Keywords : calibration

Calibration

SonarScope makes it possible to calibrate reflectivity, meaning to provide backscatter values in dB re. 1m2. This operation has to be done if one wants to realise multi-temporal or multi-sensors or multifrequency quantitative studies. Unfortunately this implies to perfectly know the response of each element in the measurement chain, in other words the transfer functions for each term of the sonar equation: the directivity diagram in transmission, in reception, the transmission loss in the water column, the seafloor reflectivity angular law… it would be pretentious to say that we master all these elements. On the other hand, one can model as well as possible each of them by establishing different assumptions. SonarScope features a data processing protocol for modelling some parameters insufficiently known. This protocol works if enough information about the sonar characteristics is already provided by the supplier: source level, array directivity diagrams measured in factory, TVG law, processing … Kongsberg-Simrad were pioneers in this field; we progressed significantly in this field thanks to them giving us access to this information.

The calibration procedure has to be done by a well-trained person because decisions have to be taken at given stages. If the user is interested only by an image with relative intensity levels, he will be able to apply a statistic compensation which will give a satisfying image if the chosen hypotheses for the compensation dominate inside the image. We met cases for side-scan sonar imagery where the TVG law and directivity diagrams have an equal level of influence on data; in this case a statistic compensation depending on one or the other parameter will not be satisfying.

Image at outputing of the sounder

Image averages according to transmission angles and transmission beam numbers.

Image provided by the sonar and changed into angular reflectivity (suppression of Lambert's law correction).

Angular reflectivity modeling

Image of transmission directivity patterns

Previous image compensated from the angular reflectivity model and from the manufacturer's compensation of directivity diagrams : the result brings out array directivity patterns.

Image averages according to transmission angles and transmission beam numbers.

Directivity patterns modelling

Modelled directivity patterns

Image entirely compensated by models (BS + DiagTx)

Comparison at the sonar output and compensated calibrated image

EM12D Image at outputing of the sounder (SEDIFAN mission)

Calibrated EM12D image (SEDIFAN mission)