Multiple source acquisition for use in 4D marine seismic
Throughout the lifecycle of an offshore oil or gas field seismic acquisition can be repeated a number of times. Within the O&G industry, seismic data enables E&P companies first to identify hydrocarbon reservoirs and then maximize the extraction of them. During the exploration phase of a new field, a seismic company is contracted to survey the area. This can be started with sparse sampling, with one source and one streamer towed in 2D mode and then with 3D over a targeted area. However, in certain circumstances the decision can be to jump straight to 3D, with multiple sources and streamers, when the uplift outweighs the cost. Fast forward to later in the life cycle, if successful, the now producing field will need routine monitoring, and this is where the area can be surveyed again – every few years – to assess changes in the sub-surface. The subsequent surveying of a field is called ‘4D monitor’ or ‘time-lapse’ seismic acquisition. To accurately evaluate changes to the property of the reservoir, such as fluid saturation or reservoir thickness, the 4D monitor survey should replicate the preceding survey as effectively as possible (Calvert, 2005). This is no mean feat, considering years could have passed and technology is likely to have changed. The surveys could be acquired in a different period of the year, so environmental factors influencing the equipment dynamics will be inconsistent. A vessel with different equipment could be acquiring the new data, and so on. The objective is to minimize the impact of these variables as much as possible. However, this can in turn lead to advances in acquisition and processing technology and techniques becoming constrained to match the baseline. In recent years, marine seismic acquisition with three or more sources has become commonplace, driven by a combination of economics, data quality and health and safety. This paper outlines how multi-source designs can be used to simultaneously repeat a baseline survey and provide a new baseline, with enhanced spatial sampling and signal-to-noise, for future monitors.