Evaluation of small-loop transient electromagnetic soundings to locate the Sherwood Sandstone aquifer and confining formations at well sites in the Vale of York, England

Abstract

Shallow-depth transient electromagnetic (TEM) soundings have been performed at six borehole locations in an intensively farmed area in northern England to evaluate their usefulness in mapping geological formations under a thick cover of glacial drift deposits. The regionally important Triassic Sherwood Sandstone (SS) Group aquifer is directly overlain by Triassic Mercia Mudstone in the eastern two-thirds of the study area and by drift deposits in the west. Owing to the difficulty of deploying large loops and the overriding need to minimize lateral effects on the depth probes, square transmitter loops of 20, 40 and 50-m side-lengths were deployed in the central-loop configuration with the Geonics EM47 and PROTEM47/57 field equipment. Using a two-stage data interpretation technique, it is found that the effective depth of mapping ranged from about 8 to 150 m at most sounding locations. Comparison of inversion models with borehole data shows that the SS and some overlying sedimentary rocks may be discerned from the TEM soundings; there is a consistent pattern of resistivity distribution within each geological formation at all the borehole sites enabling a realistic identification of the key stratal units. However, a 7–11-m-thick upper layer is found in all the constructed models, which does not correlate with any known formation boundaries, but appears to be justified by comparison with sample dc resistivity soundings at two locations; it would also appear that the earliest time windows (<0.016 ms) are somewhat distorted by the band-limitation operation of the TEM instrumentation. This pilot study demonstrates that the TEM method is a potent tool for stratigraphic mapping in the region, but the upper 5–8 m remains largely inaccessible to the method using state-of-the-art equipment and conventional data processing techniques. It may therefore be necessary to combine TEM and short spread-length ( AB/2≤25 m) dc resistivity depth soundings to accurately map the near-surface in this glacial-covered terrain.