Horizontal Component Seismograms
Authors: Balachandran Kandiah 
Published: December 2007

Wave tests, performed in an area near Tulsa, OK, using both vertical and in-line horizontal component seismometers, show some interesting events that may be interpreted as backscattered waves from inhomogeneous regions. Normal wave tests (i.e., where seismometers record the signals generated at a fixed source location) and the corresponding transposed wave tests (i.e., where a seismometer occupies the previous source location and the source points o previous source location and the source points o the previous seismometer locations progressively were performed. As previously reported, the transposed wave test seismogram sections show significant improvement in trace-to-trace coherency over the normal wave test sections. However, in this experiment the transposed horizontal component seismogram section shows some events that apparently are not discernible on the corresponding vertical component seismograms. These are believed to be due to backscattering from randomly inhomogeneous zones where considerable conversion of compressional waves to shear waves takes place. This raises the possibility of locating geological features such as possibility of locating geological features such as unconformities, lenses, fracture zones, etc. Another interesting possibility is that scattering and conversion to shear waves may take place close to the seismometers, resulting in their successful recording on a horizontal component seismometer as opposed to a vertical component seismometer. This again raises the hope that areas that have heretofore been labeled NG may be explored using appropriate seismic techniques. The different possibilities need further research and development that would also be beneficial to static corrections in routine seismic surveys.

Introduction

Seismogram sections basically are geologic cross-sections displaying the variations of elastic properties of the subsurface with depth. These properties of the subsurface with depth. These seismograms are composited from a large number of single seismic traces after appropriate corrections are made for the source-receiver geometry and the near-surface layer travel times. Corrections for the near-surface have been made traditionally with the basic assumption that the elastic waves that are recorded are compressional or P-waves. However, investigations into this problem in a particularly difficult area, the clinker area of Wyoming, show that this is not so. Failure to make appropriate allowances for this effect results in a distorted structural picture of the subsurface.

The presence of a very strong shear wave is demonstrated on horizontal component seismograms. These events may be interpreted in three different ways, each with economic value in exploration. The paper deals with illustrations and interpretation of paper deals with illustrations and interpretation of observations using scattering theory.

SCATTERING OF ELASTIC WAVES IN RANDOMLY INHOMOGENEOUS MEDIA

Lord Rayleigh pointed out that the scattered wave amplitude at distances large compared with the incident wave length is inversely proportional to the distance from the scatterer to the point of observation, directly proportional to the volume of the scatterer, and inversely proportional to the square of the wave length.

Theoretical solutions are possible only when very simplifying assumptions are made and almost always the far-field solution is obtained. Knopoff and several others have computed far-field solutions for specific models and have formulated the general problem of seismic-wave scattering. Scattering in problem of seismic-wave scattering. Scattering in a randomly inhomogeneous medium involves considerable conversion of compressional wave energy to shear wave energy.

Experimental observations are in agreement with the simplified models theoretically investigated by other workers. For example, Hudson derived formulas comparing scattered S-wave energy with scattered P-wave energy as a function of the P- and S-wave P-wave energy as a function of the P- and S-wave velocities for the simple surface wave scattering model he was investigating.