Abstract

During the last decade significant research has been performed on the possible impact of seismic surveys on marine life. Both fish and marine mammals have been studied. None of the studies have documented direct damage caused by the signals, but there may be a temporary behavior change out to a distance of a few kilometers.

Many reports dealing with this issue do not treat the acoustic principles or the measurement of signal pressure levels in a consistent manner. Therefore it may be difficult to make direct comparisons between the results given in these reports.

During the many years that I have worked with this topic, it has become clear that an understanding of acoustic principles is important for the assessment of what effect seismic surveys have on marine life. This abstract therefore have a more detailed discussion of seismic survey principles and the physics of sound than will be possible to give at the oral presentation.

Introduction:

Seismic surveys are the basis for all exploration for and development of hydrocarbon resources. Without these, the oil industry would not be able to map the reservoirs and locate possible drilling targets several thousand meters below the sea. The type of data needed varies with the different stages of work in a license, from early exploration to the development and depletion of resources in a field.

The principle for marine seismic surveys is illustrated in Figure 1. A strong acoustic pulse is sent out from a signal source towed behind the seismic vessel. This pulse is reflected from the boundaries separating the geological layers in the subsurface, and the reflected signals are recorded by many hydrophones towed in a cable several kilometers long. Generally speaking, the system operates as a large echosounder.

By recording the reflected signal at many points along the cable, it is possible to process the data and thereby remove noise and unwanted signals, in addition to the presentation of the final data in a form that makes it suitable for interpretation.

The reflection points at each strata boundary (CDP-points) from many Shot points are combined in the subsequent processing in order to achieve the required final seismic data.

The seismic surveys can be performed as either reflection or refraction surveys. Reflection surveys are predominantly used by the oil industry; the refraction survey method have only have limited use by universities for scientific purposes. Either method requires that the seismic vessel follows predetermined paths that are selected based on the objective of the survey and the geology in the area.

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