Recently, for shallow water civil engineering projects in rivers and harbors, and for offshore resource exploration on the deep-sea floor, it has become essential to develop a highly accurate structural investigation technology that can scan to depths of about tens meters under the bottom of the sea. In this paper, we present two new signal analysis methods for accurate sub-bottom profiling: dynamic swath signal stacking in an acoustic near-field, and the use of a depth-varied broad bandpass filter to clarify the stratal architecture under the sea floor. These methods are applicable to data obtained by measuring at a low altitude using a low-frequency broadband transducer. In the present research, we developed a prototype low-frequency broadband transducer and carried out a performance test for a 3-D exploration system in a very shallow area of the Hitachi-Tonegawa River in 2009. By applying the first proposed technique to the acquired data, we confirmed that these prototype systems and signal analysis methods are capable of clarifying the depositional-structure under the riverbed up to a depth of 10 m. Moreover, we explored just below the sea floor using an acoustic source fitted with JAMSTEC's Deep-Tow and Natsushima equipment as a single-channel sub-bottom profiler that was operated at low altitude in deep water in 2011. By applying the second proposed technique to the acquired data, we confirmed that our prototype exploration system and analysis methods are capable of clarifying stratal structures at depths of up to 60 m under the sea floor.
Tracking system, one of the biotelemetry systems, detects signals from pingers using a receiver located on an observation boat. This system can be used without location constraints, unlike monitoring systems and positioning systems for which a receiver must be installed in the actual sea. But such a system cannot measure the pinger position in detail. In order to measure the pinger position in greater detail using a tracking system, a new method using “inverse cross bearing” was developed. This method determines the horizontal pinger position using the intersection point of more than two bearing lines from as many observation points. The vertical position of the pinger is measured by a depth sensor on the pinger itself. This system calculates the relative azimuth of the pinger using the receiving time differences among the four transducers. The GNSS compass, to convert the absolute azimuth of the pinger's relative azimuth, measures the ship's heading. To estimate the theoretical error factors, experiments were conducted in actual seas. The estimation results were compared to the results of the field experiments. In certain conditions, the estimated positioning accuracy was 14 m, and the measured value was 21 m. This discrepancy may be due to factors such as agitation of the ship.