Method and device for velocity field inversion based on vsp travel time double weighted tomography

Abstract

The invention discloses a method and device for double-weighted tomography inversion velocity field based on VSP travel time, relates to the technical field of geophysical exploration, and can improve the accuracy and reliability of tomography inversion results. The method includes: collecting VSP seismic data in the region to obtain the initial velocity model of the region; establishing a regional credibility model according to the distance from the well and the depth from the surface at each position in the region; Observation parameters of the region, the initial velocity model of the region, and the reliability model are used to obtain the grid slowness disturbance value in the region; the velocity of the grid in the initial velocity model of the region is corrected by the grid slowness disturbance value in the region to obtain the new regional grid speed. The velocity model; based on the new velocity model, the first arrival theoretical travel time is obtained, and it is judged whether the difference between the first arrival theoretical travel time and the actual travel time satisfies a predetermined threshold, and if so, the tomographic inversion velocity field is output. The method effectively improves the accuracy and reliability of the tomographic inversion results.

Description

Technical field [0001] The invention relates to the technical field of geophysical prospecting, in particular to a method and a device for inverting a velocity field based on double-weighted tomography during VSP travel. Background technique [0002] So far, pre-stack depth migration in western China has not achieved significant results. The essential reason is that it is difficult to determine the velocity field of the underground medium. In this type of region, it is difficult to determine the existence of many factors in the velocity field, including: In western China, the thickness of the arid mountain low velocity zone is large, reaching 500-1200 meters; ordinary small refraction and micro-logging methods can only be reversed. The velocity field in the shallower part of the performance area cannot be inverted at all. The method of using deep wells to find the surface velocity field is too costly, and the velocity field obtained after such operations is not accurate enough; o...

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Problems solved by technology

The inventors of the present application found that the above-mentioned algorithms all distribute the travel time error according to the length of the ray passing through the unit grid. The travel time error is to find a ray from the firing point on the ground to reach a certain point in the VSP well, and according to the velocity of the iterative velocity field, Calculate its travel time, and then obtain the error time with the actual VSP direct wave arrival time. Although the distribution of the travel time error according to the ray length can eventually obtain the underground medium velocity in the far distance of the well more accurately, the distribution of the travel time error according to the ray length will instead The obtained subsurface medium velocity near the well is becoming more and more inaccurate. In this way, the inaccuracy of the subsurface medium velocity beside the well cannot meet the demand, which leads to the accuracy and reliability of the tomographic inversion results based on the subsurface medium velocity. sexual insufficiency

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