SHEAR‐WAVE RECORDING USING CONTINUOUS SIGNAL METHODS PART I—EARLY DEVELOPMENT

Abstract

In recent years, it has been found possible to record shear‐wave reflections and horizontally traveling shear waves using continuous signal methods. Thus paper traces the equipment development and field work performed during this research. The earliest work with a version of a swinging‐weight vibrator showed that shear‐wave reflections could be recorded. This fact provided the impetus to make modifications to equipment to meet difficulties caused by lack of energy and lack of frequency bandwidth. Examples are given which show the flexibility of the system in providing comparison between the horizontally traveling surface waves induced and recorded by the various combinations of vibrator sources and geophone types and their relative orientations. Frequency selection by the different modes is well illustrated. For most of the reflection examples, the average ratio of shear‐wave velocity to compressional‐wave velocity in the first few thousands of feet is near 0.5. Finally, to complete the early development, the version of the shear‐wave vibrator and recording system which was used for most of the additional work is described. In order to make comparison of P‐wave and SH‐wave reflection records easier, this system provided for a 2:1 compression of the shear‐wave time scale as well as a 2:1 ratio of frequency output between the P‐ and SH‐vibrator systems. A few examples of SH reflection profiles achieved with this system are presented.