Introduction
Geophysical surveys make measurements of the seabed and the sub-seabed using sound or, at close quarters, laser light. The sensors tend to fall into three categories:
Seabed measuring sensors, e.g. echo sounders, multibeam sounders.
Imaging sensors, e.g. side-scan sonar, laser-scan, acoustic scanning systems.
Sub-bottom profilers, e.g. pingers, boomers etc.
The most common combinations of system sensors for engineering applications are:
Echosounder: for measuring the water depth directly beneath the vessel. This also acts as
a calibration device to the multibeam sounder.
Swathe bathymetry: for measuring a wide swath of seabed soundings either side of the
survey vessel.
Side-scan sonar: for generating a scaled image of the seabed morphology and features.
Sub-bottom profiler: for determining the stratification of soils to a depth of, say, 50m beneath the seabed, depending on frequencies and energy levels.
Technical Description
Acoustic energy (sound) is the most common source for underwater measuring and sensing
systems. Over very short distances, in higher quality water, a new generation of scanning systems use laser light but these systems are beyond the scope of this guide.
In operation, an acoustic energy source generates a pulse of sound that travels through the
water column and, where powerful enough, penetrates into the seabed. The sound energy is
reflected back as an echo to a receiver system and the lapse in travel time from transmission to reception is converted into ranges.
The media through which the sound passes affects the acoustic signal in various ways. The
more dense a medium, the faster is the speed of sound; hence, as the wave front passes
through different water densities, its rate of progress varies. At the interface between media, a change in the properties will cause some energy to be reflected; this is most prominent at the water/soil interface and between soil strata.
The two fundamental characteristics of the acoustic wave used in geophysical survey are
amplitude and frequency. Different acoustic and seismic tools operate within different amplitude and frequency ranges, and provide information on different aspects of the physical environment.
In the simplest terms, high frequency, low amplitude signals provide high-resolution information in the water layer and shallowest depths sub-seabed, and have a shorter range. A low frequency, high amplitude signal will travel further into the earth, but has lower resolution.
To generate different frequencies and amplitudes of acoustic energy, transducers of many types are used. Electro-mechanical transducers generate acoustic pulses in echosounders, side-scan sonar, pingers, boomers and chirp sonar. Electrical discharges generate acoustic energy in sparker systems.
Air gun systems convert compressed air pressure into high-energy acoustic pressure waves in seismic sources. Returning signals are detected using pressure sensitive transducers and hydrophones. The pressure pulses are converted to electrical energy for measurement and storage.
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