Masao Komazawa

Caldera structure inferred from gravity anomalies west of Nagamachi-Rifu Fault, Northeast Japan

A gravity survey was conducted in and around the Nagamachi-Rifu Fault. The density for both terrain and Bouguer corrections was chosen to be 2,300 kg/m3, because volcanic rocks are dominant over the whole measurement area and the surface layer density is estimated to be low from geological considerations. The Bouguer anomalies are characterized by a low anomaly similar to those of a caldera and the basement structure inferred from two and three-dimensional analysis shows that the depth is often more than 1 km in the caldera region and that there is a circular structure. The gradient of basement is steepest at the southern margin, and it resembles to rim of funnel-shaped caldera.

Examination of consistency of marine gravity with land gravity in and around the japanese islands using a helicopter-borne gravimeter

This paper reports a finding that marine gravity data around the Japanese Islands are inconsistent with nearby land gravity data. The comparison between land and marine gravity was made possible by the gravity measurements from the air using a helicopter-borne gravimeter (SEGAWA Model) developed by the present authors. The ground/sea truth gravity anomaly can be checked against the gravity from the air, though it is not free air gravity anomaly but gravity disturbance. The newly-developed airborne gravimeter first manufactured in 1998 shows a good performance with a 1–2 mgal average repeatability of measurement under a 90-knot flight speed. Thus we have found disagreements between ground truth and sea truth gravity anomalies on the basis of airborne gravity data in the areas in Japan where we have so far made measurements. Among them we will report the case in the area from Saitama and Ibaraki offshore to the Kashima-Nada Sea, p]Japan. Our conclusion about this area is that the past marine gravity data obtained by surface ship gravimeters involve systematic errors of more than 10 mgals. This kind of inconsistency between marine and land gravity is expected to be found in a number of other areas around the Japanese Islands.

Density structure inferred from gravity anomalies in the eastern area of the Itoigawa-Shizuoka Tectonic Line, central Japan

A gravity survey was conducted between October 2002 and December 2003 in and around the eastern area of the Itoigawa-Shizuoka Tectonic Line (ISTL). The total number of measurement points amounted to about 436 and 326 stations were taken at the same points as the seismic survey points. The location and altitude were decided by differential GPS and the accuracy is thought to be within several ten-centimeters. The data was compiled from existing data (Geological Survey of Japan, 2000), so the total number of compiled gravity maps is about 3,540 points. All measured gravity data were referred to the International Gravity Standardization Net 1971 (IGSN71) and the normal gravity values were estimated according to the gravity formula of 1980. Terrain corrections were conducted within a range of 60 km by approximating the real topography to an assemblage of annular prisms interpolated by mesh terrain data and random terrain data of the gravity points. The effect of the sinking of the topography due to the Earth’s curvature was taken into consideration. Bouguer corrections within a range of 60 km in arc distances were made using a spherical cap crust formula. The density for both terrain correction and Bouguer corrections was chosen to be 2,670 kg/m3, because the mean surface density of the whole area is estimated to be a slightly large density by the CVUR method. The features of Bouguer anomalies is characterized by a low anomaly in Omachi city, in the northern parts of Matsumoto basin, and a high anomaly in the central highlands, Central Uplift Zone, about 10 km west of Ueda city. The low anomaly shows that the thickness of the sedimentary layer is more than 1 km from 2D/3D automatic analysis.

Airborne gravimetry — a new gravimeter system and test results

Success of airborne gravity surveys mainly depends on determining the three-dimensional (3D) position of the moving platform. Recent advances in technology, especially the Global Positioning System (GPS), have made it possible to determine the velocity and position of the moving platform more frequently and with greater accuracy. Taking advantage of these advances in GPS technology, and using a newly developed system, helicopter-borne gravity measurements were successfully carried out over the Kanto and Tokai districts of Japan. This new gravimeter system is composed of servo accelerometer sensors, a stabilised platform, an optical-fibre gyroscope to control the stabilised platform, GPS receivers, and a data processor. The 3D position of the helicopter at every second was accurately determined by the interferometric GPS method. These GPS data were also used to compute various correction factors which are applied to the measured gravity acceleration. Real-time differential GPS positioning was also conducted using a separate receiver mounted on the helicopter. These real-time positioning data were used for controlling the optical-fibre gyroscope. The gravity acceleration data were processed and all necessary corrections were applied. Numerical filtering was carried out to remove high-frequency noise in the data. The observed free-air gravity anomalies were then compared with upward continuation of the ground gravity data to the flight altitude. We also compiled an airborne gravity anomaly map from the airborne data, which was compared with upward-continued ground gravity data.

An integrated airborne gravity survey of an offshore area near the northern Noto Peninsula, Japan

An airborne gravity survey using a helicopter was carried out in October 2008, offshore along the northern Noto Peninsula, to understand the shallow and regional underground structure. Eleven flight lines, including three tie lines, were arranged at 2 km spacing within 20 km of the coast. The total length of the flight lines was ~700 km. The Bouguer anomalies computed from the airborne gravimetry are consistent with those computed from land and shipborne gravimetry, which gradually decrease in the offshore direction. So, the accuracy of the airborne system is considered to be adequate. A local gravity low in Wajima Bay, which was already known from seafloor gravimetry, was also observed. This suggests that the airborne system has a structural resolution of ~2 km. Reduction of gravity data to a common datum was conducted by compiling the three kinds of gravity data, from airborne, shipborne, and land surveys. In the present study, we have used a solid angle numerical integration method and an iteration method. We finally calculated the gravity anomalies at 300 m above sea level. We needed to add corrections of 2–5 mGals in order to compile the airborne and shipborne gravity data smoothly, so the accuracy of the Bouguer anomaly map is considered to be nearly 2 mGal on the whole, and 5 mGals at worst in limited or local areas.

Application of Gravimetry by Helicopter to Identify Marine Active Faults and Improve Accuracy of Geoid at Coastal Zones

We started the development of gravimetry using helicopter in 1998 and attained a preliminary success in gravity measurement in 2000. For this purpose we have manufactured a new gravimeter (SEGAWA Model) with a servo accelerometer as a gravity sensor and an optical fibre gyro to keep vertical. Our success owes mainly to precise positioning of helicopter, and delicate correction for the effect of helicopter’s horizontal acceleration. We have so far conducted test and/or practical measurements by helicopter (mostly using Bell 412). The advantages of a helicopter over a fixed-wing aircraft in gravity measurement is the high resolution of gravity anomaly obtained because of low altitude flight and low and stable flight speed. The main objective of the measurements is to get continuous profiles of gravity anomalies in the land-to-sea boundary zones, so that gravity void or data discrepancy across coastal zones may be removed and that the active seismic faults running across coasts, which are reflected on gravity anomalies, may be retraced as far as the sea floor. In this paper two results have been selectively described: One is the result of measurement conducted in April 2000 along west-to-east tracks from Saitama to Kashima-Nada Sea, which has disclosed inconsistency between land and marine gravity. Another is the result of survey along the east-to-west tracks in the Enshu-Nada Sea (Tokai area), which shows the changes of free air anomaly and/or gravity disturbance from the coast to the sea floor associated with active seismic faults.