Greg A. Oldenborger

The Impact on Geological and Hydrogeological Mapping Results of Moving from Ground to Airborne TEM

In the past three decades, airborne electromagnetic (AEM) systems have been used for many groundwater exploration purposes. This contribution of airborne geophysics for both groundwater resource mapping and water quality evaluations and management has increased dramatically over the past ten years, proving how these systems are appropriate for large-scale and efficient groundwater surveying. One of the major reasons for its popularity is the time and cost efficiency in producing spatially extensive datasets that can be applied to multiple purposes. In this paper, we carry out a simple, yet rigorous, simulation showing the impact of an AEM dataset towards hydrogeological mapping, comparing it to having only a ground-based transient electromagnetic (TEM) dataset (even if large and dense), and to having only boreholes. We start from an AEM survey and then simulate two different ground TEM datasets: a high resolution survey and a reconnaissance survey. The electrical resistivity model, which is the final geophysical product after data processing and inversion, changes with different levels of data density. We then extend the study to describe the impact on the geological and hydrogeological output models, which can be derived from these different geophysical results, and the potential consequences for groundwater management. Different data density results in significant differences not only in the spatial resolution of the output resistivity model, but also in the model uncertainty, the accuracy of geological interpretations and, in turn, the appropriateness of groundwater management decisions. The AEM dataset provides high resolution results and well-connected geological interpretations, which result in a more detailed and confident description of all of the existing geological structures. In contrast, a low density dataset from a ground-based TEM survey yields low resolution resistivity models, and an uncertain description of the geological setting.

3D modeling of buried valley geology using airborne electromagnetic data

Buried valleys are important hydrogeologic features of glaciated terrains. They often contain valuable groundwater resources; however, they can remain undetected by borehole-based hydrogeologic mapping or prospecting campaigns. Airborne electromagnetic (AEM) surveys provide high-density information that can allow detailed features of buried valleys to be efficiently mapped over large geographic areas. Using AEM data for the Spiritwood Valley Aquifer system in Manitoba, Canada, we developed a 3D electric property model and a geologic model of the buried valley network. The 3D models were derived from voxel-based segmentation of electric resistivity obtained via spatially constrained inversion of two separate helicopter time-domain electromagnetic data sets (AeroTEM and versatile time-domain electromagnetic [VTEM]) collected over the survey area. Because the electric resistivity do not provide unequivocal information on subsurface lithology, we have used a cognitive procedure to interpret the electric property models of the aquifer complex, while simultaneously incorporating supporting information for the assignment of lithology in the 3D geologic model. For the Spiritwood model, supporting information included seismic reflection data and borehole records. These data constrained valley geometry and provided lithologic benchmarks at specific borehole sites and along seismic transects. The large-scale AeroTEM survey provided the basis for modeling the regional extent and connectivity of the Spiritwood Valley Aquifer system, whereas the local-scale VTEM survey provided higher near-surface resolution and insight into a detailed shallow architecture of individual buried valleys and their fill.

BURIED VALLEY IMAGING USING 3-C SEISMIC REFLECTION, ELECTRICAL RESISTIVITY AND AEM SURVEYS

In the Canadian Prairies buried valleys are important sources of groundwater. Hydrological methods such as pumping tests provide very limited spatial information to efficiently predict the sustainability of these aquifers. To obtain a full assessment in three dimensions of such complex reservoir geometry, geophysical tools are an absolute necessity. The Spiritwood valley in southwestern Manitoba, is a Canada-USA transborder buried-valley aquifer. In March 2010, the Geological Survey of Canada conducted an airborne electromagnetic (AEM) survey (AeroTEM III) over a 1062 km2 area along the buried valley north of the US border. The results show multiple resistive elongated features which have been interpreted as coarse sediment filled channels inside a 15 km wide more conductive valley filled with finer sediments such as diamictons. The spatial distribution, directionality, and size of the channels are complex. Follow up ground surveys were carried out during the summer and included a ground based, multi-electrode electrical resistivity survey to calibrate the resistivity of the various units seen in the AEM data, as well as a high-resolution seismic survey to obtain detailed architectural and depth information. The seismic data were collected using a Minivib I in inline horizontal vibrating mode (20240 Hz sweep) at a shot spacing of 6 m and a 3-component (3-C) landstreamer receiver array with 48 sleds spaced at 1.5 m. These data allow us to obtain both shear wave and compressive wave profiles. The younger, less compacted channels were better imaged with P-wave data, while some areas with shallow gas or organic peats were better imaged with S-wave data. The seismic images show detailed sedimentary sequences and permit some inferences on the relative ages of channels formed during multiple ice advances. The sections also showed the presence of other channels, which are interpreted to be infilled with finer sediments based on the seismic facies, and which are not associated with resistive features in the AEM data. This combination of AEM, electric sounding and 3-C seismic profiling provides exceptional 3-D coverage which has highlighted key hydrological features such as buried channel aquifers and potential sub-surface hydraulic pathways or connections. Such information is critical to groundwater prospecting and to the accurate assessment of recharge and discharge potentials associated with buried valley aquifers.

AIRBORNE TIME-DOMAIN ELECTROMAGNETICS FOR THREE- DIMENSIONAL MAPPING AND CHARACTERIZATION OF THE SPIRITWOOD VALLEY AQUIFER

The Geological Survey of Canada commissioned a helicopter-borne time-domain electromagnetic (HTEM) survey over a 1062 km 2 area of the Spiritwood Valley in southern Manitoba to test the effectiveness of airborne time-domain electromagnetics for mapping and characterizing buried valley aquifers in the Canadian Prairies. The HTEM data exhibit rich information content; apparent conductivity maps clearly image the Spiritwood Valley in addition to a continuous incised valley along the broader valley bottom. We detect complex valley morphology with nested scales of valleys including at least three distinct valley features and multiple possible tributaries. Conductivity-depth images (CDI) derived from the TEM decays indicate that the fill materials within the incised valleys are more resistive than the broader valley fill, consistent with an interpretation of sand and gravel. Comparison of ground-based electrical resistivity and seismic reflection data allow for calibration of CDI models. Lateral spatial information is in excellent agreement between data sets. The seismic data reveal the presence of additional valley features that are not imaged by the HTEM data as having a distinct electrical signature, possibly due to diamicton fill. The CDI model underestimates the dynamic range of electrical conductivity while overestimating depths to valley bottoms; these issues may be associated with system limitations, system bandwidth, algorithm limitations and penetration depth. The integrated data sets illustrate that HTEM surveys have the potential to map complicated buried valley aquifers at a level of detail required for groundwater prospecting and management.

ELECTRICAL GEOPHYSICS FOR ASSESSING PERMAFROST CONDITIONS ALONG HIGHWAY INFRASTRUCTURE

The Yellowknife region, part of the Slave Geological Province, falls within the extensive discontinuous permafrost zone in Canada. A large degree of economic development is routed through Yellowknife from the mineral-rich North Slave. Despite the mineral-rich nature of this region, surficial sediment maps and knowledge of permafrost conditions are only now being established in detail. Permafrost and associated ground ice can significantly affect land-based infrastructure through influence on ground stability and drainage patterns. As such, geoscience information contributing to permafrost characterization is critical for understanding risks to roads which are vital to Northern economic development. The 100 km stretch of the chip-sealed Highway 3, west of Yellowknife, presently experiences instabilities including settlement, heave, and rotations related to transitions between differing terrain and drainage conditions within the discontinuous permafrost. Electrical resistivity data were collected over identified terrain types, and across potential terrain transitions and thaw fronts based on the hypothesis that permafrost distribution and conditions vary with terrain type. Processed resistivity models indicate distinct electrical signatures for most of the terrain types which would allow for extensive geophysical characterization complimentary to landscape mapping, temperature data and shallow boreholes. The resistivity models also exhibit features indicative of the base of ice-bonded permafrost, ice-rich sediment and thaw zones, which can be correlated with terrain features of sediment type and drainage. Observed resistivity anomalies indicate thaw zones related to existing and past road infrastructure, which help in understanding conditions causing highway subsidence.

HELICOPTER TIME-DOMAIN EM RESULTS OVER THE BRANDON CHANNEL AND ASSINIBOINE DELTA AQUIFERS, BRANDON, MANITOBA

VTEM helicopter time-domain EM survey results over the Brandon Channel and Assiniboine Delta aquifers, near Brandon, Manitoba, have been studied using unconstrained layered earth modeling. The resistive sand and gravel layers are distinguished from the more conductive shale basement and reveal localized thickening from blind gravel channels cut into its floor. They further distinguish a lower aquifer separated by a conductive clay layer from shallow sands of the Assiniboine Alluvial aquifer and the Assiniboine Delta Aquifer. The Brandon Channel aquifer is delineated, to the northwest and east of the city where it splits into a main south channel and smaller north channel. The western extent of Assiniboine Alluvial aquifer is defined and evidence for the Pierson Valley aquifer extending west from Brandon is also indicated.

Recent AEM Case Study Examples of a Full Waveform Time-Domain System for Near-Surface and Groundwater Applications

Early time or high frequency airborne electromagnetic data (AEM) are desirable for shallow sounding or mapping of resistive areas but this poses difficulties due to a variety of issues, such as system bandwidth, system calibration and parasitic loop capacitance. In an effort to address this issue, a continued system design strategy, aimed at improving its early-channel VTEM data, has achieved fully calibrated, quantitative measurements closer to the transmitter current turn-off, while maintaining reasonably optimal deep penetration characteristics. The new design implementation, known as “Full Waveform” VTEM was previously described by Legault et al. (2012). This paper presents some case-study examples of a Full Waveform helicopter time-domain EM system for near-surface applications