RU2436124C2 - Method of estimating change in stress condition of geological environment - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: distant earthquakes are used as a seismic wave source and three-component seismic recorders placed on the measuring site are used as receivers. The time of the first arrival of a longitudinal wave is recorded at component z of each seismic record from one earthquake. The time interval for seismic recording is selected, for example 4 seconds. Mode-converted energy is calculated on x and y components. Energy is accumulated for each of the seismic records from a given earthquake. Mode-converted energy from the next earthquakes continues to be summed up until stable amplitude-spatial non-uniformities appear in the mode-converted energy distribution. Said non-uniformities reflect the structure of the area under the measurement site and determine a quasi-stable level of background stress. Further, monitoring observations are conducted, as a result of which a series of similar structures are obtained in the mode-converted energy distribution. Change in the stress condition of the geological environment is determined from the value of deviation from the level of background stress. Relative danger of change in the stress condition of the geological environment under the measurement site is determined from the value of the amplitude of stable fluctuation of distribution of mode-converted waves - target useful signal. The target useful signal is a colour gamma.

EFFECT: high reliability and rapidness of estimating change in stress condition of the geological environment.

2 cl, 8 dwg

Description

FIELD OF THE INVENTION

The invention relates to geophysics and seismology and is intended to study the geological environment and earthquake prediction.

Description of the invention

State of the art

A known method of assessing changes in the stress state of the geological environment, based on drilling a group of wells, periodically determining changes in water level in them (Kopylova G.N. Seismicity as a factor in the formation of groundwater regime. Vestnik KRANTS. Earth Sciences Series, 2006, No. 1, issue No. 7, p. 51-66.) [1]. Changes in water level occur as a result of changes in stress state acting on an aquifer in the ground - the geological environment. In order to elucidate the mechanism of action, an aquifer in this case can be figuratively imagined as a bulb with a large total area. This representation of the aquifer allows us to explain how, in the analogue method, the large total area of the aquifer ensures the registration of even small changes in soil stresses. Currently, the analogue method is the most advanced in solving the problem of earthquake prediction. The main disadvantages of the analogue method:

- the need for drilling water wells;

- the inability to use the method promptly in places where there are no wells.

In technical essence, the closest analogue to the prototype of the present invention is a method for assessing changes in the stress state of the geological environment, based on recording seismic records from a source of seismic waves at a measuring ground and using their subsequent processing ("Earthquake prediction methods. Their use in Japan". Collection of articles under the editorship of N.V. Shebalin. M: Nedra, 1984, p.102-104) [2].

In the known prototype method for the excitation of seismic waves using periodically repeated artificial explosions. At the measuring ground, registrars and explosions are placed in such a way that seismic waves pass through the zone of the expected change in the stress state of the geological environment, for example, through the earthquake preparation zone. The method is based on the experimental dependence of the propagation velocity of elastic (seismic) waves on the pressure applied to the sample (array). In the known prototype method, the processing of seismic recording is reduced to determining the arrival times of the target waves: longitudinal or / and transverse, and the subsequent determination of their velocities. The prototype method allows to eliminate the main disadvantages of the analogue method: not to drill water wells and, due to this, to increase the efficiency of use. But the prototype method is not free from disadvantages. The main disadvantages of the known prototype method include:

- the need for explosions, which is extremely difficult in densely populated areas, and

- the difficulty of ensuring the identity of repeated explosions.

Disclosure of invention

A comparative analysis of the features of the claimed and known solutions indicates its compliance with the criterion of "novelty."

The problem solved by the claimed method is to assess changes in the stress state of the geological environment by recording seismic records by a network of seismic registrars from a source of seismic waves and processing them, obtaining a target signal whose dynamics is related to the dynamics of changes in the field of mechanical stresses of the geological environment in the source.

The solution to this problem is provided by the fact that in the method for assessing changes in the stress state of the geological environment, based on the registration of seismic records by a network of seismic registrars from a source of seismic waves and their subsequent processing to reliably detect dangerous conditions of the geological environment, use the network in the form of three-component placed on the measuring ground seismic recorders, and as a source using distant earthquakes (from distances greater than 2 °), while on the those z of each seismic record from one earthquake record the time of the first arrival of the longitudinal wave, then select the time interval of the seismic record, for example 4 seconds, calculate the energy of the converted waves from the x- and y-components and carry out its accumulation for all registered seismic records from this earthquake and then continue the summation of the energy of the converted waves from subsequent earthquakes to the appearance of stable amplitude-spatial inhomogeneities in the distribution of energy of the converted waves, which matured by definition are the elements of the seismic field structure reflecting the information about the structure of the measuring portion by a polygon that is taken as a background voltage level quasistable and further with a predetermined periodicity monitoring is carried out observation, which yield a similar structure in the distribution of energy converted waves. By the presence of deviations from the level of background stresses, the change in the stress state of the geological environment is judged, and by the magnitude of the amplitude of the stable fluctuation of the distribution of energy of the converted waves - the target useful signal, in turn, the relative danger of changes in the stress state of the geological environment under the measuring range is judged, while the target useful signal fixed in the form of a section in coordinates (x, t), where x is the distance between the registrars on the surface of the landfill, and t is the time of seismic recording, and In the context, the target useful signal is presented in a color scheme that reflects the relative risk of a future earthquake and by the area of the polygon in coordinates (x, y), determining, taking into account monitoring observations, the change in the structure of the useful signal in three spatial coordinates and in time. At the same time, in order to ensure the necessary reliability of the received information about a dangerous trend in the change in the stress state of the geological environment, a relatively stable amplitude-spatial heterogeneity is determined by at least 5 distant earthquakes.

EFFECT: obtaining the target useful signal in the form of a section in coordinates (x, t), where x is the distance between the geophones at the measuring range, and t is the time of seismic recording, and over the area in coordinates (x, y). In the section (x, t) and in area (x, y), the target useful signal is expressed in a color scheme that visually reflects the danger of a future earthquake.

Brief Description of the Drawings

The invention is illustrated by drawings, where

Figure 1 - seismic recording (seismogram);

Figure 2 - summation (accumulation) of a piece of data, a geological section along the selected profile;

Figure 3 - forecast seismic activity in the form of a deep section along the selected profile;

4 is a forecast of seismic activity in the form of a map of the measuring range;

Figure 5 - change in seismic activity for an earthquake with M = 3.5;

6 is an example of processing monitoring observations in a four-dimensional format;

7 is a change in seismic activity for an earthquake with M-5.6.

The implementation of the invention

The newly introduced operations, forming a set of essential features, ensure the achievement of such quality properties as:

- the possibility on the basis of the existing complex of technical means of continuous seismological observations on the earth's surface;

- high efficiency of obtaining and processing information for changes in the stress state of the geological environment under the measuring range;

- high reliability, since the target useful signal is the deviation from the background voltage level is directly related to the change in stresses in the geological environment under the measuring range;

- ensuring the necessary reliability of the information obtained about a dangerous trend in the change in the stress state of the geological environment under the measuring range, since a relatively stable amplitude-spatial heterogeneity is determined by at least 5 distant earthquakes.

The invention consists in the following. Around the large urban agglomerations or important industrial facilities, such as hydroelectric power stations, nuclear power plants or a chemical plant with harmful production, create a measuring ground with dimensions, preferably 60 × 60 km ² . At the measuring ground, 10-14 recorders with three-component geophones are placed and continuous seismic recordings are carried out. On the seismic records for subsequent processing, exchange waves (Fig. 1) from distant earthquakes (from distances greater than 2 °) are isolated.

Since seismic signals from distant earthquakes arrive at the measuring ground almost vertically from below, it can be assumed that all the exchange waves recorded in the X, Y plane arise directly below the seismic receiver and, therefore, carry information about the state of the geological environment under the measuring ground. At the beginning of processing on the component z of each seismic record from one earthquake, the time of the first arrival of the longitudinal wave is recorded (Fig. 1). Next, select the time interval of the seismic record, for example 4 seconds, calculate the energy of the converted waves from the x- and y-components and carry out its summation (accumulation) for each registered seismic record from this earthquake.

Figure 2 shows the result of summing the energy of the converted waves from a group (portion) of earthquakes, where the static (geological section) and dynamic (stress state) parts are summarized.

Then, the summation of the energy of the converted waves from subsequent earthquakes to the emergence of stable amplitude-spatial inhomogeneities in the distribution of the energy of the converted waves, which by definition are elements of the structure of the seismic field, reflecting information about the geological structure (geological section) of the area under the measuring range, and which serves as the definition of quasi-stable background voltage level.

And, further, as a result of monitoring observations, a time series of similar structures is obtained in the distribution of energy of the converted waves. By the magnitude of the deviation of the obtained structures from the level of background values, a change in the stress state of the geological environment is judged for the times of receipt of each structure in the energy distribution of the converted waves. By the magnitude of the relatively stable fluctuation of the distribution of energy of the converted waves - the target useful signal - in turn, they judge the relative danger of changes in the stress state of the geological environment under the measuring range.

The target useful signal is presented in the form of a section (Fig. 3) in coordinates (x, t), where x is the distance between the geophones at the measuring ground, at is the time of seismic recording, and over the area (Fig. 4) in coordinates (x, y ), moreover, in the context (Fig. 3) and in area (Fig. 4), the target useful signal is expressed in a color scheme that visually reflects the risk of a future earthquake.

As a result of continuous monitoring observations on the territory of the measuring range, the change in the stress state is determined - the "image of the stress state" of the elastic-brittle layer of the lithosphere to a depth of 30-80 km. It is in the depth range of up to 80 km that up to 90% of the hypocenters of earthquakes and released energy are concentrated.

We consider the changes in the stress state as an example of an earthquake near a measuring range on the territory of the Caucasian Mineral Waters on February 21, 1996 with a magnitude of M = 3.5 (Fig. 5).

Here, as an example, 6 states of the geological environment in the territory of the measuring range are given at different times: for 3.5 months (Fig. 5, a), for 1.5 months (Fig. 5, b), for 13 days (Fig. 5 , c), 5 days (Fig. 5, d) before the earthquake, at the time of the earthquake (Fig. 5, e) and 2.5 months after the earthquake (Fig. 5, f).

It can be seen that the new harbinger - the "image of the stress state" - reacts to the process of preparing the earthquake. Moreover, the process of relaxation of the medium after the earthquake is controlled. So after 2.5 months after the earthquake (Fig. 5, f), the state of the medium is quite close to the initial one before the earthquake (Fig. 5, a). Thus, the new harbinger displays both the geology of the environment and the change in stress state.

In the second and subsequent versions of the processing program, the geological environment with its stable / initial / state is shown by yellow lines of equal values of the converted waves (Figs. 5, 6). Deviations from the stable state of the medium are displayed by changes in the released energy on a relative scale of different colors by color.

The third version of the program was developed to create a four-dimensional observation format and process information on the territory of the measuring range. The three-dimensional format (model) reflects the actual volumetric changes in the energy of the converted waves, and the fourth coordinate is a measure of time, or, in other words, the four-dimensional format shows the changes in the volumetric indicators of the energy of the converted waves in time (Fig.6). Such materials (Fig.6) with a frequency of 7-10 days are transferred to the Earthquake Prediction Centers.

The preparation of the devastating earthquake in Dagestan on January 31, 1999, located 360 km away, was clearly observed on the territory of the measuring range (Fig. 7). An extremely dangerous hotbed of increasing stress state with a length of 50 km arose in the period from January 1 to January 15, 1999 (Fig. 7 a), and in the period from January 22 to January 30, this center was localized to a horizontal size of 10 km and another center appeared a depth of 15 to 8, a distance of 360 km from the controlled territory (Fig. 7, b, 7, c), which indicates the response of the identified precursor to the preparation of an earthquake within a radius of 360 km from the controlled territory. Over 10 years of observations in the indicated territory of the measuring range, records were obtained from earthquakes with magnitude M = 3.5 at a distance of 130 km, magnitude M = 5.6 at a distance of 360 km, and magnitude M = 6.8 at a distance of 710 km.

The proposed method has been tested for 10 years at the Kavminvodsky training ground (1995-2004) and in Iran (since 2004). It was found that, along with the registration of the seismic mode, the proposed method allows to issue a medium-term forecast (up to 4 weeks) about the level of seismic hazard in the area of the measuring range and its immediate vicinity with the issuance of an earthquake hazard forecast, i.e. Each time after processing the monitoring data, it allows you to answer the question whether there will be or will not be an earthquake in the area of the measuring range in the next 4 weeks. At the same time, this method allows canceling false forecasts issued for other earthquake precursors. For the period 1995-2005 There are four cases of canceling false forecasts: two in the Ciscaucasia (1996, 1998) and one case in the territories of Azerbaijan (Baku, 1999) and Iran (Tagheran, 2004).

Information sources

1. 1.Kopylova G.N. Seismicity as a factor in the formation of groundwater regime. Bulletin CRANZ. Earth Sciences Series, 2006, No. 1, Issue No. 7, pp. 51-66.

2. “Methods of forecasting earthquakes. Their use in Japan. " Sat articles edited by N.V. Shebalina. M .: Nedra, 1984, p.102-104.

Claims (2)

1. A method for assessing changes in the stress state of the geological environment, based on the registration of seismic records by a network of seismic registrars at a measuring range from a source of seismic waves and their subsequent processing, characterized in that the network is used in the form of three-component seismic recorders placed on a measuring range, and as sources use distant earthquakes (from distances greater than 2 °), while on the z component of each seismic record from one earthquake, they are recorded in After the first arrival of the longitudinal wave, then select the time interval of the seismic record, for example 4 s, calculate the energy of the converted waves from the x and y components and carry out its accumulation for each registered seismic record from this earthquake, and then continue to sum the energy of the converted waves from subsequent earthquakes before the appearance of stable amplitude-spatial inhomogeneities in the distribution of energy of the converted waves, which by definition are elements of the structure of the seismic field, reflecting information about the structure of the section under the measuring range, which determines the quasistable level of background voltage and then, with a given frequency, carry out monitoring observations, which result in a number of similar structures in the distribution of energy of the converted waves, and the deviation from the level of background voltages is used to judge the change in the stress state geological environment, and in magnitude of the amplitude of the stable fluctuation of the distribution of energy of the converted waves - the target useful signal, in its Next, they judge the relative danger of changes in the stress state of the geological environment under the measuring range, while the target useful signal is recorded as a section in coordinates (x, t), where x is the distance between the registrars on the surface of the landfill, at is the time of seismic recording, and section and the area of the polygon in coordinates (x, y), the target useful signal is presented in a color scheme that reflects the relative risk of a future earthquake, while determining taking into account monitoring observations Changing the structure of the useful signal in three spatial coordinates and in time. 2. The method for assessing changes in the stress state of the geological environment according to claim 1, characterized in that in order to ensure the necessary reliability of the information obtained about the dangerous trend in changing the stress state of the geological environment, a relatively stable amplitude-spatial heterogeneity is determined by at least 5 distant earthquakes.

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