Abstract
Deep-tow magnetic surveys provide detailed information about magnetization of the oceanic crust. This method, however, needs a longer measurement time than conventional sea-surface magnetic surveys, because a ship speed for the deep-tow surveys is restricted to 2-3 knots in order to keep a vehicle towing near the bottom against buoyancy on a towing cable. This sort of problem occurs not only on the deep-tow magnetic surveys but also on the other deep-tow surveys such as seismic reflection surveys, side-scan sonar surveys, and so on. As one of the solutions to that disadvantage, we have developed a new deep-tow observation system with multiple sensors which allows us to make efficient observations compared with a single measurement. In 1996, we conducted deep-tow surveys by that observation system off western Hokkaido (in the Japan Sea). The system consisted of a seismic profiling system, a proton-precession magnetometer and a side-scan sonar. The magnetic measurements were successful for three of four track lines. A summary of the results is shown below. 1) Deep-tow magnetic anomalies in the Ishikari Basin gradually increased with gradients of 18 nT/km and 29 nT/km toward ENE and NE, respectively. 2) An extremely large deep-tow magnetic anomaly (amplitude: 775 nT; wavelength: 8 km) was detected in the northeastern margin of the Japan Basin, while there was no obvious anomaly on the sea-surface. The results in the Ishikari Basin are consistent with sea-surface magnetic anomaly data obtained previously and during these surveys. The remarkable magnetic anomaly in the Japan Basin could be explained by the following two models inferred from seismic reflection data in the area: a) a layer with positive magnetization including a conical body; b) a layer with variable magnetization including negative or weak positive magnetization causing the negative deep-tow magnetic anomaly.
