Communications – electrical: acoustic wave systems and devices – Seismic prospecting – Offshore prospecting
Patent
1995-09-11
1997-09-16
Lobo, Ian J.
Communications, electrical: acoustic wave systems and devices
Seismic prospecting
Offshore prospecting
367127, 367129, 367130, G01V 138
Patent
active
056687753
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The invention concerns a method for determining the position of seismic equipment, especially seismic sources and seismic streamers, wherein the seismic equipment is adapted to transmit and receive acoustic signals in a seismic frequency range. The invention also concerns an application of the method.
Seismic surveys can be conducted at sea, on shore or in zones between sea and shore, e.g. in shallow bays, in swampy areas and the like. A common feature of the surveys is that a seismic signal is transmitted from a source suited to the purpose and this signal is reflected by the ground formation and proceeds to be intercepted by seismic sensors. The signals are transmitted to an appropriate receiver station, where these data are processed and stored, to be used finally for constructing structural maps of the rock formations. These maps facilitate the process of assessing the probability of the existence of oil or gas in the surveyed area.
In marine surveys where it is the seabed which has to be surveyed, a typical seismic tow will consist of one or more sources and one or more cables, also called streamers. The actual towing is performed by one or more vessels. The seismic equipment towed behind the vessels is usually submerged in the water, the sources at a depth of roughly 5 to 10 meters, and the streamers at roughly 5 to 30 meters. A streamer generally extends to a length of from a few hundred meters to several thousand meters, although a length of 3,000 meters is usual. Inside the streamer which has a diameter of approximately 6 centimeters there are located a large number of sensors, also called hydrophones. A source usually consists of several suitable sonic guns, e.g. airguns which are arranged in a row or in a group. This is also called a gun array. A gun array is normally from 10 to 30 meters in length. When airguns are used, the guns are filled with compressed air, this air being released at a given time, thereby forming the seismic pulse. This is also called a seismic shot or a shotpoint. It is this pulse which, after having been reflected, is intercepted by sensors in the seismic streamer. In a streamer of approximately 3,000 meters there can be from several hundred to over a thousand groups consisting of one or more sensors. This means that the groups are situated close to one another. The normal procedure is to record data concerning the ground formation every 25 meters, and with a density in the tow's width direction of from 50 to 200 meters. The signals received by the hydrophones are transmitted analogically or digitally to a receiver station via the streamer's system of transmission lines. In the receiver system the signals are processed in the desired manner. The processing of data is performed by means of a computer program and algorithms suited to the purpose.
One of the prerequisites for obtaining the best possible result from a seismic survey is to have a method for determining the position of the seismic equipment which can provide as correct an image of the equipment's positions as possible.
On the basis of the prior art in this area, reference can be made to several methods for positioning the seismic equipment. One known method is the use of magnetic compasses in order to determine position. Magnetic compasses are attached at regular intervals, e.g. every 400 meters, along the streamer, generally with slightly closer spacing in the first and last pads of the streamer. By noting the compass readings and processing these data a determination can be made of the streamer's orientation at the point where the compass is attached to the streamer. An estimate of the streamer position must be made for that pad of the streamer which lies between the compasses. This method has several drawbacks. The magnetic compasses attached to a streamer have been found to give some doubtful readings. They are expensive to purchase. Since they are attached to the outside of the streamer they are liable to fall off. Another problem is that the actual compasses may be wrongly cal
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Geco A.S.
Lobo Ian J.
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