A three-dimensional Resistivity Structure Study Using Airborne Electromagnetics : Application to GREATEM Field Survey Data

Applications of airborne electromagnetic (AEM) survey techniques have been introduced for environmental protection and natural disaster prevention in various fields. The objective of this study was to establish a method of constructing a three-dimensional(3-D)subsurface electrical resistivity model for a complicated structure using AEM data. Numerical forward modeling was performed using a modified staggered-grid finite-difference (SFD) method, and adding a finite-length electrical-dipole (FED) source routine to generate 3-D resistivity structure models of grounded electrical-source airborne transient electromagnetic (GREATEM) field survey data. The GREATEM system was introduced by Mogi et al. (1998, 2009) and uses a grounded electrical dipole source of 2- to 3-km length as a transmitter and a three-component magnetometer in the towed bird as a detector. With a grounded source, a large-moment source can be applied and a long transmitter-receiver distance can be used to yield a greater depth of investigation, although the survey area becomes limited. Other advantages include a smaller effect of flight altitude and the possibility of higher-altitude measurements. Data are recorded in the time domain, providing a raw time series of the magnetic fields induced by eddy currents in the ground after cutting off the transmitting current, with the result that a noise filter can be easily introduced. I have verified our 3-D electromagnetic (EM) modeling computing scheme, which is based on the SFD method (Fomenko and Mogi, 2002) by comparing the results of a quarter-space and trapezoidal hill models with the results of the 2.5-D finite-element method by Mitsuhata (2000), and the 3-D finite-difference program with the spectral Lanczos decomposition method developed by Druskin and Knizhnerman (1994). This method was then used to study the possibility of detecting a conductor under shallower sea, the effects of sea and topographic features. A GREATEM survey was performed at Kujukuri beach in central Japan, where an alluvial plain is dominated by sedimentary rocks and shallow water. A reliable resistivity structure was obtained at a depth range of 300 to 350 m both on land and offshore, in areas where low-resistivity structures are dominant. Another GREATEM survey was performed at a location in northwestern Awaji Island, where granitic rocks and paleogene sedimentary rocks crop out onshore. Resistivity structures at depths of 1 km onshore and 500 m offshore were revealed by this survey. I performed numerical forward modeling using a modified SFD method by adding a FED source routine to generate a 3-D resistivity structure model from GREATEM field survey data at both Kujukuri beach and the Nojima fault. Finally, I have confirmed the accuracy of our 3-D forward modeling computing scheme and evaluated the effects of complicated structures, such as sea or topography, on GREATEM data. I have used this method to generate a 3-D resistivity model from GREATEM field survey data acquired at the Kujukuri beach and the Nojima fault. As for results, I have obtained information regarding seawater invasion area 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.