Susan E. Pullan

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.

Update on Recent Observations in Multi-component Seismic Reflection Profiling

Examples of multi-component shallow seismic reflection profiling from different environments in eastern Canada are presented to examine the benefits of shear wave reflection data and the latest developments in acquisition methodology, as well as our evolving understanding of the complex nature of seismic wave propagation. Examination of the wave motion through multi-component recording shows that, regardless of the source orientation, shear wave reflections may be polarized in varying directions. In “soft” soils characterized by low shear wave velocities, extremely high-resolution shear wave reflection sections can be obtained, with the highest-resolution data related to in-line or vertical components of motion. Data recorded in the transverse (SH) direction are generally somewhat lower in frequency but may be better able to penetrate into more compact (higher velocity) sediments. Multicomponent recording provides the capacity to record all data, without having to know the main shear wave polarization direction prior to data acquisition.