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A Sea-Bottom Seismic Energy Source For Shallow Water Engineering ApplicationsNormal access

Authors: R.L. Good, R.A. Bums and J.A. Hunter
Event name: 12th EEGS Symposium on the Application of Geophysics to Engineering and Environmental Problems
Session: Engineering Forum - Case Histories Of Engineering Geophysics Ii
Publication date: 14 March 1999
Organisations: EEGS
Language: English
Info: Extended abstract, PDF ( 916.97Kb )

A sea-bottom seismic source, patterned after the so-called “Buffalo Gun” land source, has been
developed and tested for shallow water (river, lake, sea) applications. The sea-bottom source is
designed to be lowered on a tether, and to penetrate into soft bottom sediments to a selectable depth
(O-O.5 m). The device uses various strengths of water-proofed 8-gauge blank loads, and is fired
remotely from the surface. Zero time is measured by a transducer electrically connected to the
surface vessel. The source is relatively lightweight (14.4 kg) and can be operated from a small boat
or through ice. It uses environmentally friendly blank loads detonated within the bottom sediments
(i.e. not in direct hydraulic contact with fish). The loads are a fraction of the cost of seismocaps and
do not have the same handling/shipping restrictions.
Tests have been conducted in both fresh and salt water to water depths of 30 m. Comparative tests
with a seismocap in contact with the water bottom, fired beneath a 10 kg steel plate, showed that the
sea-bottom source produces significantly more energy, particularly in the low frequency range (30-
150 Hz). Based on these results, this device has potential applications in a variety of problems in
exploration, engineering, and environmental seismology in shallow water areas, such as:
i) seismic refraction surveys beneath lakes and rivers using water-bottom or surface hydrophone
arrays (mapping bedrock surface, velocity variations);
ii) seismic reflection surveys using water-bottom geophones (or hydrophones) to map gelogical
structure of surficial sediments/bedrock for through-ice applications;
iii) water-bottom Spectral Analysis of Surface Waves (SASW), utilizing the Scholte wave, whereby
the sub-bottom, shear-wave, velocity-depth profile (hence dynamic shear moduli) can be determined.

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