology and geometry of channel-belts and valley-fills: Sedimentology, v. 49, no. 3, p. 441– 449, doi: 10.1046/j.1365-3091.2002.00453.x.[CrossRef][Web of Science][GeoRef]
Baker, G.S. 1999, Processing Near-Surface Seismic-Reflection Data: A Primer: Course Notes Series: Tulsa, Oklahoma, Society of Exploration Geophysicists, 77 p.
Baker, G.S., Steeples, D.W., and Drake, M. 1998, Muting the noise cone in near-surface reflection data: An example from southeastern Kansas: Geophysics, v. 63, no. 4, p. 1332– 1338, doi: 10.1190/1.1444434.[CrossRef][Web of Science][GeoRef]
Beaney, C.L. 2002, Tunnel channels in southeast Alberta, Canada: Evidence for catastrophic channelized drainage: Quaternary International, v. 90, p. 67– 74, doi: 10.1016/S1040-6182(01)00093-3.[CrossRef][Web of Science][GeoRef]
Bergman, B., Tryggvason, A., and Juhlin, C. 2004, High-resolution seismic traveltime tomography incorporating static corrections applied to a till-covered bedrock environment: Geophysics, v. 69, no. 4, p. 1082– 1090, doi: 10.1190/1.1778250.[CrossRef][Web of Science][GeoRef]
Bersezio, R., Giudici, M., and Mele, M. 2007, Combining sedimentological and geophysical data for high-resolution 3-D mapping of fluvial architectural elements in the Quaternary Po Plain (Italy): Sedimentary Geology, v. 202, no. 1–2, p. 230– 248, doi: 10.1016/j.sedgeo.2007.05.002.[CrossRef][Web of Science]
Boyce, J.I., Eyles, N., and Pugin, A. 1995, Seismic-reflection, borehole and outcrop geometry of late Wisconsin tills at a proposed landfill near Toronto, Ontario: Canadian Journal of Earth Sciences, v. 32, no. 9, p. 1331– 1349.[GeoRef]
Boyd, R., Scott, D.B., and Douma, M. 1988, Glacial tunnel valleys and Quaternary history of the Outer Scotian shelf: Nature, v. 333, no. 6168, p. 61– 64, doi: 10.1038/333061a0.[CrossRef][GeoRef]
Bradford, J.H., Sawyer, D.S., Zelt, C.A., and Oldow, J.S. 1998, Imaging a shallow aquifer in temperate glacial sediments using seismic reflection profiling with DMO processing: Geophysics, v. 63, no. 4, p. 1248– 1256, doi: 10.1190/1.1444426.[CrossRef][Web of Science][GeoRef]
Brennand, T.A., and Shaw, J. 1994, Tunnel channels and associated landforms, south-central Ontario—Their implications for ice-sheet hydrology: Canadian Journal of Earth Sciences, v. 31, no. 3, p. 505– 522.[GeoRef]
Brouwer, J., and Helbig, K. 1998, Shallow High-Resolution Seismic Reflection; Handbook of Geophysical Exploration: Section I. Seismic Exploration: Oxford, UK, Elsevier Science, 404 p.
Buker, F., Green, A.G., and Horstmeyer, H. 1998, Shallow seismic reflection study of a glaciated valley: Geophysics, v. 63, no. 4, p. 1395– 1407, doi: 10.1190/1.1444441.[CrossRef][Web of Science][GeoRef]
Buker, F., Green, A.G., and Horstmeyer, H. 2000, 3-D high-resolution reflection seismic imaging of unconsolidated glacial and glaciolacustrine sediments: Processing and interpretation: Geophysics, v. 65, no. 1, p. 18– 34, doi: 10.1190/1.1444709.[CrossRef][Web of Science][GeoRef]
Clayton, L., Attig, J.W., and Michelson, D.M. 1999, Tunnel channels formed in Wisconsin during the last glaciation, in Michelson D.M., Attig J.W. eds., Glacial Processes Past and Present: Geological Society of America Special Paper 337, p. 69– 82.[CrossRef]
Cutler, P.M., Colgan, P.M., and Mickelson, D.M. 2002, Sedimentologic evidence for outburst floods from the Laurentide Ice Sheet margin in Wisconsin, USA: Implications for tunnel-channel formation: Quaternary International, v. 90, p. 23– 40, doi: 10.1016/S1040-6182(01)00090-8.[CrossRef][Web of Science][GeoRef]
Daily, W., Ramirez, A., Binley, A., and LaBrecque, D. 2005, Electrical resistivity tomography—Theory and practice, in Butler D.K. ed., Near-Surface Geophysics: Investigations in Geophysics: Tulsa, Society of Exploration Geophysicists, p. 525– 561.
Darling, T. 2005, Well Logging and Formation Evaluation, Gulf Drilling Guides: Boston, Elsevier, 326 p.
Denis, M., Buoncristiani, J.F., Konate, M., and Guiraud, M. 2007, The origin and glaciodynamic significance of sandstone ridge networks from the Hirnantian glaciation of the Djado Basin (Niger): Sedimentology, v. 54, no. 6, p. 1225– 1243, doi: 10.1111/j.1365-3091.2007.00879.x.[CrossRef][Web of Science][GeoRef]
Eberle, D.G., and Siemon, B. 2006, Identification of buried valleys using the BGR helicopter-borne geophysical system: Near-Surface Geophysics, v. 4, no. 2, p. 125– 133.
Ehlers, J. 2005, Quaternary climatic changes and landscape evolution, in Koster E.A. ed., The Physical Geography of Western Europe: The Oxford Regional Environments Series: Oxford, UK, Oxford University Press, p. 39– 60.
Ehlers, J., and Linke, G. 1989, The origin of deep buried channels of Elsterian age in northwest Germany: Journal of Quaternary Science, v. 4, no. 3, p. 255– 265, doi: 10.1002/jqs.3390040306.[GeoRef]
Eiriksson, J., Kristensen, P.H., Lykke-Andersen, H., Brooks, K., Murray, A., Knudsen, K.L., and Glaister, C. 2006, A sedimentary record from a deep Quaternary valley in the southern Lillebaelt area, Denmark: Eemian and early Weichselian lithology and chronology at Mommark: Boreas, v. 35, no. 2, p. 320– 331, doi: 10.1080/03009480600612377.[CrossRef][Web of Science][GeoRef]
Ellis, D.V. 1987, Well Logging for Earth Scientists: New York, Elsevier, 532 p.
Eyles, N., and de Broekert, P. 2001, Glacial tunnel valleys in the Eastern Goldfields of Western Australia cut below the late Paleozoic Pilbara ice sheet: Palaeogeography, Palaeoclimatology, Palaeoecology, v. 171, no. 1–2, p. 29– 40, doi: 10.1016/S0031-0182(01)00265-6.[CrossRef][GeoRef]
Eyles, N., Doughty, M., Boyce, J.I., Mullins, H.T., Halfman, J.D., and Koseoglu, B. 2003, Acoustic architecture of glaciolacustrine sediments deformed during zonal stagnation of the Laurentide Ice Sheet Mazinaw Lake, Ontario, Canada: Sedimentary Geology, v. 157, no. 1–2, p. 133– 151, doi: 10.1016/S0037-0738(02)00229-4.[CrossRef][Web of Science][GeoRef]
Fennel, J., Sikstrom, C., and Lukie, T. 2001, A multidisciplinary approach to resolving complex hydrological systems, in 54th Canadian Geotechnical Conference, 2nd Joint International Association of Hydrologists and Canadian Geotechnical Soceity Groundwater Conference, Calgary, p. 133– 140.
Fenton, M.M., Schriener, B.T., Nielsen, E., and Pawlowicz, J.G. 1994, Quaternary geology of the Western Plains, in Mossop G.D., Shetson I. eds., Geological Atlas of the Western Canada Sedimentary Basin: Calgary, Alberta, Canadian Society of Petroleum Geologists and the Alberta Research Council, p. 413– 420.
Fisher, T.G., Jol, H.M., and Boudreau, A.M. 2005, Saginaw Lobe tunnel channels (Laurentide Ice Sheet) and their significance in south-central Michigan, USA: Quaternary Science Reviews, v. 24, no. 22, p. 2375– 2391, doi: 10.1016/j.quascirev.2004.11.019.[CrossRef][Web of Science][GeoRef]
Francese, R.G., Hajnal, Z., and Prugger, A. 2002, High-resolution images of shallow aquifers—A challenge in near-surface seismology: Geophysics, v. 67, no. 1, p. 177– 187, doi: 10.1190/1.1451490.[CrossRef][Web of Science][GeoRef]
Gabriel, G. 2006, Gravity investigation of buried Pleistocene subglacial valleys: Near-Surface Geophysics, v. 4, no. 5, p. 321– 332.
Gabriel, G., Kirsch, R., Siemon, B., and Wiederhold, H. 2003, Geophysical investigation of buried Pleistocene subglacial valleys in northern Germany: Journal of Applied Geophysics, v. 53, no. 4, p. 159– 180, doi: 10.1016/j.jappgeo.2003.08.005.[CrossRef][Web of Science][GeoRef]
Garcia-Gil, S., Vilas, F., and Garcia-Garcia, A. 2002, Shallow gas features in incised-valley fills (Ria de Vigo, NW Spain): A case study: Continental Shelf Research, v. 22, no. 16, p. 2303– 2315, doi: 10.1016/S0278-4343(02)00057-2.[CrossRef][Web of Science][GeoRef]
Glasser, N.F., and Bennett, M.R. 2004, Glacial erosional landforms: Origins and significance for palaeoglaciology: Progress in Physical Geography, v. 28, no. 1, p. 43– 75, doi: 10.1191/0309133304pp401ra.
Hall, H.D., and Hajnal, Z. 1962, The gravimeter in studies of buried valleys: Geophysics, v. 27, no. 6, p. 939– 951, doi: 10.1190/1.1439120.[CrossRef][GeoRef]
Halliday, E.J., Barrie, J.V., Chapman, N.R., and Rohr, K.M.M. 2008, Structurally controlled hydrocarbon seeps on a glaciated continental margin, Hecate Strait, offshore British Columbia: Marine Geology, v. 252, p. 193– 206, doi: 10.1016/j.margeo.2008.04.009.[CrossRef][Web of Science][GeoRef]
Hearst, J.R., Nelson, P.H., and Paillet, F.L. 2000, Well Logging for Physical Properties: A Handbook for Geophysicists, Geologists, and Engineers: New York, McGraw Hill Higher Education, 483 p.
Hobson, G.D., Hunter, J.M.A., and Scott, J.S. 1970, A shallow seismic survey, elastic constants and surficial geology of part of Defense Research Establishment Suffield (DRES), Suffield, Alberta: Ottawa, Geological Survey of Canada Paper 69–13, 24 p.
Holzschuh, J. 2002, Low-cost geophysical investigations of a paleochannel aquifer in the Eastern Goldfields, Western Australia: Geophysics, v. 67, no. 3, p. 690– 700, doi: 10.1190/1.1484512.[CrossRef][Web of Science][GeoRef]
Hooke, R.L., and Jennings, C.E. 2006, On the formation of the tunnel valleys of the southern Laurentide Ice Sheet: Quaternary Science Reviews, v. 25, no. 11–12, p. 1364– 1372, doi: 10.1016/j.quascirev.2006.01.018.[CrossRef][Web of Science][GeoRef]
Hunter, J.A., Pullan, S.E., Burns, R.A., Gagne, R.M., and Good, R.L. 1984, Shallow seismic-reflection mapping of the overburden-bedrock interface with the engineering seismograph—Some simple techniques: Geophysics, v. 49, no. 8, p. 1381– 1385, doi: 10.1190/1.1441766.[CrossRef][Web of Science][GeoRef]
Huuse, M., and Lykke-Andersen, H. 2000, Overdeepened Quaternary valleys in the eastern Danish North Sea: Morphology and origin: Quaternary Science Reviews, v. 19, no. 12, p. 1233– 1253, doi: 10.1016/S0277-3791(99)00103-1.[CrossRef][Web of Science][GeoRef]
Huuse, M., Lykke-Andersen, H., and Michelsen, O. 2001, Cenozoic evolution of the eastern Danish North Sea: Marine Geology, v. 177, no. 3–4, p. 243– 269, doi: 10.1016/S0025-3227(01)00168-2.[CrossRef][Web of Science][GeoRef]
Huuse, M., Piotrowski, J.A., and Lykke-Andersen, H. 2003, Geophysical investigations of buried Quaternary valleys in the formerly glaciated NW European lowland: Significance for groundwater exploration: Journal of Applied Geophysics, v. 53, no. 4, p. 153– 157, doi: 10.1016/j.jappgeo.2003.08.003.[CrossRef][Web of Science]
Jianyi, H., Lirong, D., and Qiming, L. 1999, The geology of natural gas in the People's Republic of China: A review: Journal of Petroleum Geology, v. 22, no. 2, p. 191– 213, doi: 10.1111/j.1747-5457.1999.tb00467.x.[Web of Science][GeoRef]
Jorgensen, F., and Sandersen, P.B.E. 2006, Buried and open tunnel valleys in Denmark—Erosion beneath multiple ice sheets: Quaternary Science Reviews, v. 25, no. 11–12, p. 1339– 1363, doi: 10.1016/j.quascirev.2005.11.006.[CrossRef][Web of Science][GeoRef]
Jørgensen, F., and Sandersen, P.B.E. 2008, Mapping of buried tunnel valleys in Denmark: New perspectives for the interpretation of the Quaternary succession: Geological Survey of Denmark and Greenland Bulletin, v. 15, p. 33– 36.
Jorgensen, F., Lykke-Andersen, H., Sandersen, P.B.E., Auken, E., and Normark, E. 2003, Geophysical investigations of buried Quaternary valleys in Denmark: An integrated application of transient electromagnetic soundings, reflection seismic surveys and exploratory drillings: Journal of Applied Geophysics, v. 53, no. 4, p. 215– 228, doi: 10.1016/j.jappgeo.2003.08.017.[CrossRef][Web of Science][GeoRef]
Keiswetter, D., Black, R., and Steeples, D. 1994, Mapping bedrock beneath glacial till using cdp seismic-reflection methods: Geophysical Research Letters, v. 21, no. 6, p. 453– 456, doi: 10.1029/94GL00051.[CrossRef][Web of Science][GeoRef]
Kilner, M., West, L.J., and Murray, T. 2005, Characterisation of glacial sediments using geophysical methods for groundwater source protection: Journal of Applied Geophysics, v. 57, no. 4, p. 293– 305, doi: 10.1016/j.jappgeo.2005.02.002.[CrossRef][Web of Science][GeoRef]
Kirsch, R., Rumpel, H.-M., Scheer, W., and Wiederhold, H. 2006, Groundwater Resources in Buried Valleys—A Challenge for Geosciences: Hannover, Germany, Leibniz Institute for Applied Geosciences (CGA-Institut), 303 p.
Kluiving, S.J., Bosch, J.H.A., Ebbing, J.H.J., Mesdag, C.S., and Westerhoff, R.S. 2003, Onshore and offshore seismic and lithostratigraphic analysis of a deeply incised Quaternary buried valley-system in the northern Netherlands: Journal of Applied Geophysics, v. 53, no. 4, p. 249– 271, doi: 10.1016/j.jappgeo.2003.08.002.[CrossRef][Web of Science][GeoRef]
Kristensen, T.B., Huuse, M., Piotrowski, J.A., and Clausen, O.R. 2007, A morphometric analysis of tunnel valleys in the eastern North Sea based on 3D seismic data: Journal of Quaternary Science, v. 22, no. 8, p. 801– 815, doi: 10.1002/jqs.1123.[CrossRef][GeoRef]
Kristensen, T.B., Plotrowski, A.A., Huuse, M., Clausen, O.R., and Hamberg, L. 2008, Time-transgressive tunnel valley formation indicated by infill sediment structure, North Sea—The role of glaciohydraulic super cooling: Earth Surface Processes and Landforms, v. 33, no. 4, p. 546– 559, doi: 10.1002/esp.1668.[CrossRef][Web of Science][GeoRef]
Kuhlmann, G., and Wong, T.E. 2008, Pliocene paleoenvironment evolution as interpreted from 3D-seismic data in the southern North Sea, Dutch offshore sector: Marine and Petroleum Geology, v. 25, no. 2, p. 173– 189, doi: 10.1016/j.marpetgeo.2007.05.009.[CrossRef][Web of Science]
Kuhn, O. 2004, Ancient Chinese drilling: Canadian Society of Exploration Geophysicists Recorder, v. 29, no. 6, p. 39– 43.
Langenberg, C.W., Hein, F.J., Lawton, D., and Cunningham, J. 2002, Seismic modeling of fluvial-estuarine deposits in the Athabasca oil sands using ray-tracing techniques, Steepbank River area, northeastern Alberta: Bulletin of Canadian Petroleum Geology, v. 50, no. 1, p. 178– 204, doi: 10.2113/50.1.178.
Lennox, D.H., and Carlson, V. 1967, Geophysical exploration for buried valleys in an area north of 2 Hills Alberta: Geophysics, v. 32, no. 2, p. 331– 362, doi: 10.1190/1.1439871.[CrossRef][Web of Science][GeoRef]
Letourneau, J., Miller, R., Muehlenbachs, K., and Rostron, B. 2000, Stable isotope geochemistry of some natural gas macroseeps in Alberta, in GeoCanada 2000 Conference, Calgary, abstract 569, p. 4.
Lin, C.M., Gu, L.X., Li, G.Y., Zhao, Y.Y., and Jiang, W.S. 2004, Geology and formation mechanism of late Quaternary shallow biogenic gas reservoirs in the Hangzhou Bay area, eastern China: American Association of Petroleum Geologists Bulletin, v. 88, no. 5, p. 613– 625, doi: 10.1306/01070403038.
Loke, M.H. 2004, Tutorial: 2D and 3D Electrical Imaging Surveys: Panang, Malaysia, Geotomo Software, 136 p.
Loke, M.H., and Barker, R.D. 1996, Practical techniques for 3D resistivity surveys and data inversion: Geophysical Prospecting, v. 44, no. 3, p. 499– 523, doi: 10.1111/j.1365-2478.1996.tb00162.x.[CrossRef][Web of Science][GeoRef]
Lonergan, L., Maidment, S.C.R., and Collier, J.S. 2006, Pleistocene subglacial tunnel valleys in the central North Sea basin: 3-D morphology and evolution: Journal of Quaternary Science, v. 21, no. 8, p. 891– 903, doi: 10.1002/jqs.1015.[CrossRef][GeoRef]
Mathews, W.H. 1980, Retreat of the Last Ice Sheets in Northeastern British Columbia and Adjacent Alberta: Ottawa, Geological Survey of Canada 331, 24 p.
McKercher, R.B., and Wolfe, B. 1986, Understanding Western Canada's Dominion Land Survey System: Saskatoon, University of Saskatchewan, 26 p.
Miller, R.D. 1992, Normal moveout stretch mute on shallow-reflection data: Geophysics, v. 57, no. 11, p. 1502– 1507, doi: 10.1190/1.1443217.[CrossRef][Web of Science][GeoRef]
Miller, R.D., and Xia, J.H. 1998, Large near-surface velocity gradients on shallow seismic reflection data: Geophysics, v. 63, no. 4, p. 1348– 1356, doi: 10.1190/1.1444436.[CrossRef][Web of Science][GeoRef]
Møller, M.J., Olsen, H., Ploug, C., Strykowski, G., and Hjorth, H. 2007, Gravity field separation and mapping of buried Quaternary valleys in Lolland, Denmark, using old geophysical data: Journal of Geodynamics, v. 43, no. 2, p. 330– 337, doi: 10.1016/j.jog.2006.09.021.[CrossRef][Web of Science][GeoRef]
Munro-Stasiuk, M.J. 2003, Subglacial Lake McGregor, south-central Alberta, Canada: Sedimentary Geology, v. 160, no. 4, p. 325– 350, doi: 10.1016/S0037-0738(03)00090-3.[CrossRef][Web of Science][GeoRef]
Musil, M., Maurer, H., Green, A.G., Horstmeyer, H., Nitsche, F.O., Vonder Muhll, D., and Springman, S. 2002, Shallow seismic surveying of an Alpine rock glacier: Geophysics, v. 67, no. 6, p. 1701– 1710, doi: 10.1190/1.1527071.[CrossRef][Web of Science][GeoRef]
Nitsche, F.O., Green, A.G., Horstmeyer, H., and Buker, F. 2002, Late Quaternary depositional history of the Reuss delta, Switzerland: Constraints from high-resolution seismic reflection and georadar surveys: Journal of Quaternary Science, v. 17, no. 2, p. 131– 143, doi: 10.1002/jqs.645.[CrossRef]
O'Cofaigh, C. 1996, Tunnel valley genesis: Progress in Physical Geography, v. 20, no. 1, p. 1– 19, doi: 10.1177/030913339602000101.
Pawlowicz, J.G., Hicken, A.S., Nicoll, T.J., Fenton, M.M., Paulen, R.C., Plouffe, A., and Smith, I.R. 2004, Shallow gas in drift: Northwestern Alberta: Energy and Utilities Board/Alberta Geological Survey Information Series 130: Edmonton, Alberta Energy and Utilities Board.
Pawlowicz, J.G., Hicken, A.S., Nicoll, T.J., Fenton, M.M., Paulen, R.C., Plouffe, A., and Smith, I.R. 2005a, Bedrock topography of the Zama Lake Area, Alberta (NTS 84L): Energy and Utilities Board/Alberta Geological Survey Map 328, scale 1:250,000.
Pawlowicz, J.G., Hicken, A.S., Nicoll, T.J., Fenton, M.M., Paulen, R.C., Plouffe, A., and Smith, I.R. 2005b, Drift Thickness of the Zama Lake Area, Alberta (NTS 84L): Alberta Energy and Utility Board Map 329, scale 1:250,000.
Pelton, J.R. 2005, Near-surface seismology: Surface-based methods, in Butler D.K. ed., Near-Surface Geophysics: Investigations in Geophysics: Tulsa, Oklahoma, Society of Exploration Geophysicists, p. 219– 263.
Piotrowski, J.A. 1994, Tunnel-valley formation in northwest Germany—Geology, mechanisms of formation and subglacial bed conditions for the Bornhoved Tunnel Valley: Sedimentary Geology, v. 89, no. 1–2, p. 107– 141, doi: 10.1016/0037-0738(94)90086-8.[CrossRef][Web of Science][GeoRef]
Praeg, D. 2003, Seismic imaging of mid-Pleistocene tunnel-valleys in the North Sea Basin—High resolution from low frequencies: Journal of Applied Geophysics, v. 53, no. 4, p. 273– 298, doi: 10.1016/j.jappgeo.2003.08.001.[CrossRef][Web of Science][GeoRef]
Pugin, A., and Rossetti, S. 1992, Acquisition of land based high-resolution seismic profiles in glacial basins: 2. Case-studies in the Alpine foreland of Switzerland: Eclogae Geologicae Helvetiae, v. 85, no. 2, p. 491– 502.[Web of Science][GeoRef]
Pugin, A., Pullan, S.E., and Sharpe, D.R. 1996, Observations of tunnel channels in glacial sediments with shallow land-based seismic reflection: Annals of Glaciology, v. 22, p. 176– 180.[GeoRef]
Rafaelsen, B., Andreassen, K., Hogstad, K., and Kuilman, L.W. 2007, Large-scale glaciotectonic-imbricated thrust sheets on three-dimensional seismic data: Facts or artefacts?: Basin Research, v. 19, no. 1, p. 87– 103, doi: 10.1111/j.1365-2117.2007.00313.x.[CrossRef][Web of Science][GeoRef]
Rains, R.B., Shaw, J., Sjogren, D.B., Munro-Stasiuk, M.J., Skoye, K.R., Young, R.R., and Thompson, R.T. 2002, Subglacial tunnel channels, Porcupine Hills, southwest Alberta, Canada: Quaternary International, v. 90, p. 57– 65, doi: 10.1016/S1040-6182(01)00092-1.[CrossRef][Web of Science][GeoRef]
Roberts, M.C., Pullan, S.E., and Hunter, J.A. 1992, Applications of land-based high-resolution seismic-reflection analysis to Quaternary and geomorphic research: Quaternary Science Reviews, v. 11, no. 5, p. 557– 568, doi: 10.1016/0277-3791(92)90012-W.[CrossRef][Web of Science][GeoRef]
Ross, M., Parent, M., Benjumea, B., and Hunter, J. 2006, The late Quaternary stratigraphic record northwest of Montreal: Regional ice-sheet dynamics, ice-stream activity, and early deglacial events: Canadian Journal of Earth Sciences, v. 43, no. 4, p. 461– 485, doi: 10.1139/E05-118.[GeoRef]
Russell, H.A.J., Arnott, R.W.C., and Sharpe, D.R. 2003, Evidence for rapid sedimentation in a tunnel channel, Oak Ridges Moraine, southern Ontario, Canada: Sedimentary Geology, v. 160, no. 1–3, p. 33– 55, doi: 10.1016/S0037-0738(02)00335-4.[CrossRef][Web of Science][GeoRef]
Schmelzbach, C., Green, A.G., and Horstmeyer, H. 2005, Ultra-shallow seismic reflection imaging in a region characterized by high source-generated noise: Near-Surface Geophysics, v. 3, no. 1, p. 33– 46.
Schmitt, D.R. 1999, Seismic attributes for monitoring of a shallow heated heavy oil reservoir: A case study:Geophysics, v. 64, no. 2, p. 368– 377, doi: 10.1190/1.1444541.
Sharpe, D.R., Pullan, S.E., and Warman, T.A. 1992, A basin analysis of the Wabigoon area of Lake Agassiz, a Quaternary clay basin in northwestern Ontario: Géographie physique et Quaternaire, v. 46, no. 3, p. 295– 309.[Web of Science][GeoRef]
Shaw, J. 2002, The meltwater hypothesis for subglacial bedforms: Quaternary International, v. 90, p. 5– 22, doi: 10.1016/S1040-6182(01)00089-1.[CrossRef][Web of Science][GeoRef]
Sjogren, D.B., Fisher, T.G., Taylor, L.D., Jol, H.M., and Munro-Stasiuk, M.J. 2002, Incipient tunnel channels: Quaternary International, v. 90, p. 41– 56, doi: 10.1016/S1040-6182(01)00091-X.[CrossRef][Web of Science][GeoRef]
Smith, R.C., and Sjogren, D.B. 2006, An evaluation of electrical resistivity imaging (ERI) in Quaternary sediments, southern Alberta, Canada: Geosphere, v. 2, no. 6, p. 287– 298, doi: 10.1130/GES00048.1.
Sørensen, K. I., and Auken, E. 2004, SkyTEM—A new high-resolution helicopter transient electromagnetic system: Exploration Geophysics, v. 35, p. 191– 199.
Stalker, A. M. 1960, Buried Valleys in Central and Southern Alberta: Ottawa, Geological Survey of Canada 60–32, p. 13.
Thomsen, R., Søndergaard, V.H., and Sørensen, K.I. 2004, Hydrogeological mapping as a basis for establishing site-specific groundwater protection zones in Denmark: Hydrogeology Journal, v. 12, no. 5, p. 550– 562, doi: 10.1007/s10040-004-0345-1.[CrossRef][Web of Science]
van Overmeeren, R.A. 2001, Hagedoorn's plus-minus method: The beauty of simplicity: Geophysical Prospecting, v. 49, no. 6, p. 687– 696, doi: 10.1046/j.1365-2478.2001.00283.x.[CrossRef][Web of Science][GeoRef]
Wiederhold, H., Buness, H.A., and Bram, K. 1998, Glacial structures in northern Germany revealed by a high-resolution reflection seismic survey: Geophysics, v. 63, no. 4, p. 1265– 1272, doi: 10.1190/1.1444428.[CrossRef][Web of Science][GeoRef]
Yilmaz, Ö. 2001, Seismic Data Analysis: Processing, Inversion, and Interpretation of Seismic Data: Tulsa, Society of Exploration Geophysicists, Investigations in Geophysics 10, 526 p.
RECEIVED FOR PUBLICATION August 5, 2007
REVISED MANUSCRIPT RECEIVED January 22, 2009
MANUSCRIPT ACCEPTED February 24, 2009
| ||||||||||||||||||||||||||||||||||||||||||||||||||||
| JOURNAL HOME | HELP | CONTACT PUBLISHER | SUBSCRIBE | ARCHIVE | SEARCH | TABLE OF CONTENTS |
