Seismic data acquisition

Acquisition of seismic exploration data involves the synthetic generation of seismic waves, and their subsequent detection after passing through or reflecting off the region of interest (i.e., the "target"). The most frequently practiced form of seismic acquisition is the reflection seismic survey. A reflection seismic survey typically involves generating hundreds to tens of thousands of seismic source events, or shots, at different locations in the survey area. The seismic energy generated by each shot is detected and recorded at a variety of distances from the source location (Figure 3). The detectors used to transform ground movement into an electrical voltage that can be recorded are geophones and hydrophones, generically referred to as receivers. For every source event, each receiver generates a seismogram or trace, which is a time series representing the earth movement at the receiver location. Each trace has a reference time zero corresponding to the time of its source event. A record of all traces for each shot is usually written to a medium such as magnetic tape for subsequent study, including processing, display and interpretation.

Figure 3: Geophones record the movement of the ground due to the propagation of seismic energy as seismograms or seismic traces. Seismograms are freqently displayed as "wiggle traces." (Click for larger figure.)

Precise relative and absolute positioning information must also be collected and recorded for all source and receiver locations. This task has been greatly simplified since the deployment of the Global Positioning Satellite (GPS) constellation. Absolute source and receiver positions are usually known to within five meters thanks to the availability of differential GPS techniques. The relative distance between the source and receiver for each trace is its offset.

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.

Marine acquisition of seismic reflection data is generally accomplished using large ships with multiple airgun arrays for sources. Airguns are deployed behind the seismic vessel and generate a seismic signal by forcing highly pressurized air into the water. Receivers are towed behind the ship in long streamers that are several kilometers in length. Marine receivers are composed of piezoelectric hydrophones, which respond to changes in water pressure. Being pressure sensitive, hydrophones measure the acceleration of the medium as a seismic wave passes through it, unlike geophones, which respond to the velocity of the medium. Because of sensitivity and noise issues, responses from a group of 5 to 50 hydrophones are summed to produce a single seismogram, and the group is considered a single receiver. Note that the equipment just described is generally intended for probing subsurface depths of a few hundred meters to 10 kilometers. Marine seismic acquisition is also performed for shallow hazard surveys using smaller ships, higher frequency sources and much shorter hydrophone streamers, frequently with only a single receiver group.

Until the 1980's, reflection seismic acquisition was carried out by arranging the source and receivers in a line for a shot, then advancing the equipment along a linear transit as necessary to complete the survey. Geographic and cultural obstacles on the earth's surface frequently forced some deviation from this idealize acquisition pattern, but the end result was usually the acquisition of a 2-D seismic profile along a nearly linear transit. Since the mid-1980's, improvements in computational power have made the acquisition and processing of 3-D seismic surveys a practical, though expensive, endeavor. Because geological structures are three dimensional in nature, 3-D acquisition and processing are usually desirable, or even necessary, to produce a proper representation of the subsurface. As a result, most seismic surveys and the vast majority of seismic data are now collected as 3-D surveys. The shift from 2-D acquisition to 3-D acquisition has had operational consequences that have greatly changed the nature of the seismic exploration industry. Because the facilities and equipment necessary for modern acquisition are very expensive, most seismic surveying is now carried out by a handful of large contracting companies. Because terabytes of data are acquired in medium size surveys, most data processing is also performed by contractors, and the client frequently only receives a processed and greatly compressed seismic image volume as a product for use in interpretation.

Recent advances in acquisition technology are improving the quality and economics of 3-D acquisition. In land acquisition, the logistics of deploying receiver stations and cables are a significant expense, severely limiting the number of receivers that can be deployed per shot. Thanks to advances in electronics, the receiver signal is now commonly digitized at or near the receiver, and the data are either recorded near the receiver stations during a survey, or transmitted to a base station by radio or by fiber-optic cables. This improvement in data handling makes it economical to deploy more receivers during a survey. In marine surveys, the streamer mode of acquisition has traditionally allowed only nearly "in-line" source-receiver azimuths and provided only limited angular raypath coverage of the subsurface. Today, seismic vessels can deploy a number of streamers behind the ship in parallel -- as many as 20 -- allowing the acquisition of a wider azimuth of source-receiver offsets and making it possible to acquire more data in a fixed amount of time. Another new development in marine acquisition is the deployment of multiple vertical cables, each anchored at the seafloor bottom and containing multiple receivers. As the source ship shoots around and over the vertical cable deployment, data are acquired with multiple source-receiver offsets and receivers at multiple depths. The variety of receiver depths beneath the water surface makes the elimination of water surface multiples a simple task during subsequent data processing.

An important subset of reflection seismology is multi-component seismology. Multi-component seismic acquisition has recently been transformed by new technological developments that have improved the practicality of marine acquisition. In the solid earth, the complete elastic wavefield is composed of both P-waves and S-waves and is a vector quantity. Multi-component receivers that measure particle displacements in three perpendicular orientations are necessary, therefore, to detect the full elastic wavefield. In a marine environment, however, water cannot transmit shear wave energy, so direct recording of multi-component information is not possible using towed streamers. Marine multi-component acquisition is feasible using ocean bottom seismograms (OBS's), but placing individual multi-component receivers on the sea floor is time consuming and expensive. These practical constraints on ocean bottom acquisition have recently been addressed by the development of cables containing multi-component arrays that can be deployed on the sea floor more economically. This advance in acquisition technique has been matched by the recent appreciation of the importance of converted mode energy in reflection seismology. Although marine seismic airguns generate only P-wave energy, some portion of the P-wave energy is converted into S-wave energy as the direct wave travels downward into the subsurface and encounters impedance boundaries. The converted mode corresponding to conversion upon reflection from down-going P-wave to up-going S-wave is sometimes referred to as the C-wave, and is now known to have practical applications in exploration. Multi-component seismic data are discussed further in the section on seismic interpretation.

In addition to reflection seismology, there are a number of other important methods in seismic exploration. In refraction seismology, for each source event, only the initial ground movement that arrives at each receiver is significant in later analysis. This results in a data set of time versus source-receiver offset for each shot. These first arrivals or head waves followed travel paths that were refracted at the critical angle upon entering a high velocity layer. These travel paths are predominately horizontal, and these data can be interpreted to reveal the depths and seismic velocities of layers that support critical angle refraction paths. (A refraction experiment can only observe a layer if its seismic velocity is higher than all the velocities above the layer.) Because refraction seismology only uses seismic energy that propagates through critical angle travel paths, relatively large source-receiver offsets are required, and the data analysis usually assumes a layered geology without structural complications. In spite of these limitations, seismic refraction experiments are useful for shallow engineering studies and for large scale crustal studies -- depths that are generally too shallow or too deep for seismic reflection surveys.

Another subset of exploration seismology is downhole seismology. Vertical seismic profile (or VSP) surveying involves recording the responses of geophones in a borehole or well for sources at the earth's surface. Likewise, a reverse VSP is collected using a source in the borehole and receivers at the surface. A crosshole survey measures the seismic responses of geophones in one borehole to a source in another borehole. Downhole methods are especially useful for accurately determining seismic P-wave and S-wave velocities since the distance of equipment down the hole is a known value. (In the crosshole case, this implies analysis using crosshole tomography.) Additionally, either the sources or receivers, or both, are much closer to the target than in surface reflection acquisition, and the seismic travel paths avoid at least one of two passes through the near surface weathering zone. As a result, downhole data can be used to construct very high-resolution images of the subsurface target region.

Walter Kessinger

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