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HADDEN*, SCOTT, and CD GREEN, University of St. Andrews, St.Andrews, Fife, Scotland

Abstract: Remote Classification of Seabed Types Derived from Interferometric Sonar Data

In the past 5 years, the application of interferometric sonar techniques has significantly advanced bathymetric mapping for oil and gas hazard surveys and also near shore engineering studies. Furthermore, recent developments have provided significant technological advances with respect to sea-bottom type classification for the marine survey industry. These advances offer the opportunity for rapid engineering and environmental marine site evaluation and may also offer the mechanism through which time dependent bottom condition changes can be evaluated during 4D seismic acquisition.

Basics of Interferometry

The application of interferometry to remote mapping, a technique well known in airborne and satellite surveying, has now been adopted in marine sonar surveying and offers similar advantages to those seen for airborne techniques with full 3D coverage through mosaic images. Marine interferometric sonar systems derive the distance of a reflection point on a surface through a function of the 2-way time travel of sound in water. By aligning a series of receivers within a transducer stave, spaced out at intervals relative to the wavelength of the acoustic beam, the interferometric sonar is also capable of measuring the phase angle of return of the reflected wave.

This phase angle measurement is then used to pin-point the reflection point on the time (distance) arc relative to the source.

By simultaneously sampling the bathymetry and the amplitude of the reflected wave, it is now possible to create an accurate bathymetric image of the sea floor over which the amplitude or "side-scan" images can be directly overlaid - a process which immediately high-lights a significant advancement in seabed mapping, and one which also illustrates the affects that varying angles of incidence have upon the received amplitude levels. Moreover, both of these maps can be produced rapidly after multiple passes and mosaicing. Additionally, the acoustic amplitude recordings can be processed to account for several factors within the sonar equation.

The Sonar Equation

The sonar equation is used to account for the factors acting upon the acoustic wave between the transmitter and receiver emission and reception, and can be written as (from Coates)

SE = SL + 2DI - 2TL + TS - NL

Where SE = Signal Excess, SL = Source Level, DI = Directivity Index, TL = Previous HitTransmissionNext Hit Previous HitLossNext Hit (Spherical Spreading and Previous HitAbsorptionNext Hit), TS = Target Strength, and NL = Noise Level.

The values of SL, DI and NL are dependent upon the specifications of the acoustic system in use. However, numerical values for the other variables can be derived through a combi-.nation of system specification figures and the sonar geometry (see Figure 1 for acoustic geometry).

For example, the Previous HittransmissionNext Hit Previous HitlossNext Hit is dependent upon the sonar frequency of the system, the acoustic beam shape emitted, and also the distance travelled by the acoustic wave between the source and the reflection point (slant range). Knowing all of the above, it is possible to apply an accurate slant range correction figure to each value in the data set, thus replacing the time varied gain enhancements found on traditional side-scan software.

The target strength however, is dependent, among other things, upon the size of the insonified area.This area can be calculated using the beam azimuth, angle of incidence and the pulse length of the acoustic wave.

The application of the sonar equation permits the numerical accountability of all the variables affecting the acoustic wave other than those active at the water-sediment interface, namely Previous HittransmissionNext Hit into the sediment and surface scattering. Therefore, the acoustic analysis can be focused upon the affects of these two variables, and with the help of laboratory testing of sediment samples, a simple classification scheme can be proposed, based upon the Previous HitabsorptionTop and scattering potential of various sediment types.

Sediment Classification via Interferometric Sonar

Any sediment classification scheme must be initially derived from laboratory testing of sediment characteristics.

The most important parameters controlling the acoustic response of marine sediments, ranked in order of importance (according to Stoll, 1989) are -
1. Porosity
2. Density
3. Overburden stress
4. Degree and type of lithification
5. Grain size and distribution

In the acoustic impedance equation, the dominant sediment characteristic is density (although porosity can be considered a factor also), whilst in the spectral reflection equation the main factors are angle of incidence and bed roughness.

Consequently, the relationship between acoustic response and sediment type is analysed in two parts. First, the sediment type is compared to the acoustic impedance value. This comparison is used to infer a relationship between the sediment properties such as density and porosity, and the acoustic response. Secondly, an analysis of sediment type versus spectral reflection shows the impact of micro-topographic variability (surface scattering or bed roughness) within each insonified area, upon the sonar equation.

A combination of these two methods of analysis yields a sediment classification scheme derived from the acoustic response of the insonified area.

Many applications of this seabed classification are envisaged in most sectors of the marine survey industry, such as - cable/pipeline route surveys with comprehensive sediment maps for slope stability prediction; marine aggregate industry using a prediction of grain size distribution for aggregate resource development; and, hazard surveys for engineering structures such as well-heads in the deep offshore, and harbour facilities in the near shore.

Another interesting application of this technique is its possible integration with "new" 4D marine seismic surveys. In these surveys it is important that the bottom conditions are recorded both before and during repeat seismic surveys in order that the coupling of the ocean bottom cable is known for static corrections. The interferometic sonar has the ability to rapidly evaluate changes in bottom condition and thus record any possible changes in the data that may adversely affect the coupling. These changes are the subject of on-going research.

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