Influencing Factors Studies Of Crosshole Seismic Tomography Resolution And Its Application

Subsurface geological problems,including caves,abandoned goaf and fault geological anomaly zone,is a serious threat to the engineering construction and the people's life and property safety.Geophysical prospecting mainly adopts electrical prospecting and seismic exploration method to detect its position,range and depth,and provides the reference for the later management.The conventional seismic exploration is based on a uniform and smooth reflection interface.However,on the transverse direction,seismic has the inhomogeneity,so the abnormal area cannot be fully explored.Tomography can carry out inversion imaging by using information such as seismic traveltime and amplitude,so that the velocity distribution image in the detection range is obtained,whose resolution is higher than that of conventional seismic exploration.Meanwhile,it has the features of good reliability,intuitive image and a large amount of information.Tomography method includes borehole tomography,crosshole tomography and combined tomography.The crosshole tomography will excite and receive all signals in the wells,so its signal noise ratio and resolution are better than the borehole tomography,which makes it has obvious advantages in detecting subsurface geological problems.Thus,the crosshole tomography it is the main content of this paper.First,this paper outlines the mathematical theory for crosshole tomography imaging and the positive and negative selection algorithm.Using the shortest traveltime forward algorithm and SIRT algorithm,the forward numerical simulation,the paper analyzes the abnormalities and the wave field characteristics under abnormal conditions,including traveltime and amplitude of seismic preliminary wave.It also elaborates the basis of traveltime tomography and amplitude attenuation tomography.Then,through a large number of forward models,the paper focuses on three factors that influence the tomographic resolution from the observation system perspective,including the observation angle,ray density and source frequency.After the conduction of the traveltime tomography imaging and amplitude attenuation tomography imaging,the paper arrives at three following conclusions:(1)the viewing angle is bigger,tomography the result is more accurate,and the higher the resolution is.But when the viewing angle comes to a certain extent,it has little influence on the imaging results;(2)the denser the ray,the more abnormal regions through ray tomography,and the higher the resolution is.But the over-densified ray would enhance the effect very limitedly;(3)source the frequency directly affects the wave field of seismic wave,the higher the frequency,the small wavelength is,and the better the effect of traveltime tomography.But as for the amplitude attenuation imaging,when frequency reaches a certain level,it would enhance the effect very limitedly.In this paper,the crosshole tomography of traveltime of seismic preliminary wave is applied to the detection of the karst cave and the fault direction,and fine results are obtained.