Geophysical Field Schools at the University of Saskatchewan

IGOR MOROZOV, JIM MERRIAM, and SAM BUTLER

UNIVERSITY OF SASKATCHEWAN, DEPTARTMENT OF GEOLOGICAL SCIENCES,

SASKATOON, SK, CANADA, IGOR.MOROZOV@USASK.CA

Abstract

Mineral resources are a foundation of the economy in western Canada and particularly in Saskatchewan. With recent changes in the markets and emphasis on sustainability and the green economy, there is an increasing demand for geophysics professionals. This article gives an overview of the geophysics program at the University of Saskatchewan and describes how it meets these new challenges. Field schools designed around continuous data acquisition projects have been the central element of our program for many years. This article gives highlights of some of the most important results from these projects.

Introduction

The Department of Geological Sciences of the University of Saskatchewan (UofS) offers the only professionally accredited program in geophysics in the province. The key components of the geophysical skillset are data collection and interpretation. In the absence of support for large projects such as CREWES and limited time spans of major projects like Lithoprobe (2005), Weyburn CO2 storage monitoring (IEAGHG, 2012) or Aquistore (2023), our program has nevertheless been traditionally strong in collecting data and giving broad hands-on experience to the students. In this article, we start with a brief overview of the geophysics program and continue by highlighting key results from many years of field-school observations in Saskatchewan.

Geophysics Program at the University of Saskatchewan

Over several decades, the UofS geophysics program has been relatively small but versatile, covering a broad variety of topics and offering good exposure to most theoretical and field aspects of geophysical work (Table 1). UofS geophysics graduates pursue successful careers in the oil and gas industry (mostly Calgary) and mineral exploration (potash and uranium, mostly in Saskatchewan). The demand for geophysical expertise in mineral exploration has grown significantly in the recent years, particularly with the rise of cable-free multi-nodal and distributed sensing technologies. Another notable trend in recent years is the increase of interest in near-surface, engineering, and archaeological applications of geophysical methods. In our program, we try to encourage open minds and provide broad and versatile training, which allows our graduates to stay ahead of current and future trends (Table 1).

Table 1. Teaching and research areas of current UofS geophysics faculty.

The annual field school represents the core and the single most important part of our geophysics program. The field school for Geophysics majors is combined with an introduction to geophysical methods for geologists and geoengineers. Over about eight to ten days, the students participate in practical data acquisition using both basic as well as state-of-the-art methods, often with guidance from industry leaders (see the “Acknowledgments” section). The collected data are further analysed in laboratory sections of senior classes and in graduate research. In the next section, we describe some of these projects.

UofS Geophysical Field Schools

The University of Saskatchewan has operated a geophysics field school for many years, started in the 1980’s by Profs. Don Gendzwill and Zoli Hajnal. Our goal has been to expose students to as wide a range of equipment and techniques as possible and to demonstrate how data from one technique can complement another. We also emphasize continuity of studies, with each generation of students contributing to the previous geophysical characterization of the area.

Since 1992, we have focused on the Strawberry Hills area east of Saskatoon, where the University has established a permanent test site for geophysical investigations and training. At the test site, a permanent seismic station using a mid-band Taurus seismometer is located, as well as several artificial targets for practising near-surface imaging techniques. Originally, the site housed a 100-m long ground tiltmeter, which is no longer operational. The site also contains station SASK of the national geodetic network (Canadian Active Control System; CACS (2023)), at which absolute gravity measurements are periodically conducted by Natural Resources Canada (NRCAN).

The Strawberry Hills area is a mix of rural acreage and farmland, which is often in crop while field school is scheduled. For these reasons, access is mostly limited to the University property and grid roads. In recent years, the increasing density of housing in the area has limited the application of seismic because of explosives restrictions. Unfortunately, the development and traffic has also increased the noise level on the seismic station.

In addition to annual surveys at the test site, we have also used magnetotellurics (MT) and gravity to look at subsurface collapse structures near Colonsay, Saskatchewan, and the Elbow impact structure (O’Dale, 2023; Li and Butler, 2021). Recently, the City of Saskatoon has encouraged a slope stability investigation in the South Saskatchewan River valley within the central part of the city. This project occupied two years of field schools, and we are likely to return to the area in the future. The Strawberry Hills and the slope stability projects are further highlighted in the following subsections. Also, during recent field schools, we tried looking at even shallower targets and investigated an old landfill and the use of resistivity and GPR for detecting graves.

Strawberry Hills Project

In the Strawberry Hills area, we have collected about thirty line-miles of gravity profiles at 50 to 100 m station spacings. Figure 1 shows a Bouguer gravity map from a compilation of 1782 UofS field-school stations and 238 oil company stations. Ties at road intersections between surveys done in different years average about 0.05 mGal variances. The data in the early years were leveled with transit level, but differential GPS has been used exclusively since 2000.

Figure 1. Bouguer gravity in the vicinity of the UofS geophysics test site. Coordinates are relative to the NRCAN GPS pier located at 404425E, 5783787N in UTM zone 13, and gravity values are also referenced to this location. Tiny black triangles are gravity stations. Terrain density of 2.35 g/cm3 was used in Bouguer correction to emphasize the shallow geology. Hatched pattern in the NE corner indicates the interpreted gravel deposit. Grey bar labeled S indicates the location of reflection seismic line shown further in the article.

Gravity, magnetics, reflection or refraction seismic, resistivity, induced polarization, self potential, VLF and VLF-R, and time-domain and frequency-domain electromagnetics have been attempted almost every year. Ground-penetrating radar has occasionally been included, but the conductive tills limit the depth of investigation.

Some surveys can be done at small scale, but, for example, gravity and standard reflection seismic imaging are only effective at larger scale at which the effect of the terrain can be mitigated. Our electrical and magnetic targets (described below) are small, which means that the gravity and seismic results cannot be productively integrated with the electrical surveys to allow a joint interpretation. However, with detailed modeling of terrain effects, gravity might still be usefully included in high-resolution studies, as in the slope stability project in the next subsection.

In the Strawberry Hills area, a comparison of the Bouguer gravity with existing well logs suggests the relief on the Cretaceous shales to be the source of (~ 1 mGal) gravity anomalies at horizontal scales of 1 to 10 km (Figure 1). This is likely to be the case over a much wider area. When we first started this project, the Strawberry Hills area was identified as entirely the Battleford Formation (Christiansen, 1970). However, terrain density analysis from the gravity survey indicated a density more appropriate to Floral Formation. A revision of the geology using more recent well logs (The Geological Atlas of Saskatchewan, 2004) suggests that the Battleford may be thin and patchy on the Strawberry Hills, and the Floral is the dominant near-surface till, in agreement with our conclusion.

One well at a site on the northern edge of our gravity survey has intersected a thick gravel layer at a depth of 100 m. We subsequently extended our gravity survey further north and revealed a small (0.3 mGal) gravity anomaly consistent with the known gravel at that site. In subsequent years, we extended the survey and traced the path of the gravel for several miles east and west (hatched pattern in Figure 1). The gravel is tentatively interpreted as part of the Empress Formation and may have some association with the Tyner Valley.

Because no suitable natural induced-polarization (IP) and electromagnetic (EM) targets were found on the university test site, several galvanized steel sheets were buried to make artificial targets. These targets have yielded reliable and informative data from self-potential (SP), IP, magnetics, applied potential, and especially time-domain EM measurements.

The target used most often is a 1.25 m by 7 m vertical rectangular sheet with long axis tilted 22º down toward W 25º N (grey in Figure 2a). Three separate modes of the time-domain EM responses were identified for this target using different transmitters (Figure 2a). To our knowledge, this is the only case in which all three modes have been identified for the same target. The fastest-decaying field is interpreted as a channeling response from the till into the target. This interpretation is based on the polarity with a power-law decay of a half-space induction response and the crossover nature of profiles with a separation of 7 m (red in Figures 2a and 2b). The next fastest-decaying mode is interpreted to be a remanent magnetic response with an exponential decay constant of 0.3 ms and a single peak over the target with a half-height width of 1.5 m (blue in Figure 2b). The slowest mode is interpreted to be the induction response of the target (green in Figure 2b). This mode shows an exponential decay with time constant of 3 ms and a crossover separation of 1 m, consistent with the known dimensions and depth of the target (Figure 2a). The polarity of this response is opposite to that of the channeling response, which is related to the difference in transmitter loops used in the EM47 and EM57 surveys. Note that although the time-domain EM data are so rich in information, other EM techniques such as VLF or EM16 have never yielded interpretable data with this target.

Figure 2. EM profiling across a tilted buried target (galvanised steel sheet): a) position of the target and schematic of three response modes, b) three gates from vertical-component time-domain recordings using EM47 and EM57 systems. In plot b), record values in mV are divided by scaling denominators Rο equal 200 for EM47 gate 1, and 300 and 0.3 for EM57 gates 1 and 20, respectively. Times of gates are labeled in the legends.

The magnetics, IP and time-domain EM data are jointly interpreted to provide information on the target that no single method can manage. The magnetics data yields good estimates of target depth, strike and length, but only weak data on width, vertical extent and plunge. The time-domain data allows an estimate of target conductance, and ultimately of conductivity (limited by the uncertainty in the width) and corroborates the length estimates from magnetics. The IP data (Figure 3) sharpen the depth extent and could yield a better estimate of the width if tighter station spacing is used during profiling. IP responses from this shallow target also contain occurrences of negative and nonlinear IP, which are useful instructional features (Merriam, 2022).

Figure 3. Pseudo-depth sections of double-dipole IP profiling over the same target as in Figure 2: a) with 1-m dipole size, b) with 2-m dipole size.

For the target shown in Figures 2 and 3, early SP surveys suggested that the iron sheet was oxidizing at a rate that would consume it in about fifty years. However, in thirty years of observation, magnetic surveys still show no decrease in the moment of the target. Repeat IP surveys also indicate very little deterioration.

VLF surveys are also routinely conducted as part of our field schools. Saskatoon’s geographic location and the locations of the three VLF stations with the strongest signals at our location (Cutler, LaMoure, and Jim Creek) mean that a conductor with any strike can be excited. The test site contains three 100-m long buried cables aligned E-W, N-S and at forty-five degrees to N, and thus they illustrate the importance of the interaction of target strike and antenna azimuth in VLF interpretation. Jim Creek and especially Cutler transmitters excite the E-W cable strongly, and LaMoure performs better with the N-S cable. The third cable responds weakly to all three VLF transmitters.

With consistent and generous support from the industry (section “Acknowledgments”), one day of each field school is usually devoted to collecting a reflection seismic line. Including extensive seismic work in student training is also important because the collected data require extensive processing, interpretation, and report preparation efforts, which also are an important part of the learning process. To provide realistic seismic acquisition experience, we typically collect a 500¬–800 m long line with 2-m to 4-m receiver spacings using a 96-channel cable recording system and a rolling split spread. For sources, over one hundred of ¼ or ½-pound explosive charges are used.

Over the years of field schools, many locations along grid roads in the Strawberry Hills area (Figure 1) were tested. Figure 4 is an example of the migrated reflection section produced by one of the students (Jeff Bryce) in 2015 at location shown by grey bar in Figure 1. This section showed some of the best responses of the deeper structures.

Figure 4. Migrated image from a seismic reflection line east of the test site (grey bar labeled S in Figure 1). Data processing and image by Jeff Bryce (modified).

More recently, and particularly due to the unfortunate impacts of the pandemic, the emphasis of seismic investigations has shifted to smaller-scale surveys using hammer sources. An example of such shallow seismic work is given in the next subsection.

Slope Stability Project

Several locations on the banks of the South Saskatchewan River within the city limits of Saskatoon have exhibited landslide failures. A most recent prominent slumping event occurred in May 2016 near Saskatchewan crescent east, in an area located across from downtown Saskatoon (Figure 5). The damaged area extended approximately 70 m along the river and toward it. Following this event, the City has constructed remediation structures (retaining wall and drainage pipes) and installed a number of monitoring wells in this area. In 2021 and 2022, we conducted field-school studies at this site, looking for integration of multiple geophysical techniques into geotechnical site characterization and detecting possible variations of the subsurface with time.

Figure 5. Subsurface layering and riverbank slumping in the Saskatchewan Crescent area in Saskatoon (modified after Christiansen (1992)).

A detailed report on the Saskatchewan Crescent slope-stability studies will be published soon in an MSc thesis by Mark Lepitzki. Electrical resistivity and shallow seismic observations proved to be most successful in the area, providing useful ties with well data that are useful for geotechnical characterization. Resistivity imaging was the most innovative and important part of these surveys and consisted in a large-scale, high resolution, 3-D survey using a rolling distributed array collected over 450 receiver stations at about 1-m spacings (Figure 6a). The data acquisition was conducted by DIAS Geophysical, who also provided their computer facility for data inversion.

Figure 6. 3-D electrical resistivity imaging of the landslide area (Figure 5): a) source and receiver layout; b) cross-section Aʹ–A through the inverted 3-D resistivity volume. Images courtesy of Mark Lepitzki (modified).

Resistivity variations within about 2 to 10-m depths (Figure 6b) correlate with the inferred geological structure of the subsurface (Figure 5). The contact between the glacial till and the stratified drift (likely dominated by clay) is seen as an increase of resistivity at 8–10 m depths. The somewhat surprizing low resistivity immediately below the trail (Figure 6) may be due to the geogrid installed as part of the remediation measures, and/or to the resulting increase in moisture content.

Resistivity images were complemented with shallow refraction and multichannel surface-wave (MASW) imaging along the trail (line C–Cʹ in Figure 6a). For MASW, 100-Hz and 24-Hz geophones were placed at 30-cm spacing, and a 2-lb hammer with metal strike plate was used as the source. To provide continuous coverage with constant mutual source-receiver configuration, a rolling, split 72-channel receiver spread was used, similar to standard reflection data recording on land.

Figure 7 shows preliminary results of the 2021 MASW survey. MASW data are conventionally presented in the form of surface-wave dispersion spectra as functions of frequency and phase velocity of the surface wave (e.g., Park et al., 1999). However, we find that for interpretation purposes and particularly for uncertainty estimation, it is more convenient to use phase slowness instead of velocity (Figure 7a). With frequency increasing to about 30 Hz (corresponding to reducing sampling depth), the phase slowness increases to about 10 ms/m. This increase is caused by lower S-wave velocities within the uppermost 1 to 2 meters of the subsurface (Figure 7b).

Figure 7. MASW imaging along line C–Cʹ (trail in Figure 6a): a) sample phase slowness spectrum from one hammer strike, b) estimated distribution of shear modulus. Data processing and mages by Mark Lepitzki (modified).

For geotechnical analysis, the most important mechanical characteristic of the subsurface is the static (slow and generally large-strain) shear modulus. MASW testing provides an approximation for this quantity, in the form of the dynamic (fast-deformation and low-strain) shear modulus. The shear modulus inferred from the measured surface-wave dispersion is shown in Figure 7b. The inverted layering generally correlates with the resistivity image (Figure 6b); however, there is also some disagreement about the depth of the clay-till contact. This disagreement may need to be addressed in more detailed modeling and inversion, or potentially in future field work.

Conclusions

Field schools are among the key parts of the geophysics program at the University of Saskatchewan. Field schools provide training in a variety of field techniques, skills of data analysis and processing, integration of different techniques, and drawing geological and geotechnical conclusions.

Organising the annual field schools around larger ongoing projects allows us to continuously grow the volume of data and its detail, and to increase the practical impacts of studies. Two examples of such ongoing field school projects are illustrated: 1) integrated studies of the Strawberry Hills area near Saskatoon and 2) assessment of subsurface structure and slope stability of a landslide-prone location on the South Saskatchewan Riverbank in Saskatoon.

Acknowledgments

We have only been able to conduct field schools with generous support from industry. We especially acknowledge Patterson Mining Geophysics, whose crew have been with us annually since the early 1990s. Induced polarization, frequency-domain and time-domain EM would not have been possible without Patterson’s support. The state-of-the-art smart-nodes resistivity/IP equipment of DIAS Geophysical has been a regular and most valuable component of our field schools in recent years. The Potash Corporation of Saskatchewan, now Nutrien, has in the past contributed ground penetrating radar and time-domain EM, vertical seismic profiling, and seismic reflection work. David Goldak has introduced magnetotellurics on two occasions. Phoenix Geophysics contributed magnetotellurics expertise to the survey of the Elbow impact structure in 2019. Tyler Mathieson, of Discovery International Geophysics, has demonstrated HeliSam (although without the helicopter). Peter Dueck of Axiom (now of GoldSpot Discoveries) has contributed a drone magnetics survey. Recently, Randy Brehm of NSGeo Imaging helped with conducting several forensic and near-surface ground-penetrating radar surveys and data analysis.

Since the1990’s, the seismic part of UofS field schools was facilitated by donations of explosives by Dyno Nobel and drilling services by Great Plains Drilling. A 96-channel Bison seismic recorder with cables and geophones was donated by the University of Wyoming (USA). In 2018, Rafael Gonzalez (electronics technician at the Department of Geological Sciences) built a new analog of the Bison system using modern electronic components and technology. His, and previously Lloyd Litwin’s contributions were invaluable for many years of field schools and other aspects of our program.

References

Aquistore (2023). https://ptrc.ca/aquistore, last accessed 25 June 2023.

CACS (2023). Canadian Active Control System (CACS), https://webapp.csrs-scrs.nrcan-rncan.gc.ca/geod/data-donnees/cacs-scca.php?locale=en, last accessed 25 June 2023.

Christiansen, E. A., ed. (1970). The physical environment of Saskatoon, Saskatchewan Research Council in cooperation with the National Research Council of Canada, Ottawa, 68 pp.

Christiansen, E. A. (1992). Pleistocene stratigraphy of the Saskatoon area, Saskatchewan, Canada: an update. Canadian Journal of Earth Sciences, 29(8), 1767–1778.

O’Dale, C. (2023). Elbow impact structure, https://craterexplorer.ca/elbow-impact-structure/, last accessed 25 June 2023.

IEAGHG (2012). IEA GHG Weyburn CO2 Monitoring & Storage Project, https://ieaghg.org/docs/general_publications/weyburn.pdf, last accessed 25 June 2023.

Li, A., and S.L. Butler (2021). Forward modeling of magnetotellurics using Comsol Multiphysics, Applied Computing in Geosciences, 12, https://doi.org/10.1016/j.acags.2021.100073

Lithoprobe (2005). Canada’s national Lithoprobe geoscience project, https://lithoprobe.eos.ubc.ca/, last accessed 25 June 2023.

Merriam, J. B. (2022). Induced polarization: processes and properties, Samizdat Press, 207 pp.

Park, C. B., R. D. Miller, and J. Xia (1999). Multichannel analysis of surface waves, Geophysics, 64, 800–808.

The Geological Atlas of Saskatchewan (2004). CD-ROM version 7, Saskatchewan industry and Resources, Regina.

About the Authors

Igor B. Morozov received a Ph.D. (1985) in theoretical physics from Moscow State University (Russia), from a supervisor who was one of the inventors of the synchrotron radiation. In early 1990s, he switched to seismology and worked on ultra-long, nuclear-explosion sourced Deep Seismic Sounding profiles and nuclear-test monitoring. Further research covered a broad variety of topics from free oscillations and tides in planetary bodies, earthquake codas, deep crustal profiling, exploration, near-surface, and engineering seismology. Recent interests include seismic imaging and attributes, potential-field imaging, time-lapse seismic monitoring, seismic attenuation, theoretical rock physics, laboratory experiments with rock samples, seismic data acquisition and monitoring. In all applications, he emphasizes deeper understanding of physics and innovative mathematical methods, such as inversion, machine learning, and integrated geophysical software.

Jim Merriam is professor emeritus in the Department of Geological Sciences at the University of Saskatchewan. He has a BSc (Mathematics and Physics) from Sir George Williams University, an MSc from Memorial University and a PhD from York University. He has published on the Earth’s core, mantle, crust, oceans and atmosphere. At Saskatchewan, he taught senior classes in potential fields, electromagnetic methods and a graduate class in inversion. His textbook ‘Induced Polarization: Processes and Properties’ can be downloaded from Samizdat Press (samizdat.mines.edu).

Samuel L. Butler completed his Ph.D. in 2000 at the University of Toronto working on numerical models of thermal convection in Earth’s mantle, concentrating on the effects of mid-mantle phase transitions. In 2001, he joined the faculty in the Department of Geological Sciences at the University of Saskatchewan. He has also studied flows in compacting and reactive porous media and spinning liquid fluid drops as they pertain to tektites. Sam is also interested in the use of general-purpose finite-element multiphysics simulation tools and has used them to simulate a wide variety of geophysical phenomena including applied geophysics techniques.