MD DSS in depth

As with any survey sonar system, the MD DSS operation is based on transmitting sound waves into the water and receiving their reflections from obstacles such as the sea bottom and underlying material layer interfaces. The portion of energy reflected from an interface is proportional to the angle of incidence and the characteristic acoustic impedance ratio in the layer interface.

In conjunction with MD DSS, we often are asked to provide answers to two questions. First, what is multi-mode operation? Second, how much sediment penetration can we expect to get with MD DSS? Below we will address these two issues.

Multi-mode operation

Not surprisingly, with multi-mode operation we refer to the system's ability to function in different modes. MD DSS supports the following operation modes:

Mode Type Description
Pinger Active Sub-bottom profiling using a fixed frequency sound pulse in the range 1 kHz to 50 kHz
Chirp Active Sub-bottom profiling using frequency modulated sound pulses in the range of 500 Hz to 50 kHz
Seismic Active or passive Acquisition of seismic reflection data, resulting from a sound pulse generated either by MD DSS or a separate seismic source such as an air gun, boomer or sparker operating in the range of 20 Hz to 2 kHz
Side scan Passive Acquisition of acoustic reflection data from a side scan sonar system operating at a frequency of, e.g., 100 kHz or 500 kHz

Below are examples of data acquired with MD DSS in different modes:

Wideband chirp sub-bottom profile
              revealing sediment structures at a sub-decimeter level
Chirp profile (7-22 kHz)
Broadband chirp sub-bottom profile
              providing excellent sediment penetration while retaining
              high resolution
Chirp profile (3-8 kHz)
Boomer seismic reflection profile
              exhibiting deep penetration in glacial and postglacial
              marine sediments
Boomer profile (1 kHz)
Side scan sonar sonogram with high
              resolution seafloor image
Side scan sonar image (400 kHz)

MD DSS multi-mode operation allows multi-faceted, single-pass surveys to be carried out with just one system.

Penetration in sub-bottom profiling

Penetration varies with frequency and material. As a general rule, the lower the frequency, the better the penetration. With all other factors constant, a lower frequency travels a longer distance. Rougher materials are better at absorbing sound waves.

When addressing the issue of sediment penetration, many actors in the industry like to present impressive numbers. Considering the above facts, however, this approach turns out to be misleading. Being dependent on sediment composition to such a high degree, penetration can hardly be expressed in numbers applicable to more than just one particular location and its stratigraphic conditions.

To demonstrate the penetration capability of different acoustic methods, below we present some example profiles.

High resolution pinger sub-bottom
              profile displaying sediment layer structure
Pinger profile (24 kHz)
High penetration chirp sub-bottom
              profile demonstrating ability to reveal hard bottom or
              bedrock contour under marine sediment layers
Chirp profile (2-6 kHz)

The above two profiles portray the subsea geology along a fairway in the Baltic Sea.  At places, penetration differs markedly.

A very high resolution pinger
                sub-bottom profile portraying sediment layer structures
Pinger profile (24 kHz)
A deep penetration boomer
                sub-bottom profile displaying bedrock/hard till surface
                under postglacial and glacial sediments
Boomer profile (500 Hz - 1.5 kHz)

In this pair of profiles, there is an even more noticeable difference in penetration.

High resolution pinger sub-bottom
                profile in deeper water, exhibiting sediment layers
                along a submarine cable route
Pinger profile (28 kHz)
Air gun seismic reflection profile
                showing sub-bottom penetration down to bedrock
Air gun profile (100 Hz)

Finally, above is a pair of profiles from quite the opposite ends of MD DSS's operating frequency spectrum.  Although the frequencies are several orders of magnitude apart, the difference in penetration is hardly perceptible.

For acquisition of this survey data and for permission to publish views thereof, we would like to thank Tele1Europe in Finland, Geological Survey of Finland, Primatel Ltd, Stockholm university, Department of Geology and Geochemistry, and University of Sao Paulo, Institute of Oceanography.


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