Simultaneous modeling of transient electromagnetic and resistivity/induced polarization soundings to improve resolution in hydrogeological investigations.

摘要

This study presents methods of improving resolution in geoelectrical soundings applied to environmental investigations. First, a procedure is derived for calculating apparent resistivity from transient electromagnetic (TEM) data obtained in the central loop configuration, accounting for a finite transmitter shutoff ramp. Comparison of results obtained accounting for a finite ramp time with those from a step response, demonstrates that near-surface interpretation can be more accurately accomplished by taking the ramp into account. The ramp-derived apparent resistivity is two-valued, but still useful for preliminary data interpretation. The breakdown of this apparent resistivity definition is shown by field examples where, for some sample times, the apparent resistivity function is not defined. This occurs for TEM soundings collected in areas with conductive surfical sediments, such as tidal marshes.;Five computer programs are presented for the quantitative interpretation of geoelectrical soundings, allowing data input, reduction, and one-dimensional individual or simultaneous inverse modeling of resistivity, induced polarization (IP), and TEM methods.;One simulation and two groundwater exploration field examples from New Jersey illustrate resolution improvement in geoelectrical soundings achieved by simultaneous inversion of data obtained by two or more methods, and by constraining parameters of the inverse problem. Simultaneous inversion of simple IP data with conventional resistivity and resistivity-TEM data sets resulted in improved parameter resolution by (1) decoupling correlated layered-earth parameters, (2) by adding information to a geological interpretation about a second physical property, namely chargeability, and (3) by increasing the electrical information available.;Geoelectical soundings collected near Union Beach borough, where an underlying aquifer is intruded by seawater, were analyzed. The conductive tidal marsh proved by depth-penetration analysis to be no hinderance to TEM soundings; however, the close proximity to seawater in Raritan Bay created a lateral resistivity variation unable to be accounted for by one-dimensional TEM modeling. The same lateral resistivity variation is shown in a short profile of soundings in a tidal marsh on the oceanside of central Cape May peninsula, New Jersey.