Seismic survey is a key tool an oil company would use when exploring for hydrocarbons. Not only does it increase the exploration success thus reducing risk, it allows operators to monitor the reservoir through time. The principle behind the seismic survey is reflective seismology, i.e. a generated shock/acoustic wave that travels into the earth, is reflected by the earths rock and returns to the surface where it is recorded and measured by a receiving device a geophone. Shock waves are generated by either explosives, specialized vibro vehicles when done onshore, or airguns powered by a compressor in offshore applications.
By analyzing the time it takes for the seismic waves to travel between the rock formations and the surface engineers and sophisticated software allows the creation of subsurface maps. These maps provide an indication of where hydrocarbons may be, as well as providing details on the structural geology of the area explored. The degree of subsurface visual/graphical is a key consideration within the category and is largely dependent on the reservoirs stage in the lifecycle.
Seismic could be offshore and onshore, 2D, 3D and 4D. The output of 2D seismic is a single / line graphical representation of the rock. 2D is used when collecting large areas of data and 3D survey is not economically viable. When the data is obtained using 3D seismic, it is displayed as a three-dimensional cube that can be sliced in various directions and angles, to allow further detailed analysis of the reservoir and rock formations. As such, added details helps to reduce the uncertainty 2D seismic surveys present. 4D seismic is a standard 3D survey with a time interval as a 4th dimension. Comparing data over time (usually years) provides an understanding of the reservoir's behaviour and historical changes, and help to provide clarity on its future conditions and performance.
Seismic acquisition category could be divided into a segment consisting of several tiers:
In all seismic surveys, accurate positioning is key to acquiring data. Without knowing the exact time and position from where the data originated, the acquired information is of little use. Positioning is done using differential GPS (DGPS) to ensure precise positioning, as well as various software and offset data points.
When conducted onshore, a larger number of people and equipment are deployed to acquire the data. Onshore data acquisition is less sensitive to weather conditions compared to offshore operations. While in the onshore applications receivers/geophones are easily placed and positioned, the nature of offshore seismic presents a number of additional challenges. Offshore cables, known as streamers, are used to house the receivers and are set at distance intervals from each other. The length of the streamers could reach 6-7 km (or more) depending on the survey location and depth. In 2D surveys, only 1 streamer is used, whereas in 3D seismic as many as 12 may be used although it is most common to use between 6 and 8. There are different streamer configurations and towing techniques, each designed to cater for certain technical parameters. The survey area must be larger than the subsurface area being explored.
Conducting seismic surveys in shallow water or transition zones, by far, is the most challenging application. Finding a vessel large enough to accommodate all required personnel and equipment, yet with a small, enough draft to operate in the waters is a particular challenge. Using barges or shallow-draft vessels reduces the stability of the vessel, thus increases the inaccuracy/quality of acquired data. This may lead to the selection of different equipment or a combination of instruments to conserve space, weight and provide more reliable data.
The methodology of the survey largely remains the same, both marine and land. Predefined lines are set at appropriate spaces and lengths and each line is surveyed before moving to the next.
2D seismic is a relatively low-cost activity costing significantly less than both 3D and 4D seismic surveys.