Comprehensive Geophysical Evaluation of Grouting in the Goaf of an Urban Underground Coal Mine

Coal mining goaf is unconsolidated material formed by collapse of overlying strata. Water in goaf can flow into and contaminate surface water systems. An integrated geophysical investigation approach combining microseismic monitoring (MSM) and the opposing-coils transient electromagnetic (OCTEM) method was used to evaluate the effectiveness of grouting, using the Suncun Coal Mine as an example. First, MSM was used to obtain seven shear wave velocity depth profiles and 3D imaging, and the main low-velocity anomalies were delineated. Next, 12 resistivity depth profiles and 3D images were obtained by OCTEM to detect low-resistivity anomalies. Finally, the MSM and OCTEM results were verified by four boreholes drilled in the goaf areas. The results showed that the combination of MSM and OCTEM effectively identified the spatial distribution and locations of area with inadequate grouting and water-conducting channels. The geophysical results can be used for parameter selection and to design future grouting and water plugging programs.