A three-dimensional Resistivity Structure Study Using Airborne Electromagnetics : Application to GREATEM Field Survey Data [an abstract of dissertation and a summary of dissertation review]

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.