Open Cut » Geology
The Multi-channel Analysis of Surface Waves (MASW) is a recently developed seismic method to estimate S-wave velocity structure under the ground. It analyses the seismic waves in the frequency domain for the dispersion property of surface waves and estimates the distribution of the S-wave velocity. The field setting of the survey uses a linear geophone array and a small seismic source such as sledgehammer or weight drop.
This method has been mostly applied in the construction industry for investigating depth of the basement rocks, competence of the ground, cavity detection and other geotechnical problems. The application depth is typically 10 to 30m deep. There is no case history of the application of MASW to resource exploration reported to date.
The current project is a trial of application of the MASW method to open-cut coal fields, perhaps for the first time. It attempted to map the base of the weathering layer and the coal seams dipping from 15 to 50m deep. The software used for MASW and inversion is SurfSeis by Kansas Geological Survey. The result could be expressed not only as conventional 1D profiles and 2D sections, but by a 3D voxel that could be sliced in any direction by using an additional external software and data conversion program developed in this project.
The data acquisition for this experiment took place in the planned open cut mine site of the Hail Creek coal mine in central Queensland in April to May 2008. Following the analysis of the seismic data it was found that: 1. the MASW method can differentiate the weathered layer over the host rock; 2. the MASW method could find top and base of the coal at some points; 3. The current inversion software seems to need some guidance by providing initial model for iteration; 4. the current gridding software introduces some artefact between the weathered layer and the host rock, which happens to appear similar to the S-wave velocity of coal.
Several 3D presentation software packages were evaluated and Voxler by Golden Software was selected to express the result in 3D perspective view and horizontal and vertical slices. A set of small programs to interface data acquisition, data processing and data presentation is made and used for the data transfer and reformatting.
Although this first attempt of using the MASW for coal delineation did not achieve the required resolution, the method is still considered to have potential. Several directions to improve the method are identified for future study and development, some of which are already in the research arena. When those software become available, the data may be revisited for further tests.