Three-dimensional resistivity modeling of GREATEM survey data from Kujukuri beach, Japan

Abstract

An airborne electromagnetic (EM) survey using the Grounded Electrical-Source Airborne Transient Electromagnetic (GREATEM) system was conducted over the Kujukuri coastal plain in southeast Japan to assess the system's ability to provide accurate data in relatively shallow seawater. To obtain high-quality data with an optimized signal-to-noise ratio, a series of data processing techniques were used to acquire the final transient response curves from the GREATEM field survey data. These steps included movement correction, coordinate transformation, removing local noise, data stacking (to overcome the influence of changes in horizontal resistivity structure), and signal portion extraction. We used synthetic numerical modeling to construct a three-dimensional (3D) resistivity structure model from the GREATEM data. This model was based on an initial one-dimensional (1D) resistivity structure inverted from the GREATEM field survey data. A 3D electromagnetic forward-modeling scheme based on a finite-difference staggered-grid method was developed and used to calculate the response of the 3D resistivity model at each corresponding survey line. Convolution was conducted in the frequency domain to add the frequency response characteristics of the field survey instrument to the forward-model synthetic data of the EM transient response. A 3D stacking of the field data was performed to acquire transient response curves that fit the transient response curves generated from the 3D synthetic data. Both the resulting 1D inversion and 3D resistivity structures showed that the GREATEM system has the capability to map resistivity structures as far as 800 m offshore and as deep as 300–350 m underground in coastal areas of relatively shallow seawater depth (5–10 m).