REFLECTION SEISMOLOGY

Seisma Energy Research, AVV (formerly Seisma Oil Research, LLC) presents this article as part of a series of articles on understanding the energy business. We hope you enjoy this series.

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/Tovex, a specialized air gun or vibrators, commonly known by their trademark name Vibroseis. 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.

Reflection Experiments

A reflection experiment is carried out by initiating a seismic source (such as a dynamite explosion) 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.

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.

Surveying Land

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.

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.