2001 Volume 110 Issue 1 Pages 44-57
In order to evaluate the capability of one-dimensional microgravity investigations, we carried out two test surveys across the Katagihara fault in the southwest of Kyoto basin and the Fumotomura fault at the foothills of Suzuka Range. Since seismic reflection survey had already been carried out in these faults, the Bouguer anomaly due to the fault structures was expected in advance at an order of 0.1 mgal across the Katagihara fault and less than 0.1 mgal across the Fumotomura fault. It was also estimated that the spacing of the gravity points should be less than 50 meter to reveal the structures. We therefore conducted precise gravity measurements using a LaCoste & Romberg gravimeter (G-type) at about a 50 meter interval, and also carried out leveling surveys on the same points using a Wild NA3000 digital level. Moreover, we paid much attention for terrain corrections using the 50 meter DEM (Digital Elevation Model) provided by Geographical Survey Institute and partially using a 10 meter DEM compiled by ourselves. Consequently, we achieved 0.1 mgal level precisions for almost all the survey points. Using the gravity anomaly data, density structures in both survey areas were estimated and compared with the structures obtained from seismic reflection survey. The main results are as follows ; (1) comparatively simple structures are obtained to explain the gravity anomaly in the Katagihara fault, (2) the density contrast between the basement and the sedimentary layer is 0.58 g/cm3 in the Katagihara fault, (3) no gravity anomaly due to the displacement on the fault plain is observed in the Fumotomora fault, and (4) possibility of a high density layer is observed beneath the Tokai Group. Although the gravity survey has some limitations and drawbacks, it certainly gives us useful information about the density contrasts. Moreover, one-dimensional gravity survey is quite easy to conduct with very low cost. We therefore recommend that this kind of gravity surveys should be carried out whenever seismic reflection survey is conducted.
