Study On Grotto Resistivity Measurement And Application Of Time-lapse Electrical Resistivity Inversion

Grotto cultural relics are important material cultural heritage in China,which have important research significance for the study of Chinese history,religion,and culture.However,influenced by natural environment and human factors,many grotto cultural relics are currently facing diseases,such as fractures and water seepage.The grotto protection is an important and urgent issue at present.The grotto rock structure detection is the prerequisite of the grotto protection.Electrical resistivity tomography(ERT)has been widely used to detect the geological structure of grotto cultural relics because of its stable detection results and non-destructive detection process.However,there are two challenges in grotto structure detection using ERT:(1)the weak electric field anomalies generated by fractures inside grotto are usually difficult to detect;(2)the traditional ERT measurement schemes can hardly meet the requirements of high precision and high resolution in the cliff region.Therefore,a new ERT measurement scheme needs to be designed to improve the resolution of grotto rock structure detection.This paper firstly combines the 2.5-dimensional electrical field finite element method to calculate and analyze the electrical response characteristics of fracture with different water content and depth to study the feasibility of fracture electrical field detection.The results show that the potential difference decreases with the increase of water content of the fracture.When the water content of the fracture reaches a certain level,the potential difference basically keeps stable and does not change.For fractures with different depth,the potential difference will change toward the dipole potential difference with the increase of fracture depth until it keeps the same with the dipole potential difference.Based on the forward simulation,this study implements the Gauss-Newton resistivity inversion with L2-norm smooth regularization,total variation(TV)regularization and L2-TV phased regularization.After that,numerical experiments are carried out to compare the imaging effect of three constraint method.It is shown that Gauss-Newton inversion with L2-TV phased regularization can improve the stability of the inversion and anomaly boundary inscription,which provides a stable initial model and reference model for time-lapse resistivity inversion.A typical site was selected for the experiment,and the surface-to-cliff ERT measurement scheme was used and the resistance inversion was carried out for the step structure model.The inversion results have high imaging accuracy,which provides support for the detection of the cliff of grotto culture relics.The resistivity anomalies of fracture are usually small,and the fracture resistivity anomalies arising from different water content changes can be highlighted by using time-lapse ERT monitoring.Based on the Gauss-Newton resistivity inversion,three time-lapse resistivity inversion methods(parallel inversion,sequential inversion,and difference inversion)were implemented and tested.It was determined that difference inversion could improve the resolution of the variation anomalies.Difference inversion is applied to the fracture monitoring in the cliff region of the Dazu Grotto north mountain experimental site,and the results show that the difference inversion can accurately identify the resistivity changes of fracture after rainfall and infer the fracture water content changes from the observed data at different moments,which provides an important basis for the fracture monitoring of the grotto rock structure.For the special topographic features of grotto culture relics,a new borehole-tosurface-to-cliff ERT measurement scheme is proposed.After that,the sensitivity and detection capability of AM-BN array and AB-MN array are analyzed.The numerical experiments show that for a single ERT configuration,the surface-to-cliff AM-BN array has a better inversion effect and the detection range is more suitable for the detection requirements of grotto protection.For the combined ERT schemes,the combination of borehole-to-surface and surface-to-cliff schemes using AM-BN array has higher resolution and better imaging effect in grotto target structure detection,and is not affected by the symmetric effect.In addition,the numerical experiments for boreholeto-surface-to-cliff ERT difference measurement also show that it has a good imaging effect on the changes of complex stochastic media.The research in this paper can provide technical theoretical support for time-lapse ERT monitoring and the selection of borehole-to-surface-to-cliff ERT measurement scheme,and provide an important methodological technique for the grotto cultural relic protection.