Test of the borehole shear wave source SH66
Anke Dannowski, Freiberg, July 2004, Diploma thesis.
The use of cross-hole shear wave tomography is a useful tool to describe small geological structures and to give underground parameters of the subsurface especially in areas with no direct accessibility. In this work the seismic borehole sparker source SH66 generating horizontally polarised shear waves is presented and tested. A description of the source is given (Other borehole source types are mentioned.) The literature study in the theoretical part of this work discusses the most important behaviour of shear waves in loose and solid rocks is pointed out. Furthermore, it can be shown that the amplitude of generated shear waves in hard rock is smaller than in soft rock due to the higher shear modulus in the hard rock.
Two series of tests with the high-frequent sparker source have been performed in loose and hard rocks. Very good quality and well separated P- and S-wave signals of the sedimentary rocks in Hannover stand in contrast to a poor data quality and additional low vp/vs-ratio in the strongly weathered metamorphic rock in Freiberg, Germany. This very small amount of measurements indicates a source pattern of the SH66 and shows that a sufficient coupling pressure lies at about 2 bar. Furthermore, compensated shear wave geophones at the surface are tested and show a good data quality.
Cross-hole measurements or tomography have a wide field of application. They are particularly suitable for the detection of small structures in the underground due to the high-frequency signal and the resulting high resolution. Civil engineering projects favour this method because of its non-invasive character and the application in densely populated areas.
In 1999 a series of borehole measurements for a lineament study was performed in the Sunnfjord area, Norway. Beside the resistivity logging tool, the SAS 200, the optical and acoustic televiewer were lowered in several wells.
The primary objective of these measurements with the new technique of the televiewer was, to provide a better image of the borehole surface and to relieve the study of fracture systems.
The comparison between at least one of the televiewer data and the resistivity data is essential to get a picture of the water movement and the ability of the fractures to transport water. Two discussed hydrogeological models for lineaments in metamorphic and crystalline bedrock, like in the area of interest, could be ones more confirmed by the measurements, interpretation, and analysing of the data.