New practical and cost-effective digital single channel seismic recorder and data-analyzer system were developed. The recorder consists of a high precision seismic digitizer and a general purpose personal computer. The digitizer utilizes a high-speed(320 kHz), wide dynamic range(120 dB), 16 bit floating point (12 bit mantissa and 4 bit exponent) A/D converter, Micro Networks MN 5420. The personal computer controls the digitizer and acquires digital data through DMA interface. The data are recorded on high density (1 GB /cartridge) digital audio tape, DAT, data storage system through SCSI interface. Sampling intervars and trace length can be selected among 4, 2, 1, 0.5, 0.25, 0.125 msec and 4, 2, 1, 0.5, 0.25, 0.125 sec, respectively. The recorded data can be played-back and displayed on the recorder and applied further processes with a wide range of computer systems. We show a field data example with a simple imaging analysis.

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In general, pollutants discharged into the sea are often absorbed in coastal sediment in a long time. The movement of the sediment has been made rather clear in shallow waters, but not yet clear in deeper waters. The auther carried out a field survey of sediment in an offshore area and obtained thickness of sediment movement layer, accumulation rate, concentration of suspended sediment, critical water depth for sediment movement, and direction of sediment transport.

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For ocean development and off-shore construction, it is necessary to conduct a geological and geophysical survey of the seabed to select the most suitable and safe sustaining layer for the construction. Although there are several seabed survey methods available, improvements are still needed in order to reduce the amount of required manpower, and to improve the efficiency and accuracy of surveys.
As a means of solving the above-mentioned problems, Ono Sokki Co., Ltd. has developed a system called BSMP (Bottom Shear Modulus Profiler) based on the theory introduced by Pro. Tokuo Yamamoto and Dr. Tsuyoshi Torii of Miami University in 1986. Using this system, the shear modulus of a seabed can be obtained by observing hydraulic pressure differences on the seabed caused by waves and distortion of the sea bed caused by hydraulic pressure.
The author executed a performance test of this system off-shore of Urayasu in Tokyo Bay, with the cooperation of Ono Sokki Co., Ltd. As a result of this test, it was clarified that the shear modulus determined by BSMP at each depth of the seabed corresponded to the N-values of the boring samples.

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When we are going to perform soil investigation in the sea over 25 m deep, selection of a drilling scaffold platform is always accompanied with some laborious operations. The operations are, for example, to install anchor wires for supporting the platform and to guide passing ships around the drilling scaffold platform and the installed anchor wires. Also a larger scale drilling platform requires a larger scale floaing crane and work ships for installation. As a result the losses of times, labors and economical costs are comparatively large.
Recently a new type scaffold derrick with no anchor wire was developed, which is easy and simple to transport, install and operate.
This derrick is called the Spar-Buoy Scaffold Derrick. The buoyancy chamber is located at the middle portion of the derrick center pipe, which is the platform's main body. This chamber provides a powerful buoyancy to stand the derrick in a stable and upright position.
Prior to this development we studied characteristics of movement of the derrick through numerical computations and model tests using a 1/25 scaled model. The study cleared the behavior of the Spar-Buoy Scaffold Derrick against wind force, current force, and force due to wave motion, and confirmed the possibility of practical use. Based on these results we made a design and manufactured an actual size Spar-Buoy Scaffold Derrick, and performed a drilling experiment in the sea of 28.5 m deep. During this experiment no operational problem was met.
Using the Spar-Buoy Scaffold Derrick in-situ tests results and undisturbed soil samplings, which are important factors in soil investigation, showed equivalent quality to thoe with ordinary steel scaffold platforms

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On this paper, we solve the two problems on a multi-frequency echo sounder, which are frequency dependency of acoustic reflection loss Lp on marine sedimentary bottom such as soft mud and depth differences on echo-gram for each frequency, by using the critical multi-layered model.
In order to explain the first problem we set up some multi-layered models, a basic model is composed of sea water, the 1st sedimentary layer and the 2 nd layer, and an advansed model is very thin surface layer with low volume concentration on the 1 st layer. A correspondence between absolute value of complex reflectivity |R(f)| and calculated reflectivity converted from L_p in situ is examined. The calculated reflectivity is composed with echoes from each boundary and layer, so it shows that the main reflection point is changed to shallow boundary according to increase of frequency. For each model, frequency dependency of |R(f)| is in good response to converted values.
The second problem seems to be related to deepening of reflection point by sounding frequency, so we set up practical 4-layered model. The layer model is composed of a basic layer concluded 20% porous organic particles in mineral as the 1 st layer and very thin surface layer of thickness 1 centimeter covered on the 1 st layer. For this model there are some peaks on auto-correlation coefficients C(τ) obtained from |R(f)|, which expresses propagating time difference at each layer. The depth of each layer calculated from the peak agrees with it of echo sounding records in each frequency of 200, 100, 24 kilohertz because of deepening of main reflection point in inverse proportion to frequency. It can be also verified by layer's physical properties estimated from each peak of C(τ).

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