Marine acoustic profiling
enables mapping of large areas in a short time. State-of-the-art field operation normally can acquire data for 20 — 50 km per day. The method is, however, sensitive to waves and wind, and to operational conditions in the investigation area.
is that it is strongly advised to supplement this kind of survey with refraction seismic profiles and/or boreholes.
Short explanation of the method
Acoustic profiling is the common denomination for investigations with equipment working according to the principle of echo-sounding. The method is based on reflection of sound waves at a somewhat lower frequency than a bathymetry echo sounder or fish finder, resulting in enhanched penetration through sediments and to reflectors/ layer boundaries deeper down. It yields travel time of sound waves and thus relative thickness and distribution of sediments.
The equipments consist a transmitter (sound source) and a receiver (hydrophone) that is towed by a boat that drives at a slow speed (2-5 knots). A sound pulse is transmitted from the transmitter at regular intervals. These sound signals are reflected from the seabed, sediment inter-beds and rock surface and by return to the hydrophone they are transformed to electric signals. The signals are then sent through an amplifier and a filter before being digitised in a A/D converter and visualised in a two-dimensional section on the recording software.
False or undesired signals are normal elements of the picture, and often disturbing the interpretation. The most common type is multiples of the first reflector (seabed). The returning echo of the pulse then has been reflected by the water surface, traveled once more to the seabed and back to the hydrophone. By small water depth and by thicker soil layers than water depth, this phenomenon may mask out the true reflector of e.g. rock surface, which often is the main target of investigation. The accuracy of the method may be poor in such situations.
Boomer & Sparker
The most common types of sound source for marine acoustic profiling are penetration echo sounder (or pinger), boomer and sparker.
Penetration echo sounder has a transmitted pulse sound array of 3 -10 kHz. The penetration abilitynormally is limited to 5 - 25 m. Both transmitter and receiver may be fitted to one unit, normally in the boat haul or on a rod mounted to the boat side.
Boomer is a metal disc which repeatedly is being repelled from a copper coil by HV charges from a condenser battery. The boomer usually is towed on atwin haul structure in the water surface behind the boat. Filtrated frequency array is
0,6 - 2 kHz. The penetration usually is less than 200 m and the resolution is 1-2 m.
Sparker is a metal frame with fitted electrodes. Sparks are ignited between the electrodes and the steel frame by current pulses from a condenser battery, causing a sound pulse in the water. The sparker frame is towed behind the boat, just below the water surface. The signals are filtrated to utilize the frequency array of 0,05 — 1 kHz. Penetration is better
and may go down to 800 m by favorable conditions, but the solution is poorer, normally 5 — 10 m. Saline water is a requirement for the sparking effect to take place.
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