GREATEM survey in coastal area

Abstract:

To determine the electric-resistivity structure in coastal areas, airborne electromagnetic (AEM) surveys have been used many places. These results showed AEM can reveal seawater depth, sediment thickness, and depth to bedrock. However, until now, AEM could only reveal the resistivity structure to a depth of ∼200 m in coastal areas. Conventional AEM, using an airborne inductive source limits the depth of exploration because the presence of a conductor, such as saline surface water, decreases the penetration of the AEM signal. In order to enhance the depth of exploration, we used a grounded electrical source aligned parallel to the coast line and towed a magnetic receiver in the air. The method, called grounded-electrical-source airborne transient electromagnetics, or GREATEM (Mogi et al., 1998), should enhance survey depth in coastal areas, since this method galvanically injects electric current into the ground. The method has increased the depth of exploration to 800 m in on-shore areas (Mogi et al., 2009, Ito et al. 2014).

We have carried out a GREATEM survey at Kujukuri beach in central Japan, where an alluvial plain is dominated by sedimentary rocks and shallow water. Another GREATEM survey was performed at northwestern Awaji Island, where granitic rocks crop out onshore.

In these studies, we faced issues how do we evaluate an effect of sea water on the data. To circumvent this problem, we used a three-dimensional (3D) electromagnetic (EM) modeling scheme based on the staggered-grid finite-difference (FD) method (Fomenko and Mogi, 2002) to study the effects of oceanic saltwater on GREATEM survey data at coastal areas.

The models consisted of two adjacent layers where sea is a conductive thin sheet placed on top of a unifirm half space ( 100 Ohm-m). Then, we performed numerical forward modeling using the SFD method to generate a 3-D resistivity structure models from GREATEM field survey data at both of Kujukuri beach and the Nojima fault. As a result, we have evaluated the effects of complicated structure, such as sea on GREATEM survey data and have obtained information regarding seawater invasion in sedimentary rocks and a resistivity structure along an active fault. This study indicates that the GREATEM system can be used for the assessment of natural disaster areas.