Some basic concepts
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OffsetIn surface seismic acquisition, the horizontal distance from source to receiver. Offset between seismic source and receiver creates a delay, or moveout, in the arrival time of a reflection that can be corrected before stacking and can be used to determine velocity.
Acoustic Impedance The product of density and seismic velocity, which varies among different rock layers, commonly symbolized by Z. The difference in acoustic impedance between rock layers affects the reflection coefficient.
Seismogram Traces recorded from a single shotpoint. Numerous seismograms are displayed together in a single seismic section.
Seismograph A device or system that records the ground oscillations that make up exploration seismic data or earthquakes, sometimes used incorrectly as a synonym for geophone. A seismograph can include amplifiers, receivers and a recording device (such as a computer disk or magnetic tape) to record seismograms.
Geophone A device used in surface seismic acquisition, both onshore and offshore, that detects ground velocity produced by seismic waves and transforms the motion into electrical impulses. Geophones detect motion in only one direction. Conventional seismic surveys on land use one geophone per receiver location to detect motion in the vertical direction.
A type of receiver that can be positioned on the seafloor to acquire seismic data. Marine Seismic acquisition can be performed by a source vessel and a recording vessel with streamers or, as shown here, seabed geophones. Energy from the source vessel in the form of P-waves travels through the Earth as P-waves and S-waves and is recorded by the receiver groups and relayed to the recording vessel.
HydrophoneA hydrophone (Greek "hydro" = "water" and "phone" = "sound") is a microphone designed to be used underwater for recording or listening to underwater sound. A device designed for use in detecting seismic energy in the form of pressure changes under water during marine seismic acquisition. Hydrophones are combined to form streamers that are towed by seismic vessels or deployed in a borehole. Geophones, unlike hydrophones, detect motion rather than pressure.
Takeout on Seismic Cable
Streamer A surface marine cable, usually a buoyant assembly of electrical wires that connects hydrophones and relays seismic data to the recording seismic vessel. Multistreamer vessels tow more than one streamer cable to increase the amount of data acquired in one pass.
MultiplexA process that permits transmitting several channels of information over a single channel without crossfeed. Usually different input channels are sampled in sequence at regular intervals and the samples are fed into a single output channel; digital seismic tapes are multiplexed in this way. Multiplexing can also be done by using different carrier frequencies for different information channels and in other ways.
Binary and Decimal Amplitude
Air GunA source of seismic energy used in acquisition of marine seismic data. This gun releases highly compressed air into water. Air guns are also used in water-filled pits on land as an energy source during acquisition of vertical seismic profiles.
Marine seismic vessels typically tow arrays of air guns and streamers containing hydrophones a few meters below the surface of the water. The air guns are activated periodically, such as every 25 m (about 10 seconds), and the resulting sound wave travels into the Earth, is reflected back by the underlying rock layers to a hydrophone and then relayed to the recording vessel.
Vibrator An adjustable mechanical source that delivers vibratory seismic energy to the Earth for acquisition of seismic data. Mounted on large trucks, vibrators are commonly used for acquisition of onshore seismic data. Seismic data whose energy source is a truck-mounted device called a vibrator that uses a vibrating plate to generate waves of seismic energy; also known as Vibroseis data. The frequency and duration of the energy can be controlled and varied according to the terrain and type of seismic data desired. The vibrator typically emits a linear "sweep" of at least seven seconds, beginning with high frequencies and decreasing with time ("downsweeping") or going from low to high frequency ("upsweeping").
Vibrators (right) are commonly used for acquisition of onshore seismic data and are mounted on large trucks (left).
The term phase in seismic signals is often referred to as minimum phase, maximum phase, mixed phase, or zero phase. Minimum phase: The minimum-phase signal, shown in (a), is described as a front-loaded signal. This means that the energy in the signal is concentrated in the front of the pulse. The signal is not symmetrical. The phase of this signal will vary for each frequency component of the signal. Mixed phase: The mixed-phase signal, shown in (b), is described as a signal with its energy concentrated in the center of the pulse. It can be divided into minimum-phase and maximum-phase signals. The signal is usually not symmetrical. The phase of this signal will vary for each frequency component of the signal. Maximum phase: The maximum-phase signal, shown in Figure (c), is described as an end-loaded signal. This means that the energy in the signal is concentrated toward the end of the pulse. The signal is not symmetrical. The phase of this signal will vary for each frequency component of the signal. The characteristics of the maximum-phase signal are the opposite of the minimum-phase signal.Zero phase: The zero-phase signal, shown in (d), is symmetrical and centered on zero time. The zero-phase signal has the shortest duration and largest peak amplitude of any signal with the same amplitude spectrum. These characteristics make it the most desirable of all the signals because of its resolution capability. The phase of the zero-phase signal is zero for all frequency components contained within the signal.
CorrelationThe comparison of seismic waveforms in the time domain, similar to coherence in the frequency domain.
SignalThe portion of the seismic wave that contains desirable information. Noise is the undesirable information that typically accompanies the signal and can, to some extent, be filtered out of the data.
NoiseAnything other than desired signal. Noise includes disturbances in seismic data caused by any unwanted seismic energy, such as shot generation ground roll, surface waves, multiples, effects of weather and human activity, or random occurrences in the Earth. Noise can be minimized by using source and receiver arrays, generating minimal noise during acquisition and by filtering and stacking data during processing.
Coherent NoiseUndesirable seismic energy that shows consistent phase from trace to trace, such as ground roll and multiples.
Random NoiseDisturbances in seismic data that are not coherent (they lack a phase relationship between adjacent traces, unlike air waves and ground roll) and cannot be correlated to the seismic energy source. Random noise can be reduced or removed from data by stacking traces, filtering during processing or using arrays of geophones during acquisition.
Ground RollA type of coherent noise generated by a surface wave, typically a low-velocity, low-frequency, high-amplitude Rayleigh wave. Ground roll can obscure signal and degrade overall data quality, but can be alleviated through careful selection of source and geophone arrays, filters and stacking parameters.
Signal-to-Noise ratio The ratio of desirable to undesirable (or total) energy. The signal-to-noise ratio can be expressed mathematically as S/N. The signal-to-noise ratio is difficult to quantify accurately because it is difficult to completely separate signal from noise. It also depends on how noise is defined.
Multifold Fold CMP Coverage
If there are N geophones and the array moves forward a distance of nx between shots, it can be shown that the number of rays that share the same common midpoint is N/2n. This is the fold of the data. Fold is sometimes reported as a percentage of coverage (100N/2n %)
In the example below, there are 6 geophones and the array moves forward a distance of1x between shots. This gives 3-fold CMP coverage or 300% coverage.
Marine Seismic AcquisitionMarine seismic vessels are typically about 75 m [246 ft] long and travel about 5 knots [9.3 km/hr or 5.75 miles/hr] while towing arrays of air guns and streamers containing hydrophones a few meters below the surface of the water. The tail buoy helps the crew locate the end of the streamers. The air guns are activated periodically, such as every 25 m (about 10 seconds), and the resulting sound wave travels into the Earth, is reflected back by the underlying rock layers to hydrophones on the streamer and then relayed to the recording vessel.
VSPA class of borehole seismic measurements used for correlation with surface seismic data, for obtaining images of higher resolution than surface seismic images and for looking ahead of the drill bit; also called a VSP. Purely defined, VSP refers to measurements made in a vertical wellbore using geophones inside the wellbore and a source at the surface near the well. Most VSPs use a surface seismic source, which is commonly a vibrator on land and an air gun in offshore or marine environments. A VSP is a much more detailed survey than a check-shot survey because the geophones are more closely spaced, typically on the order of 25 m [82 ft], whereas a check-shot survey might include