How does seismic exploration work

Instead, marine seismic vessels use a combination of air guns, water guns and other acoustic sources to create the pulse needed to take seismic readings. The water gun or a combination array consisting of several sources is preferable to the air gun, because the air gun generates an unwanted secondary pulse after initially being fired, which obscures the waveform that can disturb the proper interpretation of the data.

The water gun injects water into the surrounding water, resolving the problem caused by compressed air. The point is to produce an acoustic vibration that can be read by the geophones and interpreted to reveal the geologic features underneath the seafloor. There are other methods as well, including a source that produces a chirp, or "swept" frequency. This method uses a series of frequencies based on "chirp radar" to supply the reflected pulse.

The various sources emit a variety of frequencies in order to give a more accurate read of the subsurface terrain. Whatever the acoustic source, the goal is to produce seismic pulses that reflect off of the boundaries between various layers of rock, all of which will cause the sound wave to react differently. It is important that the pulse be emitted in rhythmic repetition as the vessel moves to create a detailed length of mapping. Regardless of which method is used, the returning pulses are picked up by an array of geophones attached to lines towed by the ship.

These arrays are called "streamers," and consist of long net-like bands with geophones spaced evenly along the streamer. Interpreting the sound waves recorded by geophones makes it possible to determine the size and depth of crude oil and natural gas deposits. Seismics has established itself as a key process for the exploration of crude oil, natural gas and geothermal deposits. The onshore exploration for deposits with seismics is generally conducted using shot seismics or vibro-trucks.

In offshore seismics, special ships equipped with air guns are used to explore oil and gas fields under the ocean with air pressure waves. Seismic procedures are differentiated into 2D seismics and 3D seismics. For shot seismics, small charges are detonated in bore holes with a depth of 20 to 30 metres. By evaluating the sound waves using geophones, conclusions can then be drawn regarding oil, gas or geothermal deposits.

Simultaneously, these trucks then begin to generate energy of increasing frequency over the period of several seconds. Like with the dynamite method, the resulting reverberations are measured by geophones, with the data being sent to a recording truck. The rough signal is then filtered and processed to edit out background noise and produce a clean, sharp final signal.

The simplest and oldest form of seismic work is called 2D. In 2D, the seismic data is collected over a loose grid pattern, with the lines of the grid often conforming to local roads for ease of access.

The lines may be several miles apart. While 2D can give geologists a good general understanding of an area, its reliance on relatively few cross sections means that such seismic surveys can result in considerable structural uncertainty between lines.

By the early s, technological advances made so-called 3D seismic possible. Denser data and improved computer processing ensures that subsurface features are correctly located, and can reveal the previously mentioned DHIs, which indicate the presence of hydrocarbons, rather than merely the structural elements, which could possibly contain hydrocarbons.

Because of advances in computer power and recording ability, 3D is very commonplace today. While more expensive than traditional 2D seismic, the increased effectiveness and reliability of 3D seismic usually make it well worth the price. An advance on the premise of 3D seismic is called 4D seismic. This method involves 3D seismic acquired on a given area multiple times over an extended period of time.