ABSTRACT

The Birdwell 3-D Velocity Log displays photographically the amplitude of elastic waves as a function of time and of the depth of the logging tool in a borehole. The elastic waves are generated by a magneto-strictive source transducer and detected by a receiving transducer, which is at a fixed distance from the source. The output of the receiver, which is proportional to the amplitude of the received elastic waves, modulates the light that exposes the film record. The duration of the receiving time can be selected to record the entire wavetrain of late events or to record only early events at a finer time scale. When a full complement of elastic waves (i.e., the refracted compressional wave, composite shear wave, direct fluid wave, and boundary wave) is recorded, their velocities and the density of the borehole fluid provide the information necessary to compute the elastic too dull and density of the rock surrounding the borehole. The differences between the values of the moduli determined from measurements of 3-D Velocity Logs and by other better known techniques are small, well within the limits of experimental error, when the velocity log is interpreted by an experienced seismic analyst.

INTRODUCTION

Quantitative descriptions of the mechanical properties of the rock environment have long been basic requirements of planning, design, and quality control of engineering projects. Because of this need, many techniques for determining the elastic moduli from static and dynamic test measurements of cores have been introduced. In addition, other techniques have been developed for making the measurements in boreholes at the engineering site. The values of the elastic moduli calculated from static measurements of cores are sometimes significantly different from the values determined by in-situ measurements of the same rock.

These differences have been attributed to several causes. Static measurements on cores are generally performed at room temperature and atmospheric pressure whereas in-situ tests are made under the normally higher temperatures and pressures due to the depth of burial of the rocks. The appreciable effects of temperature and pressure on dynamic measurements of elastic properties of cores reported by Hughes and Jones (1) confirm this possibility.

Another cause of the differences is the amount of fracturing in the sample tested. The fractures, micro or macro, in samples tested at room temperatures lead to lower calculated values for the elastic moduli than would be obtained for the same samples at higher pressures where cracks and voids tend to close. Fracturing has another effect on comparisons between core results and in-situ results. Core samples are taken between macro fractures and are generally small compared to the size of the sample tested in-situ. The effect of fractures in the larger mass of rocks measured in-situ will depend on the depth of burial (pressure) as well as on the character of the fractures.

The recognition of the effect fractures have on measurements of rock properties led to several empirical classifications of rock masses. One such classification is the Rock Quality Designation (RQD) developed by Deere and Miller (Z).

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