Apparatus and method for carrying out seismic surveys
View Patent ↗A 4D seismic source comprising a series of sub-arrays of guns on flotation devices. Each flotation device has a GPS device. For firing, the guns in the e.g. three sub-arrays identified as being closest to the desired firing location are selected.
1. A method of conducting a seismic survey at a predetermined desired position, the method comprising:
towing a seismic source comprising an excess of individual seismic generation elements behind a marine vessel;
deploying the individual seismic generation elements as a series of equally spaced generally linear sub-arrays over an area including the said predetermined desired position;
determining the position of each sub-array using a positioning determining device associated with each sub-array;
comparing the determined positions of each sub-array with the said predetermined desired position;
selecting a sub-array most closely located to the said predetermined desired position; and
generating a desired signal using only the seismic generation elements of the selected sub-array and the nearest sub-array to the selected sub-array, while not selecting the seismic generation elements from the disregarded sub-arrays so as to minimize lateral error.
2. A method according to claim 1 , wherein each sub-array comprises a flotation device on which the generation elements are mounted.
3. A method according to claim 1 , wherein the sub-arrays are towed at a depth of from 2 to 20 meters.
4. A method according to claim 1 , wherein the sub-arrays are spaced apart by a distance in the range of from 5 to 20 meters.
5. A method according to claim 1 , wherein two sub-arrays which are centered most closely on the selected sub-array are selected and used to generate the signal in cooperation with the selected sub-array while more distant sub-arrays are disregarded and are not used to generate the seismic event.
6. A method according to claim 1 , wherein each sub-array has the capability to create a plurality of seismic events spaced along a line.
7. A method according to claim 6 , wherein the several seismic events are generated along a first series of parallel lines.
8. A method according to claim 7 , wherein the several seismic events are generated along a second series of parallel lines which cross the first series.
9. A method according to claim 1 , wherein the seismic survey is repeated after the elapse of a period of time.
10. A method according to claim 1 wherein the position determining device is a GPS device.
11. A method according to claim 1 wherein the desired signal is created by a simultaneous firing of all of the selected seismic generation elements.
12. A method according to claim 5 wherein the desired signal is created by a delayed firing pattern that includes independently firing the selected sub-array and independently firing the two nearest sub-arrays within the same pass over the desired source position.