Potential coordinate mislocations in crosshole tomography: Results from the Grimsel test site, Switzerland

Abstract

Tomographic techniques based on borehole‐to‐borehole and tunnel‐to‐borehole traveltime data are now being employed in a wide range of studies associated with the exploration and exploitation of hydrocarbons and metallic minerals, the disposal of chemical and radioactive waste, diverse civil engineering projects, and archaeology. A fundamental assumption of currently employed tomographic inversion strategies is that the coordinates of the boreholes and tunnels containing the seismic sources and receivers are accurately known. By inverting both synthetic and observed traveltime data, we demonstrate that relatively minor coordinate errors (1–2%) in the deeper parts of long boreholes (>100 m) may produce artifacts in the tomographic images that are comparable in extent and amplitude to true velocity anomalies. To address this problem, we introduce the coupled inverse method, commonly used in earthquake studies, as a means to determine simultaneously borehole coordinate adjustments and an estimate of the tomographic image. This method has been applied to traveltime data generated and collected along a tunnel and in three boreholes within a granitic body situated in the central Swiss Alps (Grimsel test site operated by NAGRA, the Swiss National Cooperative for the Disposal of Radioactive Waste). Coupled inversions of two independent subsets of traveltime data that involve a common central borehole, together with a coupled inversion of the entire data set, yield consistent coordinate adjustments for all boreholes and tomographic images that are compatible with the known geology and a sonic log from the central borehole. Further tests with synthetic data demonstrate that certain types of weak anisotropy could influence the coupled inversions. Regardless of whether minor coordinate mislocations or weak anisotropy is the dominant effect at the Grimsel test site, distinct low‐velocity zones appear to delineate fractures zones that are conduits for groundwater flow.