7 - Geophysical Methods

7 - Geophysical Methods - PETE 3053 – SUBSURFACE ASPECTS...

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Unformatted text preview: PETE 3053 – SUBSURFACE ASPECTS OF PETE 3053 – SUBSURFACE ASPECTS OF PETROLEUM ENGINEERING Geophysical Prospecting ­ 7 Geophysical prospecting is the art of searching for concealed deposits of oil and gas by physical measurements from the earth’s surface Methods Methods 1) Seismic Reflection – with this technique, the structure of subsurface formations is mapped by making use of the times required for a seismic wave (or pulse), generated in the earth by a near­surface source (explosion or vibration), to return to the surface after reflection from the formations themselves. The reflections are recorded by detecting instruments responsive to ground motion, which are laid along the ground near the shot hole. Methods Methods 2) Seismic Refraction – In refraction shooting, the detecting instruments are laid down at a distance from the shot hole that is large compared to the depth of the horizon to be mapped. The source waves travel large horizontal distances through the earth and the time required for travel gives information on the velocity and depth of certain subsurface formations. This technique is not as accurate for the structure as reflection, but it can give an indication of the stratigraphy. Methods Methods Gravity – In gravity prospecting, one measures minute variations in the pull of gravity from rocks (torsion balance, pendulum, gravity meter). Different types of rocks have different densities and the denser rocks have a greater gravitational pull. If the denser rocks are arched upward in a structural high, such as an anticline, the earth’s gravitational field will be greater over the axis of the structure than along its flanks. A salt dome, on the other hand, which is less dense than the rocks into which it has intruded, may be detected from the low value of gravity. Methods Methods Magnetic – Magnetic prospecting measures variations in the magnetic field of the earth which are attributable to changes of structure or magnetic susceptibility in certain near­surface rocks. Sedimentary rocks generally have a small susceptibility compared with igneous or metamorphic rocks. This is useful in oil and gas exploration in sedimentary layers controlled by topographic features, such as ridges or faults, on the basement surface. Some Seismic Terms Some Seismic Terms Seismic Program – the arrangement of locations for a seismic investigation Seismic crew, seismograph crew or doodlebug crew – people who do the work in the field (individuals are called doodlebuggers) Surveying – locating the positions on the ground and determining their elevation Shooting – producing sounds (seismic energy, seismic waves, energy) Energy Sources – explosives, vibrators, weight drop, air gun Some Seismic Terms Some Seismic Terms Seismic Survey – the shooting of an area Shot hole – a hole in which an explosion is fired Shot point – the geographical location of a shot hole Seismic line – a line of continuous shot points Sound detectors – geophones, seismometers, detectors (field terms – jugs, seises, pickups, phones) Seismic section – vertical cross section generated from seismic data (record section, vertical section or just section)…made up of alignments of data called traces. A trace may be a wiggly line, a lineup of light and dark patches or some other form (color!) Some Seismic Terms Some Seismic Terms Cable – electrical cable connecting geophones to the recording instruments (streamer – cable used at sea) ...
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