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Reflection seismology

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Seismic reflection data

Reflection seismology (or seismic reflection) is a method of exploration geophysics that uses the principles of seismology to estimate the properties of the Earth's subsurface from reflectedseismic waves. The method requires a controlled seismic source of energy, such as dynamite/Tovex, a specialized air gun or vibrators, commonly known by their trademark name Vibroseis. Vibrators are large trucks that shake a vibrating pad through a known frequency band. By noting the time it takes for a reflection to arrive at a receiver, it is possible to estimate the depth of the feature that generated the reflection. In this way, reflection seismology is similar to sonar and echolocation.

Contents
[hide]

• 1 Outline of the method
• 1.1 Reflection experiments
• 1.2 Reflection and transmission
• 1.3 Interpretation of reflections
• 2 Applications
• 2.1 Hydrocarbon exploration
• 2.1.1 Land
• 2.1.2 Marine (streamer)
• 2.1.3 Marine (OBC)
• 2.2 Crustal studies
• 3 Environmental impact
• 4 History
• 5 See also
• 6 Further reading
• 7 References
• 8 External links

[edit] Outline of the method

Seismic waves are a form of elastic wave that travel in the Earth. Any medium that can support wave propagation may be described as having an impedance (see Acoustic impedance and Electromagnetic impedance. The seismic (or acoustic) impedance Z is defined by the equation

Z= V ρ,

where V is the seismic wave velocity and ρ (Greekrho) is the density of the rock. When a seismic wave encounters a boundary between two different materials with different impedances, some of the energy of the wave will be reflected off the boundary, while some of it will be transmitted through the boundary.

In common with other geophysical methods, reflection seismology may be seen as a type of inverse problem. That is, given a set of data collected by experimentation and the physical laws that apply to the experiment, the experimenter wishes to develop an abstract model of the physical system being studied. In the case of reflection seismology, the experimental data are recorded seismograms, and the desired result is a model of the structure and physical properties of the Earth's crust. In common with other types of inverse problems, the results obtained from reflection seismology are usually not unique (more than one model adequately fits the data) and may be sensitive to relatively small errors in data collection, processing, or analysis. For these reasons, great care must be taken when interpreting the results of a reflection seismic survey.

[edit] Reflection experiments

A reflection experiment is carried out by initiating a seismic source (such as a dynamiteexplosion) and recording the reflected waves using one or more seismometers. On land, the typical seismometer used in a reflection experiment is a small, portable instrument known as a geophone, which converts ground motion into an analog electrical signal. In water, hydrophones, which convert pressure changes into electrical signals, are used. As the seismometers detect the arrival of the seismic waves, the signals are converted to digital form and recorded; early systems recorded the analog signals directly onto magnetic tape, photographic film, or paper. The signals may then be displayed by a computer as seismograms for interpretation by a seismologist. Typically, the recorded signals are subjected to significant amounts of signal processing and various imaging processes before they are ready to be interpreted. In general, the more complex the geology of the area under study, the more sophisticated are the techniques required to perform the data processing. Modern reflection seismic surveys require large amounts of computer processing, often performed on supercomputers or on computer clusters.

[edit] Reflection and transmission

When a seismic wave encounters a boundary between two materials with different impedances, some of the energy in the wave will be reflected at the boundary, while some of the energy will continue through the boundary. The amplitude of the reflected wave is predicted by multiplying the amplitude of the incoming wave by the seismic reflection coefficientR, determined by the impedance contrast between the two materials.

For a wave that hits a boundary at normal incidence (head-on), the expression for the reflection coefficient is simply

,

where Z0 and Z1 are the impedance of the first and second medium, respectively.

Similarly, the amplitude of the incoming wave is multiplied by the transmission coefficient to predict the amplitude of the wave transmitted through the boundary. The formula for the normal-incidence transmission coefficient (the ratio of transmitted to incident pressure amplitudes) is

.

From this, it is easy to show that

.

By observing changes in the strength of reflectors, seismologists can infer changes in the seismic impedances. In turn, they use this information to infer changes in the properties of the rocks at the interface, such as density and elastic modulus.

For non-normal incidence (at an angle), a phenomenon known as mode conversion occurs. Longitudinal waves (P-waves) are converted to transverse waves (S-waves) and vice versa. The transmitted energy will be bent, or refracted, according to Snell's law. The expressions for the reflection and transmission coefficients are found by applying appropriate boundary conditions to the wave equation, a topic beyond the scope of this article. The resulting formulas, first determined at the beginning of the 20th century, are known as the Zoeppritz equations. The reflection and transmission coefficients govern the signal strength (amplitude) at each reflector. The coefficients at a given angle of incidence vary with (among many other things) the fluid content of the rock. Practical use of non-normal incidence phenomena, known as AVO (amplitude versus offset) has been facilitated by theoretical work to derive workable approximations to the Zoeppritz equations, and by advances in computer processing capacity. AVO studies attempt with some success to predict the fluid content (oil, gas, or water) of potential reservoirs, to lower the risk of drilling unproductive wells and to identify new petroleum reservoirs.

[edit] Interpretation of reflections

The time it takes for a reflection from a particular boundary to arrive at the geophone is called the travel time. If the seismic wave velocity in the rock is known, then the travel time may be used to estimate the depth to the reflector. For a simple vertically traveling wave, the travel time t from the surface to the reflector and back is called the Two-Way Time (TWT) and is given by the formula

,

where d is the depth of the reflector and V is the wave velocity in the rock.

A series of apparently related reflections on several seismograms is often referred to as a reflection event. By correlating reflection events, a seismologist can create an estimated cross-section of the geologic structure that generated the reflections. Interpretation of large surveys is usually performed with programs using high-end three dimensional computer graphics.

[edit] Applications

Reflection seismology is extensively used in exploration for hydrocarbons (i.e., petroleum , natural gas) and such other resources as coal, ores, minerals, and geothermal energy. Reflection seismology is also used for basic research into the nature and origin of the rocks making up the Earth's crust. Reflection Seismology is also used in shallow application for engineering, groundwater and environmental surveying. A method similar to reflection seismology which uses electromagnetic instead of elastic waves is known as Ground-penetrating radar or GPR. GPR is widely used for mapping shallow subsurface (up to a few meters deep).

[edit] Hydrocarbon exploration

Reflection seismology, or 'seismic' as it is more commonly referred to by the oil industry, is used to map the subsurface structure of rock formations. Seismic technology is used by geologists and geophysicists who interpret the data to map structural traps that could potentially contain hydrocarbons. Seismic exploration is the primary method of exploring for hydrocarbon deposits, on land, under the sea and in the transition zone (the interface area between the sea and land). Although the technology of exploration activities has improved exponentially in the past 20 years, the basic principles for acquiring seismic data have remained the same.

In simple terms and for all of the exploration environments, the general principle is to send sound energy waves (using an energy source like dynamite or Vibroseis) into the Earth, where the different layers within the Earth's crust reflect back this energy. These reflected energy waves are recorded over a predetermined time period (called the record length) by using hydrophones in water and geophones on land. The reflected signals are output onto a storage medium, which is usually magnetic tape. The general principle is similar to recording voice data using a microphone onto a tape recorder for a set period of time. Once the data is recorded onto tape, it can then be processed using specialist software which will result in processed seismic profiles being produced. These profiles or data sets can then be interpreted for possible hydrocarbon reserves.

Naturally enough, the three primary exploration environments for seismic exploration are land, the transition zone and marine (shallow and deep water):

Land - The land environment is self explanatory, but can cover just about every type of terrain that exists on Earth (such as jungle, desert, arctic tundra, swamp, forest, urban settings, mountain regions and savannah).

Transition Zone (TZ) - The transition zone is considered to be the transition area between the land and sea and can present unique challenges depending on the location. This may involve setting source and receiver stations across river deltas, in swamps, across coral reefs, on beach tidal areas and in the surf zone. TZ crews often work on land, in the transition zone and in the shallow water marine environment on a single project.

Marine - The marine zone is either in shallow water areas (water depths of less than 30 to 40 metres would normally be considered shallow water areas for 3D marine seismic operations) or in the deep water areas normally associated with the seas and oceans (such as the Gulf of Mexico).

What parameters are used for each acquisition project depends on a significant number of variables specific to a particular area. For example, in the marine environment the choice of a tuned air gun array will depend on the known sub-sea geology, data from previous seismic surveys, the depth at which the main features of geological interest exist within the Earth, the desired frequency output of the source array, the amount of energy or power required and so on. For the land environment, the source choice is normally between drilled dynamite shot holes or mechanical vibrators. Again, the choice will depend on the specific geology and characteristics of the prospect area but can also be influenced by non geophysical issues, such as terrain, safety issues especially for explosive use and storage and local environmental concerns (such as working in protected areas, working close to buildings and structures or in national parks etc).

[edit] Land

Desert land seismic camp

Land crews tend to be quite large entities, employing anywhere from a few hundred to a few thousand people. They normally require substantial logistical support to cover not only the seismic operation itself, but also to support the main camp (for catering, waste management and disposal, camp accommodations, washing facilities, water supply, laundry etc), fly camps (temporary camps set up away from the main camp on large land seismic operations, for example where the distance is too far to drive back to the main camp with vibrator trucks), all of the crews vehicles (maintenance, fuel, spares etc), security, possible helicopter operations, restocking of the explosive magazine, medical support and many other logistical and support functions.

Outside of the camp personnel, the basic components of a seismic land crew are the surveyors, layout and loading crew, shooters and recorders and the pick up crew. The general principle is for the surveyors to survey in shot and receiver points on source and receiver lines (the latitude and longitude coordinates of which are pre-determined by the client / contractor) using mobile GPS stations. When a shot or receiver point is reached, this position will be staked out or marked with the shot or receiver station number and line number.

Once sufficient lines of shot and receiver points have been surveyed in and shot holes have been drilled to the appropriate depth, loaders put explosive charges into the shot holes on the source lines (according to the project specification) and the receiver stations will be laid out with geophone spreads on the receiver lines. When corresponding shot and receiver lines are ready, the shooters prepare a single shot hole ready for firing, whilst the recording shack will be hooked up to the geophone spread laid on the corresponding receiver line to record the reflected data. Once a charge is ready to be shot, the recording shack initiates the shot hole firing sequence via a radio link and records the seismic data from the whole geophone spread onto magnetic medium. Once a shot is completed, the shooters move to the next shot hole and the shoot / record sequence begins again.

Once lines have been shot, loaders continue to load shot holes on new source lines and the pick up crews pick up and relay geophone spreads onto new receiver lines as required in the acquisition plan. For vibrator crews, aka "Vibroseis" (vibrations are created by the computer-coordinated vibration of hydraulically controlled plates on vibrator trucks), the vibrator trucks move from shot hole to shot hole on the designated source line instead of the loaders and shooters.

Receiver line on a desert land crew with recorder truck

Land surveys require crews to deploy the hundreds or thousands of geophones necessary to record the data. Most surveys today are conducted by laying out a two-dimensional array of geophones together with a two-dimensional pattern of source points. This allows the interpreter to create a three-dimensional image of the geology beneath the array, so these are called 3D surveys. Less expensive survey methods use one-dimensional lines of geophones that only allowed the interpreter to make two-dimensional cross-sections.

[edit] Marine (streamer)

Seismic data collected by the USGS in the Gulf of Mexico

Deep water marine surveys are conducted using vessels capable of towing one or more seismic cables known as "streamers" (see figure). Modern 3D surveys use multiple streamers deployed in parallel, to record data suitable for the three-dimensional interpretation of the structures beneath the sea bed. A single vessel may tow anything up to 10+ streamers, each 6 km+ in length, spaced 50–150 m apart. Hydrophones are deployed at regular intervals within each streamer. These hydrophones are used to record sound signals which are reflected back from structures within the rock. To accurately calculate where subsurface features are located, navigators compute the position of both the sound source and each hydrophone group which records the signal. The positioning accuracy required is achieved using a combination of acoustic networks, compasses and GPS receivers (often used with a radio correction applied call a differential GPS or DGPS).

[edit] Marine (OBC)

Shallow water marine surveys are conducted using sensors attached to an Ocean Bottom Cable (OBC) laid out on the ocean bottom rather than in towed streamers. Due to operational limitations, these types of surveys can be conducted up to depths of about 70 meters. One operational advantage is that obstacles (such as platforms) do not affect the acquisition as much as they do for streamer surveys. Most of the OBC surveys use dual component receivers, combining a pressure sensor (hydrophone) and a vertical particle velocity sensor (vertical geophone). OBC surveys can also use 4 component, i.e. those 2 components plus the 2 horizontal velocity sensors. 4 component OBC surveys have the advantage of being able to record shear waves, which do not travel through water. Multiple component OBC surveys hence lead to improved imaging.

[edit] Crustal studies

The use of reflection seismology in studies of tectonics and the Earth's crust was pioneered by groups such as the Consortium for Continental Reflection Profiling (COCORP) [1],[2].

[edit] Environmental impact

As with all human activities, reflection seismic experiments may impact the Earth's natural environment. On land, conducting a seismic survey may require the building of roads in order to transport equipment and personnel. Even if roads are not required, vegetation may need to be cleared for the deployment of geophones. If the survey is in a relatively undeveloped area, significant habitat disturbance may result. Many land crews now use helicopters instead of land vehicles in remote areas. Most countries require that seismic surveys are conducted according to environmental standards established by government regulation. Higher environmental standards have encouraged the development of lower impact seismic vehicles and acquisition methodologies. Similarly modern seismic processing techniques allow seismic lines to deviate around natural obstacles, or use pre-existing non-straight tracks and trails with less loss of data quality than would once have been the case. The more recent use of inertial navigation instruments for land survey instead of theodolites decreased the impact of seismic by allowing the winding of survey lines between trees.