What Is A Seismic Survey?
Acoustic waves are used in seismic surveys to build images of the earth by analysing vibrations from those waves. Some seismic waves can penetrate solid rock and fluids into the earth’s deep interior layers, while others can simply move along the surface of the planet, like ripples on water. Offshore surveys are carried out by transmitting acoustic waves into different layers of rock beneath the sea bottom, recording how long each wave takes to bounce back, and analysing the various features of those waves, according to the International Association of Geophysical Contractors (IAGC).
Seismic Survey can be carried out both on land and at sea. Both types of surveys are used by the oil and gas industry to assess where oil and gas reserves are located on land and at sea. Both studies complete seismic imaging of the earth’s underlying strata using the same method. A vessel sits on the water’s surface for offshore testing, with long floating cables, or streamers, and specialised microphones, called hydrophones, attached to the cables. Acoustic wavelets are created by air guns situated behind the vessel and in front of the streamers shooting blasts of air into the water at regular intervals. These wavelets move through subterranean rock strata before returning to the surface through the water.
Data is collected based on the distance travelled by the wavelets, which is then analysed to create an image that mirrors the subsurface layers. Although onshore Seismic Survey testing is done with trucks rather than ships, all of the procedures in the process are the same.
Seismic Survey
Seismic Survey is a technique employed in the oil and gas exploration phase. The procedure provides a preliminary impression of what is under the earth’s surface. Seismic waves are released by equipment such as a seismic vibrator (on land) or an airgun (in water) that travel through the earth’s layers and “bounce back” off different rock layers. A receiver (geophone or hydrophones in water) records the reflected and refracted seismic waves, providing the initial view of the subsurface. Without needing to drill, this technology allows for the interpretation of what is beneath the earth or seabed (oil, gas, water, faults, folds, and so on).This method aids companies in determining where a well should be drilled.
Types of Seismic Surveys
Two Dimensional (2D) Survey:
One source of energy and one set of receivers create a two-dimensional image of the rock layers beneath the surface in a 2D survey. A two-dimensional survey depicts a single cross section (slice) of the earth
Three Dimensional (3D) Survey:
When a 2D survey reveals a geological structure or element that may hold oil or gas, a 3D survey is used. It provides a more detailed picture by delivering information in three dimensions about a smaller region.
Four Dimensional (4D) Survey:
A 4D survey is a 3D survey that has been conducted numerous times to track changes in the amount and position of oil and gas in a given area over time. It aids businesses in predicting the field’s future steps in development.
Marine Seismic Survey
During a marine Seismic Survey, the seismic vessel moves at roughly 5 knots in a predefined pattern while towing one or two sound sources to generate sound waves, as well as one or more long cables or “streamers” containing hundreds of uniformly spaced hydrophones. To guarantee that geological features are precisely pinpointed, the position of the vessel and equipment must be carefully managed utilising advanced navigation and acoustic systems. To decrease the effect of ocean waves on the data, each streamer can be up to 12 kilometres long and pulled six to 15 metres below the water surface. In the most technically advanced Seismic Survey, up to 12 streamers are towed at the same time, each about 50 to 150 metres aparts
A seismic acoustic source array emits a sound that lasts less than 0.1 second. It is typically repeated every 10 to 15 seconds as the seismic vessel moves along a straight ‘data acquisition’ line at a speed of about 5 knots for many kilometres. After which the vessel will move to another acquisition line and may return to the area many hours later.
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