Mapping High Resistivity Buried Channel Deposits with Airborne Electromagnetic Surveys and Other Methods

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Aggregate deposits in the shale‐dominated plains of northeast British Columbia (NEBC) are in short supply yet the demand for gravel is continually increasing due to the construction of all‐season petroleum development roads in the region. Gravels deposits close to surface with minimal overburden are preferred but such deposits are relatively rare in NEBC. Many gravels are completely buried and techniques commonly used for identifying surface features, such as geomorphological mapping, are ineffective for locating these blind deposits. Due to these difficulties, new subsurface investigation and geophysical techniques are being tested and used. Airborne, high resolution, electromagnetic (EM) surveys are particularly successful because of the high resistivity contrasts between sands and gravels and underlying shale and overlying clay‐rich glacial sediments. For example, a helicopter‐borne RESOLVE multi‐frequency EM survey, flown over a gravel deposit underlying 1 to 5 m of clay rich till, shows relatively high resistivity values (e.g. >60 ohm‐m) in the gravels and low resistivity values (e.g. \gt;15 ohm‐m) in the till. The sands and gravels occur along a gentle southeasterly slope with no obvious geomorphic indications of their presence. In the inferred core of the paleochannel deposit, the sands and gravels are at least 5 m thick. The EM survey was flown with 100 m line spacing over the deposit and 200 m spacing over a larger area (42 km2). The flat, till covered, deposit, which was originally detected only in seismic shot hole logs, was mapped remarkably well with the high frequency (115 kHz) data, which best reflects the shallow geology. Follow‐up field studies show that the depths of the deposit appear to be less than the EM data suggests, but the overall shape and location of the deposit is well represented, as is the presence and geometry of the till cap over the gravels. Mapping of buried Quaternary channels also has recently become of interest in NEBC because of their shallow gas potential and because of hazards associated with buried channels such as artesian water and pressurized gas blow‐outs. In this paper we report on geophysical and remote sensing techniques we are using to map both shallow buried channels and deeper paleo‐valleys.