The sea bed shear modulus of several coastal areas in Japan were measured by newly-developed bottom shear modulus profiler(BSMP), which actually measures the ratio of the wave-induced particle motion to the wave pressure on the seafloor. Then, a linear inversion scheme was used for calculation of the shear modulus profile. Results were compared to the values obtained at the same sites by the Standard Penetration Test(SPT) using the empirical formula. The agreement between the results with BSMP and SPT is very good. Futhermore, theoretial analysis on inversion technique is given. Then, the determination of shear modulus profile can be more accurate by comparing results from several inversions with different initial models.
The satellite altimetry is one of effective techniques for remote sensing over oceanic areas. GEOSAT, launched in 1985, is the first altimeter satellite after the success of precise altimeter observations of 10 cm level by SEASAT in 1978. The altimeter observation by GEOSAT is especially important because of its long period of continuous observation for about five years. This fact makes it possible to perform a data analysis of altimeter data in quality different from SEASAT. The time variable components in sea surface shape can be removed by averaging the repeat observations through years. Consequently, the time variable components of the sea surface can be detected as the differences between time-averaged and individual sea surface data. The sea surface variability accompanied with oceanographic phenomena such as currents and eddies in the northwest Pacific region is investigated here. Path change of the Kuroshio current and the Kuroshio Extension are clearly detected. The time-averaged altimetric sea surface shape is considered to be a quasi-stationary one, which is nearly equal to the geoid except for sea surface topography caused by large-scale ocean circulations and ocean surface variations accompanied with large-scale oceanographic phenomena such as El Niño. The precision of altimeter observations and the stability of the time-averaged sea surface shape can be tested by the crossover adjustment procedure by using time-averaged altimeter profiles. After applying corrections for orbit errors and long wave-length errors of the ocean tide model, the r.m.s. crossover difference at 1,459 crossover points of time-averaged altimeter profiles is found to be 5 cm level.
The Kirchhoff model expresses a relation between acoustic backscattering and seafloor characteristics and roughness especially for angles near normal incidence. In Sea Beam with equivalently 16 narrow acoustic beams, each beam has grazing angle from 70° to 90°, which is the range within which its approximation gives better results. Parameters of seafloor characteristics and roughness is capable to treat as a nonlinear inversion problem. This problem was solvable using BFGS formula replacing it into a forward problem. This method was applied to backscattering data collected from Japan Trench, Hachijo Depression and Mikura Basin using Sea Beam , and its usefulness for acoustical remote sensing of seabeds was confirmed.
In a deep-tow system, the real-time altitude of the tow body above the sea bottom was monitored by using a pinger attached to the tow body and a PDR (Precision Depth Recorder) on a vesssel. This allows even an off-line deep-tow system to operate safely and to keep appointed altitude. The altitude of the tow body above the sea bottom can be calculated from three data: time difference between direct arrival and bottom reflection sound wave sent from the pinger, the length of towing wire, and the inclination angle of the towing wire. Sufficient altitude data for on-board monitoring was obtained by the above calculation. Field experiments of the altitude monitoring were carried out in four areas of the Pacific side of Japan in water depths between approximately 2,000 m and 7,000 m. In every experiment, the altitude of the tow body above the sea bottom was successfully measured.
Vector data of the geomagnetic field provide more detailed information than total intensity geomagnetic data for understanding the magnetic structure of oceanic crust. In the last few years, the invention and development of the Shipboard Three Component Magnetometer (STCM) has successfully provided geomagnetic field vector data in many areas in the oceans. There are two main advantages in using vector geomagnetic anomaly field data: (1) total intensity anomaly amplitudes are often much reduced depending on the orientation of the ambient geomagnetic field and magnetic lineation while these have no effect on vector anomalies, and (2) vector geomagnetic anomaly field data provide the positions, strikes and characters of magnetic boundaries, allowing changes in these boundaries to be identified along individual ship tracks. These advantages permit tectonic interpretations to be well constrained, even in areas of sparse ship track data coverage. Furthermore, intensity variations in the vector geomagnetic anomaly field along spreading axes may be used to investigate spreading axis thermal anomalies.