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

Shallow-depth transient electromagnetic (TEM) soundings have been performed at six borehole locations in an intensively farmed area in northern Englan d to evaluate their usefulness in mapping geological formations under a thi ck cover of glacial drift deposits. The regionally important Triassic Sherw ood Sandstone (SS) Group aquifer is directly overlain by Triassic Mercia Mu dstone 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 overrid ing need to minimize lateral effects on the depth probes, square transmitte r loops of 20, 40 and 50-m side-lengths were deployed in the central-loop c onfiguration with the Geonics EM47 and PROTEM47/57 field equipment. Using a two-stage data interpretation technique, it is found that the effective de pth of mapping ranged from about 8 to 150 m at most sounding locations. Com parison of inversion models with borehole data shows that the SS and some o verlying sedimentary rocks may be discerned from the TEM soundings; there i s a consistent pattern of resistivity distribution within each geological f ormation at all the borehole sites enabling a realistic identification of t he key stratal units. However, a 7-11-m-thick upper layer is found in all t he constructed models, which does not correlate with any known formation bo undaries, but appears to be justified by comparison with sample de resistiv ity soundings at two locations; it would also appear that the earliest time windows (< 0.016 ms) are somewhat distorted by the band-limitation operati on of the TEM instrumentation. This pilot study demonstrates that the TEM m ethod is a potent tool for stratigraphic mapping in the region, but the upp er 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 less than or equal t o 25 m) de resistivity depth soundings to accurately map the near-surface i n this glacial-covered terrain. (C) 2000 Elsevier Science B.V. All rights r eserved.