The use of the Sheargun® and the Sismopressiomètre in seismic studies
Y. Bertrand, P. Bozetto, J. Lakshmanan and M. Sanchez
Journal name: First Break
Issue: Vol 5, No 9, September 1987 pp. 335 - 342
Info: Article, PDF ( 3.19Mb )
Price: € 30
There is a growing need for earthquake engineering site investigation, particularly for foundation studies of nuclear power plants. Consequently, geophysicists have developed equipment to generate and record shear waves in the ground. It has become clear during recent years that the shear wave velocity and quality factor (Q) are important parameters for the engineering classification of a lithological formation. The main parameter used in earthquake engineering is the shear modulus G related to shear wave velocity Vs by G = pV2s where p is the bulk density. Comparison of various sources, such as sparkers, explosives and purely mechanical devices, has shown us that the latter are the most efficient in creating strong shear waves at reasonable distances. Compagnie de Prospection Géophysique Française (CPGF) has built, jointly with Techniques Louis Ménard, a sonde clamped pneumatically to the walls of a borehole, the Sheargun; it is equipped with a hammer which strikes on an anvil to produce vertically polarised shear waves. The striking system is pneumatically operated from the surface. The strike instant is read with a precision of around 10 microseconds on a signal produced by a ceramic sensor and sent uphole after conversion into logical data within the sonde itself. The receiving device, lowered in a nearby borehole is a tridirectional geophone, also pneumatically clamped to the borehole wall, the Sismopressiomètre. The range of energy produced by the strike varies from 50 to 400 J, according to the diameter of the Sheargun used. The energy is then sufficient to allow a reading accuracy of arrival times (for distances from 10 to 30 m) compatible with the sampling rate of the fastest seismic recorders.