Seismic data acquisition

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Workers performing seismic tests (US, 1940s) Workers performing seismic tests, Seismic Explorations, Inc..jpg
Workers performing seismic tests (US, 1940s)

Seismic data acquisition is the first of the three distinct stages of seismic exploration, the other two being seismic data processing and seismic interpretation. [1] Seismic acquisition requires the use of a seismic source at specified locations for a seismic survey, and the energy that travels within the subsurface as seismic waves generated by the source gets recorded at specified locations on the surface by what is known as receivers (geophones or hydrophones). [1]

Contents

Before seismic data can be acquired, a seismic survey needs to be planned, a process which is commonly referred to as the survey design. [2] This process involves the planning regarding the various survey parameters used, e.g. source type, receiver type, source spacing, receiver spacing, number of source shots, number of receivers in a receiver array (i.e. group of receivers), number of receiver channels in a receiver spread, sampling rate, record length (the specified time for which the receiver actively records the seismic signal) etc. [1] With the designed survey, seismic data can be recorded in the form of seismic traces, also known as seismograms, which directly represent the "response of the elastic wavefield to velocity and density contrasts across interfaces of layers of rock or sediments as energy travels from a source through the subsurface to a receiver or receiver array." [3]

Survey parameters

Source types for land acquisition

For land acquisition, different types of sources may be used depending on the acquisition settings.

Explosive sources such as dynamite are the preferred seismic sources in rough terrains, in areas with high topographic variability or in environmentally sensitive areas e.g. marshes, farming fields, mountainous regions etc. [4] Such type of sources needs to be buried (coupled) into the ground in order to maximize the amount of seismic energy transferred into the subsurface as well as to minimize safety hazards during its detonation. An advantage of explosive sources is that the seismic signal (known as the seismic wavelet) is minimum phase i.e. most of the wavelet's energy is focused at its onset and therefore during seismic processing, the wavelet has an inverse that is stable and causal and hence can be used in attempts to remove (deconvolve) the original wavelet. [1] A significant disadvantage of using explosive sources is that the source/seismic wavelet is not exactly known and reproducible and therefore the vertical stacking of seismograms or traces from these individual shots can lead to sub-optimal results (i.e. the signal-to-noise ratio is not as high as desired).[ citation needed ] Additionally, the seismic wavelet cannot be precisely removed to yield spikes or impulses (the ideal aim is the dirac delta function) corresponding to reflections on seismograms. [1] A factor that contributes to the varying nature of the seismic wavelets corresponding to explosive sources is the fact that with each explosion at the prescribed locations, the subsurface's physical properties near the source get altered; this consequently results in changes in the seismic wavelet as it passes by these regions.[ citation needed ]

Nomad 90 vibrating Nomad 90 vibrating.jpg
Nomad 90 vibrating

Vibratory sources (also known as Vibroseis) are the most commonly used seismic sources in the oil and gas industry. An aspect that sets this type of source apart from explosives or other sources is that it offers direct control over the seismic signal transmitted into the subsurface i.e. energy can be transmitted into the subsurface over a known range of frequencies over a specified period of time. [5] Vibratory sources typically host trucks that are mounted with heavy plates which repeatedly hit the ground to transmit seismic energy to the subsurface. [6] The figure on the right shows one such Vibroseis, known as the Nomad 90. Vibratory sources are often employed where vast areas need to be explored and where the acquisition region does not feature densely populated or densely vegetated areas; highly varying topography also inhibits the employment of vibratory sources. [7] Additionally, wet regions are also suboptimal for vibratory source use since these trucks are extremely heavy and hence tend to damage property in wet terrains. [7]

Weight Drop sources, such as the hammer source, are simpler seismic sources that are typically employed for near-surface seismic refraction surveys. [8] This type of source often only involves a weight source (e.g. hammer) and a plate (alongside a trigger to initiate recording on receivers) and hence is logistically feasible at most locations. Its usage mainly being in the near-surface surveys is associated with the smaller amplitudes generated and hence smaller penetration depths compared to vibratory and explosive sources. [7] As in the case of explosive sources, weight drop sources also utilize an unknown source wavelet which offers difficulty in optimal vertical stacking and deconvolution.[ citation needed ]

Source types for marine acquisition

Air-gun is the most commonly used seismic source in marine seismic acquisition since the 1970s. [9] The air-gun is a chamber that is filled with highly pressurized, compressed air which is rapidly released into the water to generate an acoustic pulse (signal). [9] The factors contributing to its common use include the fact that the pulses generated are predictable, controllable and hence repeatable. [9] Additionally, it uses air to generate the source which is readily available and free of cost. Lastly, it also has a relatively smaller environmental impact for marine life compared to other marine seismic sources; an aspect that deters the use of vibratory sources for marine acquisition. [9] [10] Air-guns are typically used in groups or arrays (i.e. multiple air-guns of different volumes) to maximise the signal-to-noise ratio and to minimise the appearance of bubble pulses or oscillations on the traces.[ citation needed ]

Receiver type

Hydrophone

A hydrophone is a seismic receiver that is typically used in marine seismic acquisition, and it is sensitive to changes in pressure caused by acoustic pulses in its surrounding environment. Typical hydrophones utilise piezoelectric transducers that, when subjected to changes in pressure, produce an electric potential which is directly indicative of pressure changes. [11] As is the case with air-guns, hydrophones are often also employed in groups or arrays which consist of multiple hydrophones wired collectively to ensure maximum signal-to-noise ratio.[ citation needed ]

Geophone

Geophone SM-24 Geophone SM-24.jpg
Geophone SM-24

A geophone is a seismic receiver that is often chosen in land acquisition to monitor the particle velocity in a certain orientation. [12] A geophone can either be a single-component geophone which is designed to record p-waves (compressional waves), or it can be a multi-component geophone designed to record p-waves and s-waves (shear waves). [13] Geophones require sufficiently strong coupling with the ground to record the true ground motion initiated by the seismic signal. [14] This is of considerable importance for higher frequency components of the seismic signals, which can be altered substantially with respect to their phase and amplitude due to poor coupling. [14] In the figure on the right, a geophone is shown; the conical spike on the geophone is dug into the ground for coupling. As is the case with hydrophones, geophones are often arranged in arrays as well to maximise the signal-to-noise ratio as well as to minimise the influence of surface waves on recorded data. [1]

Sampling interval and Nyquist criterion

The seismic signal that needs to be recorded by the receivers is inherently continuous and hence needs to be discretised. [15] The rate at which this continuous signal is discretised is referred to as the sampling interval or sampling rate (see Sampling (signal processing) for more details). According to the Nyquist criterion, the frequency with which the seismic signal needs to be sampled should be at least equal to or greater than twice the maximum frequency component of the signal i.e. fsample ≥ 2fmax,signal. [16] The challenge that remains is that the highest frequency component is usually not known during acquisition to be able to calculatedly determine the sampling rate. Therefore, estimates need to be made of the highest possible frequencies contained within the signal; usually, sampling rates higher than these estimates are preferred to ensure that temporal aliasing does not occur. [17]

Record length

Despite the term length, the record length refers to the time duration (typically listed in milliseconds) over which the receivers are active, recording and storing the seismic response of the subsurface. [1] This recording time should usually start slightly before the source is initiated to ensure that the direct waves are received as the first arrivals on the near-offset receivers. [2] Additionally, the record length should be long enough to ensure that the latest expected arrivals are recorded. [2] Typically, for deeper exploration surveys, the record length is adjusted to the order of multiple seconds (6 seconds is common). [1] [18] 15 to 20 seconds is common for deep crustal exploration. [18] Since the recorded traces can always be clipped for later arrivals during data processing, the record length is normally preferred longer than necessary rather than shorter. [2]

Related Research Articles

<span class="mw-page-title-main">Seismogram</span> Graph output by a seismograph

A seismogram is a graph output by a seismograph. It is a record of the ground motion at a measuring station as a function of time. Seismograms typically record motions in three cartesian axes, with the z axis perpendicular to the Earth's surface and the x- and y- axes parallel to the surface. The energy measured in a seismogram may result from an earthquake or from some other source, such as an explosion. Seismograms can record many things, and record many little waves, called microseisms. These tiny microseisms can be caused by heavy traffic near the seismograph, waves hitting a beach, the wind, and any number of other ordinary things that cause some shaking of the seismograph.

<span class="mw-page-title-main">Reflection seismology</span> Explore subsurface properties with seismology

Reflection seismology is a method of exploration geophysics that uses the principles of seismology to estimate the properties of the Earth's subsurface from reflected seismic waves. The method requires a controlled seismic source of energy, such as dynamite or Tovex blast, a specialized air gun or a seismic vibrator. Reflection seismology is similar to sonar and echolocation.

A geophone is a device that converts ground movement (velocity) into voltage, which may be recorded at a recording station. The deviation of this measured voltage from the base line is called the seismic response and is analyzed for structure of the earth.

<span class="mw-page-title-main">Amplitude versus offset</span> Relation between seismic amplitude and wave travel distance

In geophysics and reflection seismology, amplitude versus offset (AVO) or amplitude variation with offset is the general term for referring to the dependency of the seismic attribute, amplitude, with the distance between the source and receiver. AVO analysis is a technique that geophysicists can execute on seismic data to determine a rock's fluid content, porosity, density or seismic velocity, shear wave information, fluid indicators.

<span class="mw-page-title-main">Vertical seismic profile</span> Vibration measurement using boreholes

In geophysics, vertical seismic profile (VSP) is a technique of seismic measurements used for correlation with surface seismic data. The defining characteristic of a VSP is that either the energy source, or the detectors are in a borehole. In the most common type of VSP, hydrophones, or more often geophones or accelerometers, in the borehole record reflected seismic energy originating from a seismic source at the surface.

Nigel Allister Anstey, British geophysicist, has made major contributions to seismic exploration, which are the foundations for many of the techniques used in today's oil and gas exploration. Anstey's contributions impact every major area of seismic exploration -– from seismic acquisition to seismic processing to interpretation to research. He is the holder of over 50 multinational patents. He is best known by many geoscientists for distilling the geophysical concepts of the seismic method into non-mathematical teachings for seismic interpreters.

Geophysical survey is the systematic collection of geophysical data for spatial studies. Detection and analysis of the geophysical signals forms the core of Geophysical signal processing. The magnetic and gravitational fields emanating from the Earth's interior hold essential information concerning seismic activities and the internal structure. Hence, detection and analysis of the electric and Magnetic fields is very crucial. As the Electromagnetic and gravitational waves are multi-dimensional signals, all the 1-D transformation techniques can be extended for the analysis of these signals as well. Hence this article also discusses multi-dimensional signal processing techniques.

<span class="mw-page-title-main">Seismic source</span> Device that generates controlled seismic energy used for seismic surveys

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 migration is the process by which seismic events are geometrically re-located in either space or time to the location the event occurred in the subsurface rather than the location that it was recorded at the surface, thereby creating a more accurate image of the subsurface. This process is necessary to overcome the limitations of geophysical methods imposed by areas of complex geology, such as: faults, salt bodies, folding, etc.

Sonic logging is a well logging tool that provides a formation’s interval transit time, designated as , which is a measure of a how fast elastic seismic compressional and shear waves travel through the formations. Geologically, this capacity varies with many things including lithology and rock textures, most notably decreasing with an increasing effective porosity and increasing with an increasing effective confining stress. This means that a sonic log can be used to calculate the porosity, confining stress, or pore pressure of a formation if the seismic velocity of the rock matrix, , and pore fluid, , are known, which is very useful for hydrocarbon exploration.

ION Geophysical provides acquisition of equipment, software, planning and seismic processing services, and provides seismic data libraries to the global oil & gas industry. The company's technologies and services are used by E&P operators and seismic acquisition contractors to generate high-resolution images of the subsurface of Earth during exploration, exploitation and production operations. Headquartered in Houston & Texas, ION has offices in the United States, Canada, Latin America, Europe, Africa, Russia, China and the Middle East.

<span class="mw-page-title-main">Normal moveout</span>

In reflection seismology, normal moveout (NMO) describes the effect that the distance between a seismic source and a receiver has on the arrival time of a reflection in the form of an increase of time with offset. The relationship between arrival time and offset is hyperbolic and it is the principal criterion that a geophysicist uses to decide whether an event is a reflection or not. It is distinguished from dip moveout (DMO), the systematic change in arrival time due to a dipping layer.

The seismoelectrical method is based on the generation of electromagnetic fields in soils and rocks by seismic waves. This technique is still under development and in the future it may have applications like detecting and characterizing fluids in the underground by their electrical properties, among others, usually related to fluids.

In geophysics, seismic inversion is the process of transforming seismic reflection data into a quantitative rock-property description of a reservoir. Seismic inversion may be pre- or post-stack, deterministic, random or geostatistical; it typically includes other reservoir measurements such as well logs and cores.

A synthetic seismogram is the result of forward modelling the seismic response of an input earth model, which is defined in terms of 1D, 2D or 3D variations in physical properties. In hydrocarbon exploration this is used to provide a 'tie' between changes in rock properties in a borehole and seismic reflection data at the same location. It can also be used either to test possible interpretation models for 2D and 3D seismic data or to model the response of the predicted geology as an aid to planning a seismic reflection survey. In the processing of wide-angle reflection and refraction (WARR) data, synthetic seismograms are used to further constrain the results of seismic tomography. In earthquake seismology, synthetic seismograms are used either to match the predicted effects of a particular earthquake source fault model with observed seismometer records or to help constrain the Earth's velocity structure. Synthetic seismograms are generated using specialized geophysical software.

<span class="mw-page-title-main">Seismic interferometry</span>

Interferometry examines the general interference phenomena between pairs of signals in order to gain useful information about the subsurface. Seismic interferometry (SI) utilizes the crosscorrelation of signal pairs to reconstruct the impulse response of a given media. Papers by Keiiti Aki (1957), Géza Kunetz, and Jon Claerbout (1968) helped develop the technique for seismic applications and provided the framework upon which modern theory is based.

<span class="mw-page-title-main">Surface wave inversion</span>

Seismic inversion involves the set of methods which seismologists use to infer properties through physical measurements. Surface-wave inversion is the method by which elastic properties, density, and thickness of layers in the subsurface are obtained through analysis of surface-wave dispersion. The entire inversion process requires the gathering of seismic data, the creation of dispersion curves, and finally the inference of subsurface properties.

In reflection seismology, the anelastic attenuation factor, often expressed as seismic quality factor or Q, quantifies the effects of anelastic attenuation on the seismic wavelet caused by fluid movement and grain boundary friction. As a seismic wave propagates through a medium, the elastic energy associated with the wave is gradually absorbed by the medium, eventually ending up as heat energy. This is known as absorption and will eventually cause the total disappearance of the seismic wave.

Semblance analysis is a process used in the refinement and study of seismic data. The use of this technique along with other methods makes it possible to greatly increase the resolution of the data despite the presence of background noise. The new data received following the semblance analysis is usually easier to interpret when trying to deduce the underground structure of an area. Weighted semblance can be used for increasing the resolution of traditional semblance or make traditional semblance capable of analyzing more complicated seismic data.

Multidimensional seismic data processing forms a major component of seismic profiling, a technique used in geophysical exploration. The technique itself has various applications, including mapping ocean floors, determining the structure of sediments, mapping subsurface currents and hydrocarbon exploration. Since geophysical data obtained in such techniques is a function of both space and time, multidimensional signal processing techniques may be better suited for processing such data.

References

  1. 1 2 3 4 5 6 7 8 Yilmaz, Öz (2001). Seismic data analysis : processing, inversion, and interpretation of seismic data (2nd ed.). Society of Exploration Geophysicists. ISBN   978-1-56080-094-1.
  2. 1 2 3 4 Stone, Dale (1994). Designing surveys in two and three dimensions. Society of Exploration Geophysicists. ISBN   978-1560800736.
  3. ↑ SCHLUMBERGER. "seismic trace - Schlumberger Oilfield Glossary". www.glossary.oilfield.slb.com.
  4. ↑ Keary, Philip (2013). An Introduction to Geophysical Exploration (3., Auflage ed.). John Wiley & Sons. ISBN   9781118698938.
  5. ↑ SEG wiki. "Dictionary:Vibroseis or vibroseis - SEG Wiki". wiki.seg.org. Retrieved 16 July 2020.
  6. ↑ KIT. "Basic Geophysics: Landseismics - Vibroseis". youtube.com. Retrieved 17 July 2020.
  7. 1 2 3 SEG wiki. "Acquisition - SEG Wiki". wiki.seg.org. Retrieved 16 July 2020.
  8. ↑ USGS. "A Hammer-Impact, Aluminum, Shear-Wave Seismic Source" (PDF). pubs.usgs.gov. Retrieved 16 July 2020.
  9. 1 2 3 4 GEO ExPro (1 January 2010). "Marine Seismic Sources Part I". GEO ExPro. Retrieved 16 July 2020.
  10. ↑ Weilgart, Lindy (2013). A Review of the Impacts of Seismic Airgun Surveys on Marine Life (PDF). Retrieved 16 July 2020.
  11. ↑ AZoSensors (20 June 2012). "What is a Hydrophone?". AZoSensors.com. Retrieved 16 July 2020.
  12. ↑ Pamukcu, Sibel; Cheng, Liang (2017). Underground Sensing: Monitoring and Hazard Detection for Environment and Infrastructure. p. 190. ISBN   9780128031391.
  13. ↑ SCHLUMBERGER. "multicomponent seismic data - Schlumberger Oilfield Glossary". www.glossary.oilfield.slb.com. Retrieved 17 July 2020.
  14. 1 2 Krohn, Christine E. (June 1984). "Geophone ground coupling". Geophysics. 49 (6): 722–731. Bibcode:1984Geop...49..722K. doi:10.1190/1.1441700.
  15. ↑ SEG wiki. "Geophysical signal processing - SEG Wiki". wiki.seg.org. Retrieved 17 July 2020.
  16. ↑ Smyth, Tamara. "Nyquist Sampling Theorem". musicweb.ucsd.edu. Retrieved 17 July 2020.
  17. ↑ Herres, David. "Temporal and spatial aliasing in signal processing". www.testandmeasurementtips.com. Retrieved 17 July 2020.
  18. 1 2 xsgeo. "SEISMIC ACQUISITION". www.xsgeo.com. Retrieved 17 July 2020.
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