ACOUSTIC SENSOR MODULE FOR DEPLOYMENT ON THE SEABED

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Authors
  1. Dodds, D.J.
Corporate Authors
Defence Research Establishment Atlantic, Dartmouth NS (CAN);GeoAcoustics Inc, Aurora ONT (CAN)
Abstract
This report describes an acoustic sensor module (ASM) for an ocean bottom seismometer (OBS) assembled by Defence Research Establishment Atlantic. It is designed to make quantitative measurements of sea floor vibrations at frequencies 1-50 Hz, ranging in level from ambient noise to explosions. To make accurate measurements, the ASM must have a noise floor below the ambient noise level, and it must fithfully follow the sea floor motion. These considerations led to a instrument using 4.5 Hz geophones, contained in an 0.25 m diameter spherical pressure vessel, attached to an 0.66 m diameter "coupling disk", with a total mass of 20 kg. The geophones are the smallest available types which give a noise floor 20 dB below typical ambient noise levels. The coupling disk mass is minimized by using a tetrahedral cell structure to stiffen it. It is effectively rigid at 50 Hz. Good coupling to verical sea floor motion is achieved. A theoretical model of sea floor coupling suggests that good coupling to horizontal motion should be obtained up to 50 Hz with sediment shear wave speeds as low as 10m/s, but measurements performed with a vibrator source within the ASM at continental shelf sites show that horizontal coupling above about 10 Hz is dependent on sea floor characteristics. While a reduction in mass would improve horizontal coupling in soft sediments, such a reduction cannot be achieved using commercially available geophones.
Keywords
Geophones;Sea floor vibrations
Report Number
DREA-CR-94-459 — Contractor Report
Date of publication
01 Nov 1994
Number of Pages
56
DSTKIM No
95-02848
CANDIS No
151397
Format(s):
Document Image stored on Optical Disk;Hardcopy

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