Reflection Seismology

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.

The three primary exploration environments for seismic exploration are land, the transition zone and marine (shallow and deep water).

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, security 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).

Seismic surveys may also have a positive impact by reducing the number of unsuccessful wells drilled while exploring for hydrocarbon deposits and by increasing the amount of hydrocarbons produced from existing wells.

Conceptual diagram of how ground vibrations stimulate increased production of oil and gas. The increase is transient but can be produced by surface vibroseis truck or vibration source in a borehole.

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 a specialized air gun. 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.

Depths are calculated from reflection records by timing the reflections for a mean receptor distance, and multiplying the time by one-half of the average velocity. This is true for nearly vertical incidence. For greater distances a “spread correction” is applied. Although reflection rays are curved, it is usually satisfactory to calculate depths on the basis of straight ray propagation.

Relative depth determination may be made by plotting travel time only. For absolute depth determinations the average velocity must be known. It may be determined by recording reflections from known depths, by shooting in wells, or by surveying a long reflection profile at the surface.

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 impedance. The seismic (or acoustic) impedance Z is defined by the equation Z= V ρ, where V is the seismic wave velocity and ρ (Greek rho) 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.

(Wikipedia. The free encyclopedia. http://www.wikipedia.org/)

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