Geophysical Methods

exploration, near surface and marine methods


seismic source

A seismic source is a device that generates controlled seismic energy used to perform both reflection and refraction seismic surveys. A seismic source can be simple, such as dynamite, or it can use more sophisticated technology, such as a specialized air gun. Seismic sources can provide single pulses or continuous sweeps of energy, generating seismic waves, which travel through a medium such as water or layers of rocks. Some of the waves then reflect and refract and are recorded by receivers, such as geophones or hydrophones. Seismic sources may be used to investigate shallow subsoil structure, for engineering site characterization, or to study deeper structures, either in the search for petroleum and mineral deposits, or to map subsurface faults or for other scientific investigations. The returning signals from the sources are detected by seismic sensors (geophones or hydrophones) in known locations relative to the position of the source. The recorded signals are then subjected to specialist processing and interpretation to yield comprehensible information about the subsurface. A seismic source signal has the following characteristics: Generates an impulse signal Band-limited The generated waves are time-varying Sledge hammer, striking the ground directly or more commonly striking a metal plate on the ground. Useful for refraction surveys down to about 20 m below surface. Explosives, such as dynamite, can be used as effective sources of seismic energy. Generally, the explosive charges are placed between 6 and 76 metres below ground referred to as "Shot Hole Drilling". An air gun is used for marine reflection and refraction surveys. It consists of one or more pneumatic chambers that are pressurized with compressed air at pressures from 14 to 21 MPa. Air guns are submerged below the water surface, and towed behind a ship. Air gun arrays may consist of up to 48 individual air guns with different size chambers, fired in concert, the aim being to create the optimum initial shock wave followed by the minimum reverberation of the air bubble. A plasma sound source (PSS), otherwise called a spark gap sound source, or simply a sparker, is a means of making a very low frequency sonar pulse underwater. For each firing, electric charge is stored in a large high-voltage bank of capacitors, and then released in an arc across electrodes in the water. The underwater spark discharge produces a high-pressure plasma and vapor bubble, which expands and collapses, making a loud sound. Most of the sound produced is between 20 and 200 Hz, useful for both seismic and sonar applications.
Thumper technique is a weight dropping technique that was introduced in the 50s as an alternative to dynamite sources. A thumper truck (or weight-drop) truck is a vehicle-mounted ground impact system which can be used to provide a seismic source. A heavy weight is raised by a hoist at the back of the truck and dropped, generally about three meters, to impact (or "thump") the ground More advanced Thumpers use a technology called "Accelerated Weight Drop" (AWD), where a high pressure gas is used to accelerate a heavy weight Hammer (5,000 kg) to hit a base plate coupled to the ground from a distance of 2 to 3 m. Several thumps are stacked to enhance signal to noise ratio. AWD allows both more energy and more control of the source than gravitational weight-drop, providing better depth penetration, control of signal frequency content. Thumping may be less damaging to the environment than firing explosives in shot-holes, though a heavily thumped seismic line with transverse ridges every few meters might create long-lasting disturbance of the soil. Electromagnetic Pulse Energy Source (Non-Explosive). EMP sources based on the electrodynamic and electromagnetic principles.
A Seismic vibrator propagates energy signals into the Earth over an extended period as opposed to the near instantaneous energy provided by impulsive sources. The data recorded in this way must be correlated to convert the extended source signal into an impulse. The source signal using this method was originally generated by a servo-controlled hydraulic vibrator or shaker unit mounted on a mobile base unit, but electro-mechanical versions have also been developed. The "Vibroseis" exploration technique was developed by the Continental Oil Company (Conoco) during the 1950s and was a trademark until the company's patent lapsed. Seismic reflection surveys are acquired in both land and marine environments. Although the fundamental principles of the two survey types are the same, the acquisition equipment and procedures differ by necessity. Moving-coil electromagnetic geophones that sense vertical velocity are usually used as receivers in land acquisition. The seismic source on land is usually either dynamite planted in a borehole or Vibroseis, a vibrating mechanism mounted on large trucks. Unlike dynamite, the Vibroseis signal is not impulsive, but rather lasts from 7 to 40 seconds. To emit its signal, the Vibroseis source sweeps through a range of frequencies from about 10 Hz to 60 Hz. Because seismic reflectors in the earth are more closely spaced than the length of the Vibroseis signal, the reflections in raw Vibroseis records overlap, making raw Vibroseis data uninterpretable. A Vibroseis trace must be processed to produce a replacement trace with a signal equivalent to that of an impulsive source. This is accomplished by cross-correlating the raw seismogram with the Vibroseis sweep. Boomer sound sources are used for shallow water seismic surveys, mostly for engineering survey applications. Boomers are towed in a floating sled behind a survey vessel. Like the plasma source, a boomer source stores energy in capacitors, but it discharges through a flat spiral coil instead of generating a spark. A copper plate adjacent to the coil flexes away from the coil as the capacitors are discharged. This flexing is transmitted into the water as the seismic pulse.
Noise sources
Correlation-based processing techniques also enable seismologists to image the interior of the Earth at multiple scales using natural (e.g., micro seism) or artificial (e.g., urban) background noise as a seismic source. For example, under ideal conditions of uniform seismic illumination, the correlation of the noise signals between two seismographs provides an estimate of the bidirectional seismic impulse response. Passive surface wave techniques measure noise; surface waves from ocean wave activity, traffic, factories, wind, etc. These techniques include the array microtremor and refraction microtremor (REMI) techniques. The array microtremor technique typically uses 7 or more 4.5- or 1-Hz geophones arranged in a two-dimensional array. The most common arrays are the triangle, circle, semi-circle and L arrays. The triangle array, which consists of several embedded equilateral triangles, is often used as it provides good results with a relatively small number of geophones. With this array the outer side of the triangle should be at least as long as the desired depth of investigation. Typically, fifteen to twenty 30-second noise records are acquired for analysis. A technique called spatial autocorrelation (SPAC) is used to obtain the Rayleigh wave dispersion curve. For a particular frequency the phase velocity is equal to that which best fits a first order Bessel function to the SPAC function.