The Effect Of A Conductive Surface Layer On Tem Central-Loop Soundings In Groundwater Exploration

One of the most popular geophysical methods of groundwater exploration is electromagnetic<br>sounding. In particular, central-loop configuration transient electromagnetic (TEM) soundings have<br>been shown to be quite effective in mapping horizontal strata of hydrogeological significance.<br>However, conductive surficial layers can interfere with what seems to be a straightforward application<br>of this method. Three effects of surficial conductive material can be addressed. First, the two-valued<br>apparent resistivity function becomes significant when surficial conductive layers are present,<br>necessitating a thorough understanding of this phenomena. Second, a breakdown in the definition of<br>apparent resistivity can occur in which there is no equivalent halfspace resistivity which would produce<br>the observed voltage in the measurement coil. Third, when collecting soundings near a lateral change in<br>surficial conductivity, migration of induced currents toward the conductor results in a sounding curve in<br>which transient voltages can drop very steeply, and can even change sign. This effect has been<br>explained in previous work in two ways. First, there can be a paramagnetic effect within soils near the<br>transmitter wire (mostly for the coincident loop configuration, but also for roving receiver locations near<br>the transmitter wire when using a large transmitting loop), and second, this can be due to an induced<br>polarization effect. A third explanation is developed here in which multiple current density rings are<br>induced in the subsurface. One ring corresponds to induction in material directly below the transmitter<br>loop, and a second ring can be developed non-symmetric with the transmitter loop, resulting from a<br>lateral surficial conductive layer.<br>These effects in TEM central-loop soundings are illustrated by analyzing data obtained at two<br>tidal flat environments, near Union Beach, and on the oceanside of Cape May Peninsula, New Jersey,<br>and from the surface of a lava flow resulting from the August 1999 eruption of Cerro Negro volcano,<br>Nicaragua.