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The Vibroseis Source

Series: Advances in Exploration Geophysics Volume: 3

By: G Baeten and A Ziolkowski

Elsevier

Hardback | Dec 1990 | #101494 | ISBN: 0444888799
Availability: Usually dispatched within 1 week Details
NHBS Price: £116.00 $148/€138 approx

About this book

Approximately 50% of all seismic surveys on land are performed using a seismic vibrator as a source. To obtain an accurate image of the earth, one needs to know the input signal to remove its effect on the seismogram. One of the main advantages of the Vibroseis source over impulsive sources like dynamite is the control one has over the emitted signal. To establish which signal on the vibrator needs to be monitored for this purpose (feedback signal), both the behaviour of the earth and of the vibrator must be understood. Discussion in the literature on the choice of feedback signal started in the 1980's, when Lerwill (1981) and Sallas and Weber (1982) raised this issue. This book presents a sound theoretical foundation to evaluate existing and new methods of signal control, and compares theoretical results with real data. Another slightly more theoretical issue discussed in the literature is the general problem of a plate (or disk) vibrating on the earth. The most famous paper in this respect is by Miller and Pursey (1954). The non-uniform distributions of traction and displacement under the plate that are measured in the field have never been included before, but this volume presents new models which account for this non-uniformity. One of these new models includes the bending of the baseplate. The behaviour of the vibrator can only be understood when vibrator and earth are coupled. Usually, either the earth model is very simple (e.g. a spring-dashpot model), or the vibrator model is very simple (e.g. uniform traction under the plate), or both. Included are accurate models for both vibrator and earth, and results for the coupled earth-vibrator model. Most articles on marine vibroseis only deal with the problem of smearing of the data due to the movement of the source and receiver through the water. Many other aspects of the marine vibrator (for example far field relations, power output, feedback signal) are considered in this volume.


Contents

Introduction. The Wavefield of a Surface Source in a Horizontally Layered Elastic Medium. Configuration. Basic relations. Decomposition into upgoing and downgoing wavefields. The local reflection of the wavefield at a layer interface. The propagation of the wavefield within a single layer. The global reflection response. The particle velocity and traction components at an arbitrary point in the medium. The transformation to the space domain. The elastic halfspace. The horizontally layered acoustic medium. Far field relations. The Seismic Vibrator. Mechanics of the land vibrator. The vibrator signal. The baseplate behaviour. The Combined Earth-Vibrator Model. The earth model. The combined earth-vibrator model. The numerical procedure. Numerical results. Test of the Theory By Experiment. The data set. The earth model. The vibrator model. The feedback signal on the seismic vibrator. The Marine Vibrator Source. Configuration. Vibroseis: land and marine. The earth model. The mechanical model of the marine vibrator. The combined earth-vibrator model. The numerical procedure. The distribution of pressure and displacement at the surface of the vibrator. The average power factor. Far field relations. Test of the theory by experiment. Processing of Vibroseis data. Description of the data set. Conventional processing. Proposed processing sequence. Concluding remarks. Appendices: Determination of Green's Matrices for Acoustic and Elastic Media. Solution of the Differential Equation Describing the Plate Bending. The Numerical Procedure for the Combined Earth-Vibrator Model. References. Author Index. Subject Index.

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