Field comparison of 3-C geophones and microphones to high-precision blasting sensors
Alejandro David Alcudia León, Robert R. Stewart, Kevin W. Hall, Eric V. Gallant
It is well known in seismic exploration that geophone amplitudes do not directly represent the actual magnitude of ground velocity. Certain applications of seismic data require knowledge of the exact magnitude of ground velocity (e.g. fracture characterization). A field test was undertaken during the summer of 2008 at the west end of the University of Calgary campus. Seven sensors were used for measuring particle velocity and pressure in the vicinity of the geophones. This experiment was unique in that two high-precision sensors designed for blast monitoring and engineering purposes (Blastmate) were used as a reference. Amplitude analysis of particle velocity in three directions of motion was performed on uncorrelated Vibroseis and hammer data. Amplitude difference factors were obtained for each receiver gather between 0 - 52.5 m nominal offset for Vibroseis shots, and 0 - 25 m nominal offset for hammer shots relative to the Blastmate peak particle velocity (PPV) measurements. In all cases, the conventional geophone amplitudes were smaller than the calibrated geophone amplitudes. Vibroseis shots yielded amplitude difference factors of 6.42 and 2.82 for the vertical component of the ION spike geophone at 0 m and 52.5 m nominal offset, respectively. The corresponding values for the radial and transverse components were 10.61, 2.85, 19.87 and 3.05, respectively. Hammer shots yielded 2.11 and 3.90 for the vertical component at 0 m and 25 m nominal offset. The corresponding values for the radial and transverse component were 3.89, 2.84, 6.04 and 3.01, respectively. The Oyo GS-3C geophone yielded similar results with actual values slightly smaller than the ION spike values due to their different sensitivities.