Reflection Seismic

Boomer

High Voltage

Cable

3 cups

Blueberries

Crane

for heavy lifting

Boomer

Streamer

Cable

High Voltage

Cable

Marine acoustic profiling

Our beleif

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.

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.

Example of a CAD presentation of results from interpreted marine acoustic profiling. Soil layer thickness is here differentiated by color codes on a bathymetry contour map.

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.

Marine acoustic profiling (Sub-bottom profiler)

Acoustic profiling is the common denomination for investigations with equipment working according to the principles of echo-sounding. The method is based on reflection of sound waves from geological reflectors/layer boundaries below under the boat. It yields travel time of sound waves and thus relative thickness and distribution of sediments. The amplitude of the reflected signal is dependent on the acoustic contrast across the boundary, thus allowing the identification of possible lithologies of different layers.

The equipment consists of transmitter (sound source) and receiver (hydrophone) and is towed after a boat traveling at slow speed (2 – 5 knots). At a constant pace, sound pulses are transmitted. The pulses are reflected from material boundaries like seabed, sediment inter-beds and rock surface, and by return to the hydrophone they are transformed to electric signals. The signals form an echo-gram, which acts as a vertical section.

The sub-bottom profiler (also known as “SBP”, “penetration echo sounder” or “pinger”) is a single unit with combined transmitter and receiver, which is usually mounted in the haul or rod mounted on the side of the boat. It works best for the areas with bathymetry from 2 to ca 200 meters, but can be used up to 400 m water depth. The usual frequency range of the SBP is 2-22 kHz, with the penetration depth around 20 – 40 meters below the seafloor. The special resolution of SBP is ca 10 – 50 cm alone track and 5 – 25 cm vertically (it strongly depends on the geological settings and water depth).

For processing of the sub-bottom data we are using state-of-the-art methods developed in the oil industry as well as “know-how” inhouse expertise to enhance the data quality to its maximum possible extend.

At present, SBP is the acoustic geophysical method which provides the best resolution at a cost of depth penetration. It is the main method to investigate the structure of the shallow sediments and bedrock topography.

Example of the sub-bottom profiler data. Depth range 14 meters, profile length 1600 meters. Frequency 10kHz. The topography of bedrock and intra-sedimentary layering is easily identifiable.