Geophysics

Geophysics | Drilling

Geophysics surveys the physical characteristics of underground structures and the whole earth using indirect measuring techniques such as gravimetry, geomagnetism, seismology, resistivity, seismic reflection, and seismic refraction.

The main geophysics technique used to find underground reservoirs for natural gas storage—and to conduct oil and gas exploration—is seismic reflection. This exploration method creates a visualization of deep underground geological structures by analyzing echoes of seismic waves.

Seismic Reflection Methodology

Seismic reflection uses the reflection of waves on the interfaces between multiple geological levels. The waves propagate and are partially reflected at the edges of each rock layer, which have different reflection speeds and densities. The reflections are then picked up by receivers on the surface—either hydrophones on water, or geophones on land.

Generally, one or more vibrators are placed on the surface to create elastic waves. The back waves recorded are generally relatively complex. They include a signal directly related to the source of the waves, a signal related to the underground reflections, and a signal related to certain refractions.

The seismic wave’s return time is used to locate the position of this transition in space. Wave amplitude can provide information on certain physical parameters of the geological framework in contact. However, due to the rapid attenuation of high frequencies underground, the depth of investigation quickly decreases in relation to the center frequency of the source used.

The signal associated with the source generally includes compressional waves, shear waves, and surface waves. The last two types of waves are generally eliminated during data processing.

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Interpreting Data

Geologists and geophysicists interpret the data by following this distinctive feature along seismic lines. Interpreting a seismic profile is a delicate process because neither the nature of the surfaces reflecting the waves nor their exact depth is known at the onset. The data collected includes the round trip time delay (double time) of the waves. These times must then be converted into depths (e.g., that are viable for drilling). These conversions are difficult, because they depend on wave velocity in the various types of rocks traversed. These speeds are not always known, which means a number of hypotheses must be made.

Based on the seismic profiles obtained, specialists get an idea of the structure associated with the sedimentary layer. If this sedimentary layer has a porous reservoir and its local structure has the properties of an anticlinal fold, then it has good potential for containing or storing hydrocarbons.

Seismic surveys may provide an image of the underground structure and, in some cases, even give information about the nature and properties of certain sedimentary layers of economic interest. These technologies make it possible to draw three-dimensional maps of the subsurface at a resolution of within one-tenth of a meter. The cost for seismic studies is therefore largely offset by the decreased need for exploratory drilling and the increase in success rates.

Intragaz has tested these techniques for many years and has continually built on its know-how by maintaining a superior level of expertise, constantly improving its equipment, and actively collaborating with scientific research centers.

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