The plasma sparker has been applied in oceanic high-resolution seismic exploration for decades. Normally it is towed on the water surface. This is suitable for shallow water, but if the water depth is great, the resolution will decrease dramatically, especially in the horizontal direction. This paper proposes the concept of a deep-towed plasma sparker and presents an experimental study of plasma sparker performance in terms of electric parameters, bubble behavior, and acoustic characteristics. The results show that hydrostatic pressure at a source depth ranging from 1 to 2000 m has a negligible influence on the electric parameters but a strong influence on bubble behavior, wherein both the maximum bubble radius and oscillation period are decreased. The collapse pulse vanishes when the source depth reaches 1000 m or deeper, and no bubble oscillation can be distinguished. The source level (evaluated by the expansion pulse) is also decreased as the source depth increases; moreover, the greater the discharge energy, the smaller the source level loss. The discharge energy per electrode should be greater than 20 J for the deep-towed plasma sparker, which can make the source level loss induced by hydrostatic pressure smaller than the transmission loss. The fast Fourier transform (FFT) results show that the dominant energy is around 20 kHz, which is mainly induced by the expansion pulse and its oscillation. According to the simulation results, the fundamental frequency of the acoustic waveform increases with source depth in accord with a log linear trend, and also reaches tens of kilohertz in deep water. So, before the development of deep-towed plasma sparker, a new technical solution will need to be developed to solve this problem.

Electroacoustic Process Study of Plasma Sparker Under Different Water Depth

A seismic reflection profiling system utilising a surface air gun source and a deep-towed horizontal hydrophone streamer has been developed for high resolution studies in the deep ocean. The instrument is deployed on a conventional armoured single conductor cable at depths of up to 6 km. Seismic data from the 30 m long streamer is wide-band frequency modulated up the towing cable to the ship together with a high frequency monitor from a 3.5 kHz echo-sounder mounted on the instrument package. The geometry of the system allows an order of magnitude improvement in spatial resolution compared with that obtained from standard surface source/receiver configurations. The summed hydrophones of the streamer attenuate cable-generated mechanical noise, and the 3.5 kHz sea-surface and bottom reflected returns provide receiver positioning information. The system has been successfully deployed at depths of 5 km in the Vema Fracture Zone in the North Atlantic, and although initially difficulties were experienced in balancing the streamer, subsequent profiles across the transform fault show details of sub-bottom structure which on conventional surface records are generally masked by diffraction hyperbolae.

A high-resolution seismic profiling system using a deep-towed horizontal hydrophone streamer

: The development of a unique Deep-Towed Array Geophysical System (DTAGS) is nearing completion at NORDA. Newly developed hardware for the system includes: a low frequency sound source; a multichannel hydrophone array; a high data rate digital telemetry system; a microprocessor-controlled, dual tape transport, digital data record system; a real-time engineering sensor display system; a short baseline navigation system; deployment/retrieval tow vehicle handling system; and portable instrumentation vans. The system is towed near the bottom (altitude of 100 m) in the deep ocean (design depth 6000 m) and provides the capability to determine detailed geophysical character of the sea floor and upper subbottom structure (acoustic penetration depth of 500 m). This publication describes the characteristics of the major DTAGS hardware suite and reviews a field engineering test during which the system was operated at a depth of 4500 m. A list of system specifications is included.

Deep-Towed Array Geophysical System (DTAGS): A Hardware Description

This paper presents the system, controller, and field test results of an unmanned marine vehicle (UMV) developed to acquire real-time ocean exploration information. The system consists of an unmanned surface vehicle (USV), an unmanned underwater vehicle (UUV), and an underwater cable. The developed platform is a research platform for ocean exploration and was proposed to compensate for the shortcomings of the existing USV, UUV, and towing system. This paper describes the proposed UMV, including the design considerations such as electronics architecture, data management, sensors, and actuators and explains the path control algorithm and controller used to follow the specified path for ocean exploration. In order to confirm the performance of the developed platform and control algorithm, a way-point tracking experiment was performed, and the results are shown.

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Development and Field Test of Unmanned Marine Vehicle (USV/UUV) With cable

The development of a deep-towed seismic profiling system has been completed after four years of design, fabrication, and testing. This system provides the capability to determine the detailed acoustic character of the sea floor and upper 500–1000 m of subbottom structure. The system is designed for tow depths of 6000 m while operating at an altitude of 100 m above the sea floor. The major components of the system include a sound source located in the deep-towed vehicle, which operates over a frequency range from 260 Hz to 650 Hz at a peak source level of 201 dB re 1 micropascal at 1 m; a 24 channel, 1000 m long array attached as a tail to the deep-towed vehicle; a duplex digital telemetry link to communicate with the deep-towed system over a coaxial tow cable; a digital data record system; a real time engineering data display system; a short baseline navigation system; handling hardware to deploy and retrieve the system; and two instrumentation vans to support system operations. This paper discusses the unique hardware developed for the deep-towed seismic system and focuses on the acoustic subsystems. Also reviewed is a performance prediction analysis initially performed to define geoacoustic parameter measurement improvements achievable with a deep-towed configuration. Field measurement engineering results acquired during the system’s final performance evaluation are presented. Data was acquired at tow depths of 4500 m during the September 1984 evaluation.

Development of a Deep-Towed Seismic System A New Capability for Deep-Ocean Acoustic Measurements

Deep-Towed Seismic System Design For Operation At Depths Up To 6000m

: A program has been initiated to develop a deep-towed system consisting of a low frequency sound source, hydrophone array, communication and data recording system, and data processing capability for measuring in the deep ocean (6000 m depth) the detailed spatial variability of the geological, geophysical, and geoacoustic parameters of the ocean floor and first 500 m of subbottom structure. This high resolution data set will allow computing definitive geoacoustic model input parameters for performance prediction of naval fleet systems with acoustic bottom interaction. The technology developments of this program will provide both the Navy and the marine science and technology communities with a new deep-ocean measurement capability. Performance studies show an order of magnitude improvement in geoacoustic parameter determination and spatial resolution for the deep-towed system over conventional surface-towed systems operating in deep water. System configuration trade-offs, optimized for compressional wave speed measurement precision, have been performed utilizing a statistical error approach. This approach provides quantitative error prediction for such varying system parameters as array length, offset (sound source to array distance), group spacing (hydrophone separation), and system tow altitude based on statistical variance of reflection energy arrival times.

Deep-Towed Geophysical Array System Development Program Review.

: This publication presents a new approach developed for analyzing geophysical acoustic reflecting data. The forward problem of estimating source to receiver travel time, and the inverse problem of estimating a sound speed versus depth relationship are addressed using a moments approach. The moments approach provides a simple tool for estimating source to receiver reflection path travel time for a laterally homogeneous medium with an arbitrary sound speed versus depth relationship. The moments approach also provides a useful tool for inverting reflection data to obtain an estimate of the sound speed versus depth relationship. A nearly closed form technique for estimating a linear sound speed versus depth relationship is presented. Derivation of the moments approach and numerical examples are included. (Author)

A moments approach for analyzing geophysical reflection data

: Development of a deep-towed seismic profiling system has been completed after four years of design, fabrication, and testing. This system provides the capability to determine the detailed acoustic character of the sea floor and upper 500-1000 m of subbottom structure. The system is designed for tow depths of 6000 m while operating at an altitude of 100 m above the sea floor. The major components of the system include: a sound source located in the deep-towed vehicle, which operates over a frequency range from 260 Hz to 650 Hz at a peak source level of 201 dB re 1 micropascal at 1 m; a 24 channel, 1000 m long array attached as a tail to the deep-towed vehicle; a duplex digital telemetry link to communicate with the deep-towed system over a coaxial tow cable; a digital data record system; a real time engineering data display system; a short baseline navigation system; handling hardware to deploy and retrieve the system; and two instrumentation vans to support system operations. This paper discusses the unique hardware developed for the deep-towed seismic system and focuses on the acoustic subsystems. Also reviewed is a performance prediction analysis initially performed to define geoacoustic parameter measurement improvements achievable with a deep-towed configuration. Field measurement engineering results acquired during the system's final performance evaluation are presented. Data was acquired at two depths of 4500 m. Keywords: Acoustics; Seismic profiling; Deep-ocean measurements.

Martin G Fagot

Papers

Deep-Towed Seismic System Design For Operation At Depths Up To 6000m

Deep-Towed Geophysical Array System Development Program Review.

A moments approach for analyzing geophysical reflection data

Development of a Deep-Towed Array Geophysical System: A New Capability for Deep-Ocean Acoustic Measurements

Deep-Towed Seismic System: A Hardware Description