Seismic methods are the most important geophysical survey technique used in the oil and gas exploration industry. The first field demonstration of seismic for oil and mineral exploration took place in the 1920s at Oklahoma in the United States of America. The method is particularly suited for mapping sedimentary rocks and is popular also for engineering and environmental applications where mapping near surface sedimentary layers to determine:
- depth to bedrock
- soil and weathered layers
- water table
- other subsurface horizons.
In seismic surveying, seismic waves are created when an energy source is used to send a low frequency acoustic pulse into the the Earth's crust to measure distances in terms of the time it takes an echo of the pulse to return to sensors. The energy is reflected or refracted at geological boundaries. By repeating the operation along a predetermined path the recorded data can be used to provide a continuous vertical profile of the geologic strata.
Refraction seismology is used in geotechnical studies where the mechanical characteristics of the subsurface layers and bedrock are important. Refraction wave fronts move horizontally along the sedimentary boundaries rather than reflected at them. In marine seismic operation refraction is primarily used to determine the base of the strata of large scale sedimentary basins. The horizontal distance between the source and the receive array can be of the order of tens to hundreds of kilometres. To achieve the best wide angle separation between source and receive systems two ship surveys are sometimes undertaken. Alternatively devices such as ocean bottom seismometers and sonobuoys fitted with radio transmitters can be used. At the other end of the scale small geotechnical studies with arrays of geophone sensors spaced over tens of metres is used to determine features such as the depth of the weathered layer for foundation studies or the extent of groundwater saturated strata.
The reflection method involves measureing the two-way travel time of seismic waves reflected to the surface from the interfaces of geologic layers. Like the refraction method there has to be an acoustic contrast between the layers in order to detect a measurable signal. Reflection is identical in principle to echo sounding used on ships, or to radar systems, except that separate source and receiver systems are required. It is the most common technique used in marine seismic exploration.
Marine seismic is used for a variety of investigations, including:
- resource (petroleum and mineral) assessment
- geotechnical investigations
- submarine hazard detection.
Hydrophone arrays called streamers are used as detectors and the hydrophones are grouped into so called channels which can vary from 1 to a thousand or more. For a petroleum seismic system, typical frequencies recorded are 8 to 250 Hz. In contrast systems using sparker sources instead of airguns operate at frequencies in the 100 Hz to 1 kHz range and are generally used on small vessels for intermediate depth and higher resolution purposes. They often use smaller receive arrays. Other sub-bottom profilers use even higher frequency piezo-electric transducers which operate in the 500 to 5Khz range. These will typically penetrate between 10 and 100 metres but obtain decametre resolution.
Commercial streamers can be over five kilometres in length and in the case of oil exploration, can be deployed behind the seismic vessel as 3D systems where eight or more streamers are deployed side-by-side. The signal sources are deployed as arrays of airguns which hang from floats metres below the sea surface immediately behind the survey vessel. They are fed by compressed air from shipboard compressors, are synchronised and typically fire at 37-50 metre intervals timed by Global Positioning System (GPS) controlled navigation systems.
The seismic signals are received by hydrophones in the streamer and digitised in electronic modules before being transmitted to the ship's computer controlled seismograph. Here they are recorded on tape or hard drive for further processing.
Geoscience Australia currently conducts regional multi-channel 2D marine seismic surveys via commercial contracts as well as the more geotechnical focussed seismic identified above, using short streamers around 24 to 48 channels in length. These surveys are conducted with either Geoscience Australia or hired equipment on research and chartered vessels. Large commercial surveys are used to gather regional basin data used to select petroleum acreage release and provide useful data for potential bidders. A recent example is Geoscience Australia's Offshore Energy Security Program targeting frontier and other potentially prospective areas in the Pacific and Indian oceans.
Smaller high resolution seismic studies are used for a variety of purposes and past surveys include:
- continental slope stability studies (for tsunami hazards) along the south east margin
- substrate classification in Tasmania and Western Australia
- identification of gas seeps and diagenic zones for petroleum exploration
- monitoring earthquake movement and tracking gas hydrate horizons.
Topic contact: firstname.lastname@example.org Last updated: November 23, 2010