Study On The Feasibility Of The SPAC Method Using Microseism In The Exploration Of Underground Structure

As a convenient survey method, the Microtremor Survey Method (MSM) that uses background ambient noise, is very suitable for surveying environment-sensitive areas such as urban. It has already found its broad application. Nevertheless, because of using ambient ground noise, the accuracy of that method still would not be able to compare with ordinary seismic method. This treatise mainly studies the problem of accuracy in the spatial autocorrelation (SPAC) method-a kind of the MSM. The SPAC method detects underground S-wave velocity structure by using a small scale seismic circular array, it has been known as a very rapid and economized survey method. It has been mainly applied to the underground research work for earthquake engineering. But, because of low accuracy level of the results, the method has still not found its application in the high-grade survey work such as oil exploration. It means that there still exist some imperfect aspects to be study in each stage such as measuring and data processing in the SPAC method, and that it would be needed to introduce some useful schemes for to accomplish acculate survey under the random wave-fields. To solve the problems has become major objectives in this work.In this treatise, attention was paid in every aspects of the method, such as the forward calculation & inversion of Rayleigh-wave dispersion curve, the measuring of microseism and extracting the phase velocity dispersion in order to find the origins of error and corresponding way from it. In Chapter 2, it discusses briefly a calculation method of Rayleigh-wave dispersion curve and its accuracy. In Chapter 3, it analyses some results of invertion using genetic algorithm and discusses a way of overcoming its weakness. The main works in Chapter 4 of this treatise are related with the study of errors and their properties which would occur in the courses of field observation and in the extracting of Rayleigh-wave dispersion from microtremor records, for such complicated study, this work mainly adopts simulated experiment method, it is found to be a convenient approach for the study of self-passive SPAC method. Through the simulated experiments and accompanied analyses, it become clear some important aspects of the method that could not distinguish through ordinary field experiment.This paper shows some useful results and proposes such as joint inversion of the global optimizing GA with the LM algorithm in the local researching technique and the idea of the combination measurement of ambient noise with the measurement of waves from an active source in a given special direction, which would essentially improve the SPAC method.Main results obtained through this study on SPAC method can be summarized as follows:1)In SPAC method, there are three parts of implementation. Those are theprocedure of extracting of dispersion feature of Rayleigh-wave from the array recordsmicrotremors, the procedure of forward calculation of dispersion curve using theparameters of layered earth, and the procedure of inversion of ground layerparameters from the dispersion curve. Through the study, it can be accomplished thebases for carrying such procedures: the base of study on the estimation of dispersionby accomplishing the algorithm based Haskell's transfer matrix method; the base ofinversion calculation using GA and LM algorithm; and the numerical procedureextracting SPAC coefficients.2)It presents some methodological issues for enhancing the accuracy in forward andinversion calculation of dispersion. It includes the estimation of phase velocity usingcurve-fitting and the idea of joint inversion of genetic algorithm in the globalsearching and LM algorithm in the local searching.3)In this study, it introduces the technique of simulation of the array-based recordsmicrotremors. For the study of the deviation properties of the SPAC coefficientsestimates incurred by various conditions, it provides convenience.4)By using the technique of simulated experiment, it can be able to obtain someattributes of deviation in SPAC coefficients estimates that is difficult to detect in fieldsurvey. Through the simulated experiments and accompanied analysis, it has beenmade clear the combined effect of finite array nature and azimuth-restricted waveincidence (directional aliasing) on the deviation of SPAC coefficients, and obtainedfigures exhibiting the structures of such deviations. And, it presents some usefulsuggests and implications for field survey, an important one in them is the suggestionof combination of the traditional measurement of microtremors with the additionalmeasurement of artificial seismic waves in the SPAC method, which will provide us apossibility to positively overcome the deviation in high frequency range of SPACcoefficient estimates which inccured in relation with random wave-field and limitedmumber of stations in array. In this paper, the effect of irregular installing of circulararray on the deviation of SPAC coefficients is exhibited in intuitive manner. It mainlycomes from radial irregularity. In the end of chapter 4, a particular consideration ismade on the deviation features occurred in the case of omni-directional waveincidence. Main attention is paid on the systematic deviation of SPAC coefficients inlow frequency ranges below its first minimum. This fact provides us a new view onthe SPAC method: provided that microseism sources are distributed in all azimuthes,the array measerved SPAC coefficients exhibits a definite deviation from the targetideal SPAC coefficients, which is not incurred by the directional aliasing effect andhas no relation with the mumber of stations in arrays. The deviation effect diminisheswith decreasing the azimuth range in which wave sources distributed, and so, whenthe sources restricted in a direction, the observed SPAC curves just closely coinside with the ideal Bessel curves. Although still having not plenty of field verification, the deviation property that has not yet discussed in earier studies seems to be worth of further study. Additionally is mentioned the problem of appropriate selection of array radius.In brief, errors in the exploration works using SPAC method that would be incurred by various reasons would have their more deep interpretation through this study. It will give significant contribution for development and improvement of the method.