Velocity Structures of Kakkonda Area Estimated by 3-D Simulations

Abstract

The purpose of this paper is to clarify the velocity structures within the Kakkonda geothermal field, northern Japan. For this purpose, 3-D numerical simulations, using the staggered grid finite-difference method, were carried out. To verify simulation programs, we adopted a model consisting of two layers: velocities of P and S wave were 3600 m/s and 2144 m/s for the first layer, and 4272 m/s and 2415 m/s for the second layer respectively, at a grid interval of 10 m. This model covered an area of 1.6 (N-S) x 1.6 (E-W) km 2 at a depth of 1.8 km. The dominant frequency of the source was assumed to be 4 Hz. Calculated P wave first arrivals agreed well with theoretical values for this model. Consequently, a four layer model, for which the dominant frequency of the source was higher and the grid interval smaller (=5 m), was calculated. Therefore, the errors between observed first arrivals of P waves and those calculated were ±1% within the model where the velocities of P and S wave were about 20 % larger than the 1-D inversion model and a partial low velocity zone existed.