Buried Valley Imaging Using 3‐C Seismic Reflection, Electrical Resistivity and AEM Surveys

Abstract

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 (20–240 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.