Seismic Data Acquisition
Seismic data originates with a source of impulsive acoustic energy near the surface of the ground. Dynamite, air guns, or vibrating plates are used on land or marine surveys. In addition, a falling weight is sometimes used for shallow land surveys and a sparker survey is sometimes used for shallow marine work. The acoustic energy passes through the earth in all directions. Some is reflected back to the surface by acoustic impedance barriers, as shown below. Here, the returned energy is sensed by geophones planted on the ground or floating near the surface of the water.

 


Geometry of seismic acquisition, 2-D survey (top) and 3-D survey (bottom)

Some reflected energy bounces back and forth more than once. These events are called ghosts if they occur in the near surface, and multiples if they come from deeper reflectors. Multiples and ghosts are a form of interference which is usually eliminated by suitable data processing.

Each geophone signal is recorded on digital magnetic tape or disc and presented as a wiggly trace of energy amplitude versus arrival time. Raw traces are seldom delivered as the final product. Considerable data processing is performed to correct for geometry, the filtering effect of the earth, and amplitude decay with depth.

In 2-D seismic, the source and geophones are located in a straight line, resulting in a seismic cross section. If the line cannot be straight due to topography, the data is processed to collect data in short approximations to straight lines.

For 3-D seismic, receivers and sources are set up in a pattern which allows simultaneous recording of many intersecting lines of data. These can be processed to provide a volumetric view of the subsurface.

4-D seismic is a term used to describe surveys taken on the same grid several years apart and are used to show changes in reservoir properties over time. These can only be due to changes in fluid content from production or injection. The results are used to evaluate production efficiency, the effectiveness of waterfloods, or monitor aquifer influx.

4-C seismic is a relatively new form of marine survey and refers to four component recording of the seismic signal. The components are the usual compressional or P-wave from a geophone, plus in-line and cross-line shear arrivals, as well as a compressional wave recorded on a hydrophone. The different response of the geophone and hydrophone to reverberations in the water allow specialized processing to remove interference.

Anywhere from 24 to several hundred surface points are recorded for each surface shot. The energy source is then moved a multiple or sub-multiple of the geophone spacing and the signal recorded again. If the movement is less than half the geophone spread length, then the same subsurface points will be recorded more than once, resulting in multi-fold coverage. The seismic traces from different surface layouts that fall at common depth points are collected and stacked together to improve signal to noise ratio. The improvement in data quality with increased coverage is shown below.


Multi-fold (stacked) seismic sections

The presentation of seismic sections has evolved over the years, from plain wiggly traces to variable intensity black and white or color displays. The color can represent signal amplitude, frequency content, or any other desired (and determinable) property of the seismic signal. 3-D seismic is often presented in color as vertical or horizontal slices, as isometric views, or as contour maps.


Seismic presentation options


Mapping, slicing, and 3-d isometric views of 3-D seismic data

Most log analysts are unfamiliar with these displays, making it difficult for them to communicate well with geophysical interpreters.
 

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