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About seismic emission tomography

Seismic emission tomography method allows reconstruction of 3-D distribution of deep seismic or acoustic signal sources, estimation of emission power and its frequency range. Input data is wave fields recorded by the seismic profile, 2D or 3D array. To determine a space-coherent component of a wave field and to reconstruct a weak seismic source image, studied volume is sampled over a space grid. For each grid point, travel time differences are calculated according to a given velocity model. Records obtained at different seismic stations (digitizers) of a seismic array are stacked - shifted by the calculated time differences and summed. The records are accumulated by channels and time. To create a 3-D space model of a site, a well-known semblance coherent measure is applied. A reconstructed image of background noise has uniform distribution of brightness throughout the image. In presents of an emission source, the signal amplitude at the output of the algorithm increases sharply resulting in a bright spot in the reconstructed image. When the signal is weak (SNR is low), the numerical value of measure at the point of source location is a product of the number of channels and the SNR value measured at the surface.

Seismic mission tomography provides location of direct sources, for example earthquakes, surface natural and man-caused events, deep seismic emission sources connected with structural defects and stress concentration (passive monitoring), as well as sources of scattering signal connected with contrast velocity inclusions and very heterogeneous and fractured volumes (diffractional emission tomography). For passive monitoring, the input data are records of seismic noise, for diffractional seismic emission tomography – codа waves the of explosions or earthquakes. A great advantage of emission tomography over other methods of seismic sources location is that it does not require picking of the arrival times of seismic phases, which excludes laborious data processing and allows to successfully localizing sources with fuzzy and interfering events.

We used seismic emission tomography method:

  1. For local earthquake hypocenters location based on data from regional networks with aperture of several hundred kilometers (Israel and Kirgizia (KNET) networks), and from small aperture seismic arrays with aperture of less than 10 kilometers. Different methods of source location allowing for signal decorrelation with the increase of the distance between the receivers were used for large-scale and small-scale station groups.
  2. For well bore position location during drilling operations at a West Siberia oil fields.
  3. For monitoring of seismic emission dynamics in the Earth’s middle crust.
  4. For detection of areas with high emission activity in the middle and upper crust in a seismic volcanic front.
  5. For reconstruction of 3D distribution of scattering heterogeneities in the upper and middle crust with coda wave data of local earthquakes.
  6. For reconstruction of 3D distribution of scattering heterogeneities in the crust and upper mantle with low-frequency microseismic background and earthquake’s coda waves.

 

Application of Emission Tomography Method

Weak spots in the massif, such as fracture zones, big cracks, boundaries of contacts of blocks, porous and fractured reservoirs, in the course of their formation and subsequent development in a variable field of tectonic and tidal stress, are sources of constant weak noise-like seismic emission. A noise-like seismic signal is generated in the process of crack extension and friction of boards of non-uniformly scaled fractured structure. Signal frequency is determined by parameters of the weak zone and character of on-going processes; the power of signal registered at the surface is comparable to the power of background seismic noise. The nature of such endogenous seismic signal called seismic emission lies in high-frequency response of energy-saturated medium to low-frequency deformation impact – tidal, baric, tectonic deformations. Seismic endogenous emission signal is generated by pulsation of accompanying gas accumulation in the course of circulation of fluid in the channels of filtration.

In the course of oil-and gas-field development and hydraulic fracturing process, the abrupt change in the intensity of stress of massif may also cause seismic emission from the area of structural defects and stress concentration. Emission tomography reconstructs source images of seismic signals, generated by the activity of fractured structure, in the form of a “noise cloud” contouring the “sounding” system. Therefore emission seismic tomography is a powerful and convenient tool for monitoring of dynamic state of medium and detection of deep local structures, especially in volcanic areas and areas with high seismic activity, at oil- and gas-fields in the course of their development and supplementary exploration, process of preparing and forming of hydraulic fracture and process of relaxation in the area of generated rupturing deformations.

Here are some examples illustrating the method:

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