Reflection 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.
History
Reflections and refractions of seismic waves at geologic interfaces within the Earth were first observed on recordings of earthquake-generated seismic waves. The basic model of the Earth's deep interior is based on observations of earthquake-generated seismic waves transmitted through the Earth's interior. The use of human-generated seismic waves to map in detail the geology of the upper few kilometers of the Earth's crust followed shortly thereafter and has developed mainly due to commercial enterprise, particularly the petroleum industry.Seismic reflection exploration grew out of the seismic refraction exploration method, which was used to find oil associated with salt domes. Ludger Mintrop, a German mine surveyor, devised a mechanical seismograph in 1914 that he successfully used to detect salt domes in Germany. He applied for a German patent in 1919 that was issued in 1926. In 1921 he founded the company Seismos, which was hired to conduct seismic exploration in Texas and Mexico, resulting in the first commercial discovery of oil using the refraction seismic method in 1924. The 1924 discovery of the Orchard salt dome in Texas led to a boom in seismic refraction exploration along the Gulf Coast, but by 1930 the method had led to the discovery of most of the shallow Louann Salt domes, and the refraction seismic method faded.
After WWI, those involved in the development of commercial applications of seismic waves included Mintrop, Reginald Fessenden, John Clarence Karcher, E. A. Eckhardt, William P. Haseman, and Burton McCollum. In 1920, Haseman, Karcher, Eckhardt and McCollum founded the Geological Engineering Company. In June 1921, Karcher, Haseman, I. Perrine and W. C. Kite recorded the first exploration reflection seismograph near Oklahoma City, Oklahoma.
Early reflection seismology was viewed with skepticism by many in the oil industry. An early advocate of the method commented:
The Geological Engineering Company folded due to a drop in the price of oil. In 1925, oil prices had rebounded, and Karcher helped to form Geophysical Research Corporation as part of the oil company Amerada. In 1930, Karcher left GRC and helped to found Geophysical Service Incorporated. GSI was one of the most successful seismic contracting companies for over 50 years and was the parent of an even more successful company, Texas Instruments. Early GSI employee Henry Salvatori left that company in 1933 to found another major seismic contractor, Western Geophysical. Many other companies using reflection seismology in hydrocarbon exploration, hydrology, engineering studies, and other applications have been formed since the method was first invented. Major service companies in recent years have included CGG, ION Geophysical, Petroleum Geo-Services, Polarcus, TGS and WesternGeco, but since the oil price crash of 2015, providers of seismic services have continued to struggle financially such as Polarcus, whilst companies that were seismic acquisition industry leaders just ten years ago such as CGG and WesternGeco have now removed themselves from the seismic acquisition environment entirely and restructured to focus upon their existing seismic data libraries, seismic data management and non-seismic related oilfield services.
Summary of the method
s are mechanical perturbations that travel in the Earth at a speed governed by the acoustic impedance of the medium in which they are travelling. The acoustic impedance, Z, is defined by the equation:where v is the seismic wave velocity and ρ is the density of the rock.
When a seismic wave travelling through the Earth encounters an interface between two materials with different acoustic impedances, some of the wave energy will reflect off the interface and some will refract through the interface. At its most basic, the seismic reflection technique consists of generating seismic waves and measuring the time taken for the waves to travel from the source, reflect off an interface and be detected by an array of receivers at the surface. Knowing the travel times from the source to various receivers, and the velocity of the seismic waves, a geophysicist then attempts to reconstruct the pathways of the waves in order to build up an image of the subsurface.
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 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.
The reflection experiment
The general principle of seismic reflection is to send elastic waves into the Earth, where each layer within the Earth reflects a portion of the wave's energy back and allows the rest to refract through. These reflected energy waves are recorded over a predetermined time period by receivers that detect the motion of the ground in which they are placed. On land, the typical receiver used is a small, portable instrument known as a geophone, which converts ground motion into an analogue electrical signal. In water, hydrophones are used, which convert pressure changes into electrical signals. Each receiver's response to a single shot is known as a “trace” and is recorded onto a data storage device, then the shot location is moved along and the process is repeated. Typically, the recorded signals are subjected to significant amounts of signal processing.Reflection and transmission at normal incidence
When a seismic P-wave encounters a boundary between two materials with different acoustic impedances, some of the energy in the wave will be reflected at the boundary, while some of the energy will be transmitted through the boundary. The amplitude of the reflected wave is predicted by multiplying the amplitude of the incident wave by the seismic reflection coefficient, determined by the impedance contrast between the two materials.For a wave that hits a boundary at normal incidence, the expression for the reflection coefficient is simply
where and are the impedance of the first and second medium, respectively.
Similarly, the amplitude of the incident 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 is
As the sum of the energies of the reflected and transmitted wave has to be equal to the energy of the incident wave, it is easy to show that
By observing changes in the strength of reflections, 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 wave velocity, by means of seismic inversion.
Reflection and transmission at non-normal incidence
The situation becomes much more complicated in the case of non-normal incidence, due to mode conversion between P-waves and S-waves, and is described by the Zoeppritz equations. In 1919, Karl Zoeppritz derived 4 equations that determine the amplitudes of reflected and refracted waves at a planar interface for an incident P-wave as a function of the angle of incidence and six independent elastic parameters. These equations have 4 unknowns and can be solved but they do not give an intuitive understanding for how the reflection amplitudes vary with the rock properties involved.The reflection and transmission coefficients, which govern the amplitude of each reflection, vary with angle of incidence and can be used to obtain information about the fluid content of the rock. Practical use of non-normal incidence phenomena, known as AVO 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 of potential reservoirs, to lower the risk of drilling unproductive wells and to identify new petroleum reservoirs. The 3-term simplification of the Zoeppritz equations that is most commonly used was developed in 1985 and is known as the "Shuey equation". A further 2-term simplification is known as the "Shuey approximation", is valid for angles of incidence less than 30 degrees and is given below:
where = reflection coefficient at zero-offset ; = AVO gradient, describing reflection behaviour at intermediate offsets and = angle of incidence. This equation reduces to that of normal incidence at =0.
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 from the surface to the reflector and back is called the Two-Way Time and is given by the formulawhere is the depth of the reflector and 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.