Acoustics – Geophysical or subsurface exploration – Seismic wave detection
Reexamination Certificate
1997-07-03
2001-07-03
Eldred, J. Woodrow (Department: 3644)
Acoustics
Geophysical or subsurface exploration
Seismic wave detection
C367S038000, C367S048000, C367S040000, C367S054000
Reexamination Certificate
active
06253870
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
The present invention relates to methods for measurement, analysis and assessment of ground structure.
2. Background Art
To determine the structure and characteristics of the surface layer of the ground, it has heretofore been necessary to drill a plurality of holes in the ground and insert or indwell instruments and ancillary devices into the holes and also to employ a source of oscillation for generating waves to be measured. It should, therefore, be obviously of great benefit if it were ever possible to know the structure and characteristics of the surface layer of the ground without any of such efforts and contrivances. To this end, it might be contemplated to measure and analyze the oscillatory waves or microseisms observed in the surface layer of the ground. However, up to the present time, there is not known a measuring technology or a system of instrumental analysis, by means of which short-period microseisms could be measured with sufficient accuracy to estimate the tectonic and other characteristics of the surface layer of the ground inclusive of the velocity of shear (S) waves.
Furthermore, the prior art system designed to simultaneously measure short-period microseisms at a plurality of points to thereby delineate the ground structure has the following disadvantages.
A. Since the spacings of oscillation sensors must be large, that is to say the scale of measurement must be large, a large free area is required and it is time-consuming to connect the cords from the respective oscillation sensors for cental analysis. Moreover, because the large number of oscillation sensors are required, the analytical procedure is scaled up to the extent that the method cannot be utilized advantageously in the field.
B. Since the network of oscillation sensors is not tailored to improvement in measuring accuracy or resolution of high-order modes, the reliability of the measured dispersion curve is low.
C. Short-period microseisms include not only substantive waves such as dilatational (P-) waves and S-waves but surface waves such as Rayleigh and Love waves. Since the confirmation or judgement of whether the detected waves are surface waves or not is not performed at measurement or analysis, it is impossible to judge whether the dispersion curve obtained is relevant to the surface waves and, hence, to accurately determine the ground structure.
D. Since short-period waves include higher-modes surface waves, it is impossible to compute an accurate surface wave dispersion curve and, hence, to ascertain the surface layer structure of the ground unless some means for removing or separating these waves are available.
E. Since the dispersion curve cannot be computed real-time from field values, there is no knowing whether measurements were made with acceptable accuracy.
OBJECT OF THE INVENTION
It is an object of the present invention to provide a method for measurement, analysis and assessment of ground structure whereby the laminar structure and characteristics of the ground can be determined quickly, expediently and accurately through simultaneous ground-level observation and analysis of short-period microseisms even in a confined available land.
It is another object of the present invention to provide a method for measurement, analysis and assessment of ground structure which does not call for drilling of holes or the use of an oscillation source.
It is a further object of the invention to provide a method for measurement, analysis and assessment of ground structure in which the accuracy of measurement and analysis has been improved by using a network of substantially equi-spaced oscillation sensors which facilitates field sensor positioning as well.
It is another object of the present invention to provide a method for measurement, analysis and assessment of ground structure by which the laminar structure and characteristics of the ground can be ascertained with accuracy through selective extraction of surface waves.
It is still another object of the present invention to provide a method for measurement, analysis and assessment of ground structure by which the accuracy of assessment of the ground structure can be improved by a collective analysis of data obtained by repeated measurement using dissimilar sensor-to-sensor distances.
It is still another object of the present invention to provide a method for measurement, analysis and assessment of ground structure by which the laminar structure and characteristics of the ground can be evaluated conveniently and accurately on a real-time basis.
SUMMARY OF THE INVENTION
The present invention is predicated on the facts that when surface waves such as Rayleigh waves are propagated through a horizontal multi-layer ground structure, phase velocity (the velocity of propagation) varies with different wavelengths (dispersions), that the apparent phase velocity can be computed by averaging the correlation coefficients of the waveforms measured by equi-spaced oscillation sensors in different directions, that this apparent phase velocity varies with different sensor-to-sensor differences due to the influence of higher-order modes, and that when the horizontal and vertical components of the motion of a particle on the ground surface due to propagation of oscillations are detected and synthesized, the locus of rotation or orbit of a particle on the ground surface can be constructed, and that such surface waves are present in short-period microseisms as well. As such, the present invention is directed to a method for measurement, analysis and assessment of surface wave dispersion curves which serve as bases for evaluation of the tetonic and elastic characteristics of the ground by repeated simultaneous measurement of short-period microseisms at a plurality of points in a simple small-scale measuring system wherein 3 or more oscillation sensors are disposed at substantially equal sensor-to-sensor distances within a comparatively small area and inverse analysis of the surface wave dispersion curves at the points of measurement to compute the S- wave velocity of the ground and evaluate the laminar structure and features of the ground.
In the first aspect, the present invention provides a method for estimating the laminar structure and characteristics of a ground which comprises disposing an oscillation sensor adapted to detect the vertical component or the vertical and horizontal components of oscillations at each of at least 3 points within a comparatively narrow area and thereby measuring microseisms at such plural points simultaneously, if necessary changing the points of measurement and repeating the measurement, and analyzing the vertical oscillation data or vertical and horizontal oscillation data.
In the second aspect, the present invention is directed to a method for estimating the laminar structure and characteristics of a ground according to said first aspect wherein said microseisms to be measured are short-period microseisms observed at ground level.
In the third aspect, the present invention is directed to a method according to said first aspect wherein two oscillation sensors are disposed on each of 2 or more straight lines passing through the ground to be measured and dividing the azimuth of 360 degrees into an even-number of sections or straight lines paralleled thereto in such a manner that all sensorto-sensor-distances are equal on each of said straight lines.
In the fourth aspect, the present invention provides a method according to said first aspect wherein said oscillation sensors are disposed at 3 or more points with equal spacings on the circumference of a circle and at one point in the center of the circle, or at a total of at least 4 points.
In the fifth aspect, the present invention provides a method according to said first aspect wherein said oscillation sensors are disposed at apices of a regular triangle and the center of gravity thereof or a total of 4 points.
In the sixth aspect, the present invention is directed to a method for estimating the laminar structure and characteristics
Kuwayama Shinichi
Tokimatsu Koji
Eldred J. Woodrow
Jordan and Hamburg LLP
Tokimatsu Koji
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