Sonia Continuous Profiler
Authors: van Overeem A.J.A. 
Published: October 2008

Shallow penetration continuous profiling is carried out to supply information for the exploitation of offshore mineral deposits, for the exploration of these deposits and for marine civil engineering studies. Unlike seismic exploration for oil and gas, deep penetration is secondary in these applications, to the detail of information obtained, i.e. to discrimination. The most straightforward way of improving discriminations is to shorten the sound pulse which is possible by increasing the sound frequency but which is limited at the same time because of the rapid increase of attenuation with sound frequency. It follows that the balance between discrimination and penetration is largely determined by the efficiency and the design of the apparatus used. In the Sonia system the optimal frequency is as high as 3000 e.p.s. which means a marked improvement in the usefulness of the technique of shallow penetration continuous profiling. The improvement in efficiency is obtained by using a 10 kilowatt sound source and a hydrophone both of which have marked directional characteristics. The signals are fed into a programmed amplifier which features variable gain, variable dynamics, variable time, varied gain with variable start of the sweep, multiplex, matched filter, etc.

Usually, the sea seriously hampers any effort to obtain direct geological information, but in a few cases this inhospitable environment offers surprising possibilities to develop indirect techniques for the collection of data which are not practicable on land. One of these techniques is continuous profiling. Essentially, a continuous profiler is an echo sounder which is a well-known and comparatively simple apparatus mounted in any merchant vessel and which does not need to be described here. However, continuous profilers used as geophysical equipment, are much more complicated from the technical point of view than echo sounders. Not only must the sea-floor show in the recordings, but reflecting surfaces beneath the sea-bed have to be recorded as well. We expect the recordings to resemble geological profiles, depicting geological features, such as stratification, type of sedimentation, submerged and filled-in valleys, bedrock surfaces, faults, folding etc. Moreover, we would also like to obtain rapidly information on the nature of rocks and sediments and on their weathering, hardness, porosity and plasticity. Sometimes, expectations even go beyond that.

Contrary to water, sediments and consolidated rocks are poor conductors of sound energy, and this particularly applies to the high-frequency portion of the sound spectrum. Attenuation is different in various kinds of rock, but whatever this variation may be, in all rock types attenuation increases very rapidly with an increase in the frequency of sound. It follows that if sound is required to travel long distances in rock, as in seismic exploration for oil and gas, low sound frequencies have to be used. The corresponding wave-lengths may be as long as 150–200 ft.

Consequently, one should not expect to find any small detail in the seismic recordings, even though very advanced processing techniques have succeeded in extracting optimal information from the seismic signal. Because seismic methods are mostly used to locate geological structures of large dimensions this lack of detail is not necessarily a serious drawback.