Estimation of the top of the saturated zone from airborne electromagnetic data

Abstract

In an airborne electromagnetic (AEM) data set acquired in unsaturated and saturated zones, the depth of the top of the saturated zone (TSZ) at the time of data acquisition should be accounted for in the resistivity-to-lithology transform. We have developed and tested a methodology for estimating the TSZ from AEM data, using data collected in three survey areas in the Central Valley of California and water-table elevation (WTE) measurements from nearby wells. The methodology is based on the difference in the distribution of resistivity values above and below the TSZ, using the WTE measurements to optimize two components of the general workflow. From the AEM data acquired in Tulare County, in the southern portion of the Central Valley, where the WTE measurements were acquired two to four weeks before the AEM data acquisition, we have found estimates of the TSZ with a root-mean-square (rms) error of 10.6 m when compared to the WTE measurements. From the two survey areas in Butte and Glenn counties, in the northern portion of the Central Valley, where WTE measurements were available at the time of, and closer to the locations of, AEM data acquisition, we have found estimates of the TSZ with an rms error ranging from 3.8 to 5.3 m, depending on the form of inversion. The level of error found in the three survey areas is comparable to the thickness of the layers in the resistivity models at the depths of the TSZ. Because the intended use of these estimates is to locate the TSZ for use in developing and applying the resistivity-to-lithology transform, the level of error associated with this new methodology is acceptable.