Abstract

Offshore technology operations often require knowledge about the properties of seabed and sub-seabed to aid in their drilling operations, identifying geo-hazards and building of sub-sea structures. The analysis of marine sediments from the core samples is also routinely carried out to detect any presence of near-surface hydrocarbon seepage, which is a strong indicator of presence of active petroleum system in the area under investigation. The most conventional and also much practiced way of doing this is by collecting seabed samples using corers. The core sample analysis provides valuable information with regard to the above requirements. However, the process of coring is complex and heavily resource intensive. As the core analysis can only provide localised information, to survey an area of interest many such core samples would have to be collected to have a clear understanding of the seabed properties. Acoustic technologies have been extensively used for remotely monitoring the seabed properties. A coring operation is often preceded by a seismic survey, which again involves huge resources. The explosive sound sources (air guns) often used with these systems have always been a target of criticism from marine mammal conservationists as it compromises on the safety of marine mammals. The advancement of AUV technology has opened up alternatives to remote monitoring of the seabed. Commercially available AUVs are operationally reliable and are often equipped with many acoustic sensors such as high-resolution side scan sonar and multi-beam sonar. As AUVs can operate pretty close to the seabed, it places less demand on the source levels of the sound sources reducing the risk for marine mammals. In this review paper we look at the feasibility of adding a towed array sensor to the AUV platform so that some of the benefits of conventional seismic surveys can be obtained at a relatively lower cost. Initial results from a shallow water indicate that the seabed type can be classified to a first level using this approach and comply well with the model predictions. The study also made use of the acoustic modem signals from the AUV as the signal source so that there was no need for a custom made source. Though this study was targeted more towards understanding the effect of seabed on the sonar performance, the technique can be adapted and extended for offshore applications.

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