Method for obtaining seismic data.

p0001The invention relates to a method for recording seismic data of a central structure having the area according to the preamble of claim 1.

p0002The detection Marine-seismic data is now essentially by towed behind ships Streamer. The vessel is preferably performed in parallel courses of the area to be detected. At each end of a journey, the vessel is then led around in a sheet and carried out at a distance parallel to the previously ridden course in the opposite direction of the area to be detected.

p0003Reversing at the ends of course be traversed tracks is time-consuming and thus kostenverursachend without data can be collected during the reverse timing.

p0004To avoid the reverse-way pattern different ways have been proposed, which shorten the reverse routes. From US-PS 4,486,863 is known a method in which a streamer is guided over the area to be detected in each circles along a progress line offset. The tugboat then travels from circular courses, the Streamer relatively accurately follows the wacky course. To move from one county to the next, leaving the ship as soon as his course to progress towards the moves parallel from vehicles approaching circle, the circle sensed tangentially and then moves tangentially on the next loop, which is then in turn traversed completely, etc.

p0005The method shown is intended only for the detection of a large connected region and allows the avoidance of inversion routes, which do not contribute to the data acquisition.

p0006The illustrated "surface seismic" is no longer readily usable when obstacles such as islands prevent the detection of a contiguous area. Just the immediate environment to a very small island, however, can be of particular importance for seismic surveys.

p0007Starting from this prior art, the invention is therefore based on the object of specifying a method for recording seismic data of an area which has a central structure, one of the structure adapted data acquisition and a simple answer Tung the acquired data allows and in a simple way can be carried out, without that would not be to depart to the desired volume of data contributing course routes.

p0008This object is achieved by the invention defined in claim 1. Advantageous developments of the invention are specified in dependent claims.

p0009The invention is particularly suitable for seismic data acquisition to central structures of an object, such. As small islands, underground salt present finger or similar punctate structures. Data acquisition is essentially polar by the shutdown of a spiral course, so that can be obtained from a 3D recording, starting from the central structure, vertical sections in any desired azimuthal direction. In this manner, a high data density for the central structure are obtained which could not be achieved in other ways.

p0010In a preferred embodiment of the invention, a spiral-shaped course is selected in which the radial distance between the spiral lines according to an Archimedean spiral is constant.

p0011The invention is illustrated below with reference to an embodiment.

p0012The figure shows a central structure in a sea area, z. B. an island whose edge structure to be detected.

p0013In certain sea areas occur structures, contact the z. B. as small islands in appearance or protruding like fingers from deeper underground layers. An areal detection with the aid of customary Streamertechnik is only possible if the structures ends below the water surface. From a thus manufactured 3D data volume even vertical sections can be created as radial sections starting from the structure, however, a considerable computing effort is necessary for this. When the structure emerges as an island from the water, sufficiently accurate detection of the flanks of the structure is not readily possible.

p0014In the novel process, a spiral track is selected for the streamers trailing ship. This allows the ship to be carried out from a starting to an end point over a large area without reversing courses must be driven.

p0015Since the inventive method is based on a data acquisition during a revolution around a structure, it is to be used in particular for central structures with advantage.

p0016To avoid stretching and reverse adjustment ranges, such as are necessary with a circular acquisition, a spiral-shaped course is selected according to the invention, the spiral is an Archimedian spiral in a particular embodiment of the invention. This is characterized in that the distance between adjacent helical lines over the entire periphery is constant. Considering a radial line, starting from the structure, so the intersection of the radial line with the spiral lines are distributed equidistantly. This distribution is valid for all radial lines.

p0017The use of an Archimedean spiral has the particular advantage that due to the constant spacing of the spiral lines along a radial line immediately the sampling rate results in cross-heading. It follows that the final 3D analysis of the acquired data in a relatively simple manner, horizontal cuts in any azimuthal direction possible. This is of particular importance when the edges of the structure, in particular in the transition from the vertical to the horizontal (in particular with respect to the Anschlepppunkte) are to be investigated. The used spiral line is relative to the being dragged Horizonte is a continuously registered in the omission of these horizons profile. Therefore, the corresponding stack speeds are no longer tilt sensitive for being dragged Horizonte.

p0018The arc length of the spiral determines the number of dots during a revolution of the streamer to the structure.

p0019The figure shows the principle of the invention. is on a central structure 1, z. B. an island, placed a spiral line 2, whose origin 3 is located approximately in the center of the structure. Since an Archimedean spiral is used, the distances of the individual spiral line segments, for. Example, between the portions 7, 8 and 9 are equidistant. The radial line 6 thus intersects the spiral line at equal intervals 5 to the end point 4 of the spiral line. 2

p0020The size of the structure 1 requires that a seismic acquisition by means of a towed streamer only on the spiral line sections 7, 8 and 9 is possible.

p0021Preferably, the distance of the spiral line is selected to be less than 50 m in order to enable a sufficiently dense data acquisition. The final radius of the spiral line depends on the type of the detected structure, the depth to be detected and the radial spread of the structure in the underground.

p0022The inventive method enables fast, accurate, particularly meaningful acquisition of seismic data to a central structure around.

LIST OF REFERENCE NUMBERS

p0023<ul><li>1 structure</li><li>2 spiral</li><li>3 center</li><li>4 endpoint</li><li>5 distance</li><li>6 radial line</li><li>7 spiral section</li><li>8 spiral section</li><li>9 spiral section</li></ul>