The three-component (3C) geophone is used for direct measurements of shear waves at the seafloor. Conventional seismic surveys on land use one geophone or a group of geophones per receiver location to detect motion in the vertical direction. Geophones, unlike hydrophones, detect motion rather than pressure. The geophone is a device used in surface seismic acquisition, both onshore and on the seabed offshore, that detects ground velocity produced by seismic waves and transforms the motion into electrical impulses. A typical length of a streamer is about 4-6 km where a single receiver section is typically 75 m long and contains 96 hydrophones which are grouped in arrays of a pre-defined length, mostly 12.5 or 25 m. Hydrophones are combined to form streamers that are towed by seismic vessels or deployed in a borehole. In this way the influence of movements due to currents, wave action and so on will be minimised. The two piezoelectric elements in one hydrophone are connected and polarised so that voltages due to pressure waves (returning signal) add and voltages due to one-directional acceleration will cancel. It measures pressure variations with the aid of piezoelectric material, which generates a voltage upon deformation. The hydrophone is a device designed for use in detecting seismic energy in the form of pressure changes in water during marine seismic acquisition. Hydrophones and geophones serve as receivers for seismic signals. It includes a variety of energy sources based on varying explosive out put parameters to meet geological and climatic conditions. Dynamite can generate usable signal strengths and a bandwidth that covers a wide spectrum of seismic energy. It is customary to drill a hole to load dynamite and fill it with heavy mud before shooting. It provides an impulsive energy that can be converted into ground motion. When dynamite ignites, a shock wave propagates with a speed of 3,000–10,000 m/s. The detonator helps to ignite the explosives. Dynamite a combination of explosive and detonator, is used as a seismic source. The recorded data are then convolved with the original sweep to produce a usable signal. A vibrator source sends a controlled-frequency sweep into the ground. A vibrator is an adjustable mechanical source that delivers vibratory seismic energy into the ground. An air-gun is a device that releases highly compressed air (at typically 2,000–5,000 psi) into the water surrounding the gun. In marine environments seismic energy is normally generated using arrays of air-guns, whereas in land seismic one often uses explosives or vibrators. Different seismic sources are usually used in land and marine acquisitions.
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