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Array Seismic Emission Tomography

This project is oriented toward the application of the mobile seismic array data analysis technique in seismic investigations of the Earth (the "noise-array method"). The aim of the this project is the development of the seismic noise array method, "ASET" [...] and using the ideas of this method for processing of real seismic array data. The technique falls into the class of emission tomography methods but, in contrast to classic tomography, 3-D images of the microseismic activity of the media are obtained by passive seismic antenna scanning of the half-space, rather then by solution of the inverse Radon's problem.

It is reasonable to expect that areas of geothermal activity, active faults, areas of volcanic tremors and hydrocarbon deposits act as sources of intense internal microseismic activity or as effective sources for scattered (secondary) waves. The traditional approaches of seismic investigations of a geological medium include measurements of time-limited (narrow-time window) determinate signals from artificial or natural sources. However, the continuous seismic oscillations, like endogenous microseisms, coda and scattering waves, can give very important information about the structure of the Earth.

The presence of microseismic sources or inhomogeneities within the Earth, result in the appearance of coherent seismic components in a stochastic wave field recorded on the surface by a seismic array. By careful processing of seismic array data, these coherent components can be used to develop a 3-D model of the microseismic activity of the media or images of the noisy objects. The processing is mainly based on calculations of the relative energy of coherent radiation of spherical waves originating from different points or small volumes below the seismic array.

Thus in contrast to classic seismology where narrow windows are used to get the best time resolution of seismic signals, our model requires a long record length for the best spatial resolution. The processing algorithm is based on a linear additive model of signals and noises. The "Semblance" (S) or "Signal/Noise Estimation" (RO) procedures are used for calculation of the relative energy of coherent radiation. These estimates are very sensitive to the presence of coherent components in the multichannel seismic recording. The set of S (or RO) values calculated for number point (Xi, Yj, Zk) of the scanned volume reflect the spatial distribution of seismic activity (radiated or scattered) of the medium beneath the array.

The applications of this technique are directed toward:

  1. The formation 3-D spatial images of the intensity distribution of noisy objects (and/or secondary wave sources) by using microseisms or the coda-waves from the both artificial and natural sources;
  2. The study and monitoring of the seismic source zones and in areas of potential the environmental impact;
  3. The monitoring of well drilling or of mineral deposit exploitation processes etc.

During 1988 and 1989 we projected and carried out seismic noise studies in the Axarfjordur regionin NE-Iceland within the framework of the expedition of the Russian Academy of Sciences. The mobile arrays were located over a high temperature geothermal field. For this work it was developed MS DOS oriented ASET 1.0 package including special software for numerical modelling, field data aquisition, 1 and 3 components seismic array data processing.

In 1992-1993 according to agreement with Prof. C.B.Archambeau (University at Boulder, Colorado) at the Moscow Data Center (SYNAPSE) it was prepared SUN UNIX oriented powerful version ASET 2.0 (in cooperation of system programmer V.A.Gurevich). This sytem apply CSS 3.0 compatible standard for input and output of seismic data.

Fig. illustrate representation some results of ASET 2.0 processing of mobile seismic array data from the Axarfjordur region.
Some aspects of the application of the above-described technique have been illustrated by the results of seismic noise studies in the Axarfjordur region in NE-Iceland. We carried out these investigations during the summers of 1988 and 1989 within the framework of the expedition of the Russian Academy of Sciences.

The main directions of this technique's applications are:

  1. The formation of the spatial distribution's 3-D images of noisy objects (and/or secondary waves sources) intensity by using of the microsism
  2. The same task, but by using of the coda-waves from the both artificial and natural sources
  3. The epicentral researches and monitoring of the active seismic noisy zones and objects
  4. The check of the wells drilling, and so on

During the summer 1988 and 1989 we carried out seismic noise studies in Axarfjordur region in NE-Iceland. The arrays are located over high temperature geothermal field.

Fig.1 and Fig.2 shows same examples of the arrays data processing. Fig.1 shows the results of processing of the several registrations from the same array at different times (levels of depth 200, 400, 600, 800, 1000m). It shows a good stability of the microseismic activity images.

Fig.2 gives an example of microseismic activity map as a vertical cross-section. This is from single registration.

Fig.3 gives an example of image obtained by using registration from all (12) the arrays. The picture is made by averaging the results from the individual arrays where they are scanning the same volume (depth 1800m). These pictures show clearly an area of stable emission of seismic noise. It is especially worth attention that the data are obtained partly in the summer 1988 and partly a year later, but both these data sets give a similar picture of the seismic noise emission.

Fig.4 vertical cross-section entire volume of investigation (6 x 4.5 x 2 km.). The distance between vertical cross-section planes is 500 m. Dark areas corresponding more high level of seismic emission.

During 1992 it was prepared the last powerful version of the ASET (Arrays Seismic Emission Tomography) software by team of programmers: V.L.Kiselevich, B.M.Shubik and V.A Gurevich. The software is realized for SUN workstation, UNIX system, XWindow and based on the CSS geophysical data standard. The package is oriented on processing of the one and three components data from big seismic arrays. It could be applied to any seismic array data and wide range of array data processing. This is powerful geophysical package (about 30000 C-lines) with its own subsystems of the Graphic, Command Language, the elements of the Data Base.


 
 
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