Three-dimensional resistivity modeling of GREATEM survey data from the Nojima fault, Awaji Island, Japan

Abstract

An airborne electromagnetic (AEM) survey using the grounded electrical-source airborne transient electromagnetic (GREATEM) system was conducted over the Nojima fault on Awaji Island, southeast Japan, to assess GREATEM survey applicability for studying coastal areas with complex topographic features. To obtain high-quality data with an optimized signal-to-noise ratio, a series of data processing techniques was used to acquire the final transient response curves from the field survey data. These steps included movement correction, coordinate transformation, removal of local noise, data stacking, and signal portion extraction. The 1D inversion results were feasible in that the horizontal resistivity contrast was not much higher than the true contrast, but they were not reasonable in that the horizontal resistivity values were greatly changed. To circumvent this problem, we performed numerical forward modeling using a finite-difference staggered-grid method (Fomenko and Mogi, 2002) adding a finite-length electrical-dipole source routine to generate a three-dimensional (3D) resistivity structure model from GREATEM survey data of the Nojima fault area. The 3D model was based on an initial model consisting of two adjacent onshore and offshore layers of different conductivity such that the sea was a highly conductive thin sheet on top of a uniform half-space, assuming the presence of topographic features on the inland side. We verified the model by fitting the magnetic transient responses between field data and 3D forward-model computed data, which were convolved with frequency characteristics of the instruments. The results of the 3D resistivity structure model revealed that the GREATEM system can detect underground resistivity structures to a depth of 500 m onshore and offshore. The GREATEM survey delineated how seawater intrudes on the landside of the fault and indicated that the fault is a barrier to seawater invasion.