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 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 (e.g., Mohorovičić, 1910). 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:
"As one who personally tried to introduce the method into general consulting practice, the senior writer can definitely recall many times when reflections were not even considered on a par with the divining rod, for at least that device had a background of tradition." 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 (GRC) as part of the oil company Amerada. In 1930, Karcher left GRC and helped to found Geophysical Service Incorporated (GSI). 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 Seismic waves 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 (or seismic) impedance, Z, is defined by the equation:
Z = v ρ {\displaystyle Z=v\rho \ } , where v is the seismic wave velocity and ρ (Greek rho) 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 (such as geophones, hydrophones, and DAS) 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 (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.
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