Incorporating ancillary data into the inversion of airborne time-domain electromagnetic data for hydrogeological applications

Helicopter time-domain electromagnetic (HTEM) surveys often suffer fromsignificant inaccuracies in the early-time
or near-surface data—a problem that can lead to errors in the inversemodel or limited near-surface resolution in the
event that early time gates are removed. We present an example illustrating the use of seismic data to constrain
the model recovered from an HTEM survey over the Spiritwood buried valley aquifer in Manitoba, Canada. The
incorporation of seismic reflection surfaces results in improved near-surface resistivity in addition to a more
continuous bedrock interface with a sharper contact. The seismic constraints reduce uncertainty in the resistivity
values of the overlying layers, although no a priori information is added directly to those layers. Subsequently, we
use electrical resistivity tomography (ERT) and borehole data to verify the constrained HTEM models. Treating
the ERT and borehole logs as reference information, we perform an iterative time-shift calibration of the
HTEM soundings to achieve regional-scale consistency between the recovered HTEM models and the reference
information. Given the relatively small time-shifts employed, this calibration procedure most significantly affects
the early-time data and brings the first useable time gate to a time earlier than the nominal first gate after ramp
off. Although time shifts are small, changes in the model are observed from the near-surface to depths of 100 m.
Calibration is combined with seismic constraints to achieve amodel with the greatest level of consistency between
data sets and, thus, the greatest degree of confidence. For the Spiritwood buried valley, calibrated and constrained
models reveal more structure in the valley-fill sediments and increased continuity of the bedrock contact.