The Suspension PS Velocity Logging System

Abstract

ABSTRACT The Suspension P-S Logging System is a new technique for measuring P and S-wave velocities in deep boreholes. The characteristic feature of this technique is that the downhole sonde contains both the electromagnetic vibration wave source and two receiver units separated by flexible tubing. The sonde suspended in a fluid-filled borehole generates and records high quality shear and compression wave velocities at specified depths without having to clamp the probes to the borehole wall. The system, as presently developed, can be applied to depths of 500 meters (1600 feet) and has been used at various site locations with different geological conditions. The system has been used on land and off-shore projects. The conventional cross-hole and "downhole" technique have been well developed for measuring insitu S-wave velocities for engineering studies, and the authors compare results of the Suspension P-S Logging technique to these conventional techniques. Generally, the comparison tests show good coincidence for both the S and P wave velocities. The velocity variances between the different methods as analyzed approximate 5%. INTRODUCTION IMPORTANCE OF THE S-WAVE MEASUREMENT Elastic properties of a geological stratum or formation can be determined from Sand P-wave velocities and from supplementary information of densities. S-wave velocity is directly related to the rigidity modulus which represents strength parameter of the formation. Generally, low S-wave velocity means low strength of the formation. Therefore, measuring the velocity of a subsoil layer is important, not only for resources exploration, but also for foundation and earthquake engineering studies. In earthquake engineering, a response analysis of the seismic waves can be made by using S-wave velocities of the surface layers. For most structures, the required depth for S-wave velocity investigation is between 30 meters and-100 meters. For structures subject to dynamic or cyclic stress conditions, S-wave velocity information can be used when making a dynamic design for the foundation. For example, an offshore structure that will receive a cyclic force from wind, waves, or tidal currents, it is necessary to study a soil-structure interaction under such a cyclic loading condition. A necessary depth to be investigated for such an engineering problem may be between 30 meters and 100 meters. For a structure such as a nuclear power plant, more careful investigation is required and investigations from 200 to 500 meters deep need to be made. For the purpose of exploration for the fluid reservoirs including oil, gas, ground-water and geothermal fluid, reflection seismic techniques are well established. Interest in deep S-wave logging in this area may be based on the fact that the propagation properties of S-wave are sensitive to conditions of pores and fractures in formations regardless of the contained fluid condition, and by which P-wave properties are strongly affected. Both P and S-wave velocities would be useful when evaluating the pore and fracture conditions of a formation. The required depth for this area may be very deep for up to 3 to 5 thousand meters.