Application Of Crosswell Seismic Tomography In Detecting Buried Fault

The common methods in detecting Buried active Fault are Shallow Seismic Exploration and Drilling Profiles,it means that, first we use Shallow Seismic Exploration to detect target fault and confirm the depth of the Up-breakpoint, then we use Drilling Profiles and Dating of Stratigraphic samples to determine the activity of this fault. However, because of surface wave and Quaternary unconsolidated sediment,we don't receive availably information of Reflection in shallow stratum. We always use Drilling Profiles to ascertain the configuration of this fault,it increases the cost of detection and decreases its efficiency,also it exists a lot of problems in many fields.In order to solve the problem in detecting Buried active Fault,this paper attempts to make use of Crosswell Seismic Tomography to receive P wave Speed Chart of shallow active-fault and determine the depth of Up-breakpoint.Actually,based on the profiles of Shallow Seismic Exploration,we make two holes near fault plane and lay epicenter and hydrophone in different hole,then we explode wave in one hole and we receive signal in another hole.We use the travel time of first break wave to invert P wave Speed Chart near target fault.This paper discusses Crosswell Seismic Tomography and introduces the application status of the Crosswell Seismic technology at home and abroad. Currently, the method of Crosswell Seismic Tomography contains Ray Tomography and Wave equation Tomography.This paper will adopt Ray Tomography to invert P wave Speed Chart. We test in Luhuatai active-fault in Yinchuan rift valley and get field data, in course of data processing, first we delete waste track and bad track and edit observation system, then get the travel time of first break wave by hand and select an initial model to caculate, invert P wave Speed Chart near fault plane.In the process of invertsion,we select homogeneous model(mean velocity is 1650m/s),layered model and different grid(1m×1m,1m×2m,0.5m×1m,0.5m×0.5m,O.5m×O.25m,0.25m×0.5m and 0.25m×0.25m).It shows that the result of homogeneous model is better than the effect of layered model and defined model;Also different grid little affects the result.This paper will adopt Curved-Ray tracing algorithm for invertsion,that is to say,first we assume original path, then based on guidelines for minimum travel time,we correct the path continuously and get the travel time and the real path in receiving point.The method of invertsion is genetic algorithm,it solves target function immediately and it does not need future knowledge or auxiliary information,it will search model groups and figure out large-scale,multivariate non-linear inversion problem.With several experiments,this paper adopts homogeneous model(mean velocity is 1650m/s),0.25m×0.25m grid size and 6 iteration,this result is the best consequence.However,because of the rupture of the same speed stratum,we deduce the position of fault plane and the character of this fault,also,with the continuous stratum, the depth of the Up-breakpoint is about 7m.The result is highly similar to drilling profile.Crosswell Seismic Tomography will provide another reference for detection of Buried active Fault.