Masters Theses

Date of Award

5-1996

Degree Type

Thesis

Degree Name

Master of Science

Major

Geology

Major Professor

Richard T. Williams

Committee Members

M. A. Breazeale, Jon Nyquist

Abstract

Geophones are commonly planted on the surface or at shallow depth to record the ground motion produced by various types of seismic waves propagating through the earth's interior. It is generally assumed that the surficial material is heavy rock, and that the instrument is massless by comparison, in which case the instrument passively follows the displacement of the ground caused by the seismic wave. However, it is not always possible to attach the instrument on solid rock, and not all geophones and seismometers are light. Indeed, when a seismometer is installed on a pier, the mass of the pier must be counted as part of the instrument. Depending on the mass and size of the geophone and local soil conditions, the recorded motion may differ considerably from that of the ground without the perturbing instrument mass. This problem is important for various types of seismic sensors used in different fields of seismology, namely marine seismology, earthquake seismology, and exploration geophysics. In this study I consider the dynamic response of a three-component geophone subjected to obliquely incident seismic waves. My theoretical considerations are based on the equations of the linear theory of elasticity. The theory shows that the motion of a geophone in contact with the ground differs from that of the ground without the instrument, with the difference attributed to the rigid body translations and rotations of the geophone and existence of a secondary wave field produced by the instrument interaction with an incident seismic wave. It is shown that a particular geophone-soil combination exhibits several characteristic frequencies at which the recorded seismic signals can be considerably amplified or attenuated. These frequencies are shown to depend on the geophone mass, size and the soil conditions of the installation site, with the amount of amplification depending on all of the above parameters and the angle at which an incident seismic wave approaches the ground surface. The inertial effects of the geophone response are analyzed for the center of mass arbitrarily positioned inside the instrument case. The offset of the center of mass from the vertical axis of the geophone is shown to introduce a coupling between different geophone components and is associated with the increase of the signal amplifications, resulting from the additional displacements recorded due to the rotations of the instrument base. The validity of the theory was tested using the seismic reflection data recorded by three component geophones used in exploration geophysics. The experimental results prove the validity of the theory, in that it is capable of predicting sharp amplifications of the seismic signals recorded by different components of a seismic recorder, at specific frequency ranges. It therefore provides a theoretical basis for developing frequency filters, suitable for a particular installation site, to remove the amplification effects resulting from the coupling phenomenon from the seismic data, and obtain an improved estimate of the ground motion.

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