RU2346299C2 - Method for geophysical and geochemical field sources distribution survey - Google Patents

Abstract

FIELD: physics; measurement.

SUBSTANCE: invention relates to methods of geological environment survey and permits to study subsurface distribution of gephysical and geochemical field sources. The latter may be various types of mineral deposites, fault zones, archaeological sites and other subsurface objects. Geophysical fields are measured and their intensity distribution is mapped. Field component anomalies are detected, which are then used to select anomaly sources. For these sources fields are calculated. Selection is validated according to relationship of actual and estimated fields. In addition, for the same area, distribution of mobile occurence forms of chemical elements and gases is studied and geochemical fields are obtained. Arbitrary points under surveyed area are selected. Band-pass filtering of obtained geophysical and geochemical fields are carried out with account for depth of previously selected points. Covariances of estimated anomalies from field sources placed at selected points and actual fields determined by filtering are calculated. Subsurface covariation distribution is mapped. Field sources are located according to matching local extreme values of geophysical and geochemical fields' covariation.

EFFECT: newly developed method, which implies surveying day surface only and provides for field source location and qualitative estimation.

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Description

The invention relates to methods for studying geological environments and allows you to study the spatial distribution in the earth of sources of physical and geochemical fields, which may be various types of mineral deposits, zones of tectonic disturbances, archaeological sites and other underground objects.

A known method of magnetic exploration, in which the intensity of the full vector of the magnetic field strength (for example, ΔT or its vertical component ΔZ) is measured over the area under study, followed by the construction of maps of this field (Geophysical methods of prospecting and exploration / L.M. Gorbunova, V.P. Zakharov, V.S. Muzylev and others. - L .: Nedra, 1982. P.27) On the maps obtained, anomalies of the observed field are distinguished, and objects are selected according to the characteristic features of these anomalies (intensity, location of “special points”, length in plan) which may be the source of these anomalies. Using the well-known formulas given in the same book, the magnetic field of these anomalous objects is calculated and the calculated and observed fields are compared. If these fields coincide, the selection of objects is considered completed. In case of mismatch of the fields, the selected anomalous objects are corrected, the field is recalculated from them, and the calculated and observed fields are again compared. Such an operation is carried out repeatedly, until the calculated and observed fields coincide with a given accuracy. If they are, they consider the interpretation of the data complete. The disadvantage of this method is that it allows you to get information only about the distribution in the space under the investigated area of the magnetic field sources and does not make it possible to separate objects that are close in magnetic properties, but differ in other physical properties, for example rocks in density. Another disadvantage is that when processing data, many equivalent solutions can be obtained, i.e. the same anomalies can be created with different distribution of field sources, and for the interpretation of the data it is necessary to attract additional information.

Closest to the proposed method is a comprehensive study of physical fields over the studied area with subsequent interpretation of the results (Znamensky VV General course of field geophysics. M .: Nedra, 1989. S.131-132). In this method, magnetic and gravity observations are performed on the test area. A joint interpretation of the obtained fields is carried out with the construction of sections and the selection of objects that differ in magnetic and density properties. The disadvantage of this method is that it does not allow to evaluate the material composition of the selected deep objects, i.e. the same anomalies can be created by objects similar in density and magnetic properties, but different in material composition, for example sulfide ores, which are the objects of searches and pyritization zones. It is not possible to distinguish these objects when using this method. Another disadvantage of this method is that when processing complex geophysical data, as well as when interpreting magnetic prospecting data, many equivalent solutions are obtained, and to select the only one that most fully reflects the structure of the studied formation, it is necessary to attract additional information. Often there are cases when different sources of physical fields located in the earth in different areas create anomalies at the same points on the day surface. When interpreting such data, subjectively often combine the location of the sources of anomalies in the earth, which leads to erroneous conclusions.

The objective of the invention is to provide a method for studying the spatial distribution in the earth of sources of geophysical and geochemical fields, allowing observations only on the surface to uniquely determine the position and material composition of these sources.

The problem is solved as follows. In a method for studying the spatial distribution of field sources, including taking measurements of physical fields with constructing maps of the distribution of their intensity, highlighting the anomalous components of the fields by which the sources of these anomalies are selected, the fields of these sources are calculated and the correctness of their selection is judged by the proximity of the observed and calculated fields, of the same area, the spatial distribution of the concentrations of chemical elements and gases in mobile forms of location is additionally studied, obtaining geochemical la. Then, arbitrary points in the space under the area under study are selected, band-pass filtering of the obtained physical and geochemical fields is carried out taking into account the depth of the selected points, and covariances of the calculated anomalies from the field sources placed at the selected points are calculated with the components of the observed fields highlighted by filtering. By the spatial coincidence of the areas of local extreme values of the covariance of physical and geochemical fields, the distribution of field sources in the earth is obtained.

The calculation of covariances used in this method for geophysical and geochemical data when comparing the spatial position of the plots with the physical and chemical properties characteristic of the desired objects allows us to unambiguously establish without any additional information the distribution of the intensity of the field sources in the earth, which is directly proportional to the values of the covariances of the corresponding fields, and way to assess the presence and position under the investigated area of objects that are sources of olei.

Figure 1 presents an example of detecting a source of gravitational and magnetic fields, presumably associated with a hydrocarbon reservoir (circled by a solid bold line - G). A section along one of the profiles shows the regions of reduced covariances of the calculated anomalies and the corresponding actually observed fields for the gravitational (horizontal shading - A) and magnetic (vertical shading - B) fields, as well as the regions of increased covariances for the nickel content field determined by thermomagnetic geochemical method (TMHM) (oblique hatching - B), and the position in the section along the profile of the site corresponding to the spatial coincidence region of the reduced covariance values for gravity field and magnetic fields and the region of increased covariances for the TMHM nickel field.

Figure 2 presents an example of detecting a source of gravitational and magnetic fields, which in this case is an oil reservoir (black fill - G), installed according to the drilling data. A section along one of the profiles shows the isolines of the covariance values of the calculated and corresponding actually observed fields for the magnetic field (thin lines with dashed lines in the direction of decreasing covariance values - A), the area of reduced covariance values for the gravitational field (horizontal shading - B), region increased values of covariances for the bromine content field TMHM (oblique hatching - B).

To implement the method carry out the following operations.

1. Physical fields (for example, magnetic and gravitational) are measured in the study area using a network of observations.

2. In the study area, the concentration fields of mobile or weakly fixed forms of finding chemical elements are measured in the study area (by surveying using geoelectrochemical methods, for example, TMGM, diffusion extraction, the method of organometallic soil forms) or gases (atmochemical methods), obtaining geochemical fields in the form matrices or cards. The network of geochemical observations may differ from the network of geophysical observations.

3. In the space under the investigated area, arbitrary points are selected. For the convenience of calculations, the selection of points can be carried out according to the same network of observations along which geophysical or geochemical observations were carried out.

4. Conduct band-pass filtering of the measured physical and geochemical fields, taking into account the depth of the selected points.

5. Calculate the covariance of the calculated anomalies from the sources of the corresponding fields placed at the selected points, with the components of the observed fields highlighted by filtering. Maps of the distribution of covariance values for various fields are built.

6. Identify areas of coincidence of local extreme values of the covariance of physical and geochemical fields in the lower half-space, which estimate the spatial distribution of field sources, which may be ore objects, hydrocarbon deposits, zones of tectonic disturbances, archaeological sites and other underground objects.

For example, hydrocarbon deposits are characterized by reduced magnetization, reduced density and increased contents of indicator elements of hydrocarbons (nickel, vanadium, bromine, etc.) and hydrocarbon gases. Therefore, the location of hydrocarbon deposits is recorded in the ground only by a comprehensive examination of areas of local decreases in covariance values for magnetic and gravitational fields and increases in covariance values for distribution fields of hydrocarbon and gas satellite elements. Ultrabasic massifs are characterized by increased values of magnetization and density of rocks, as well as increased contents of some indicator elements characteristic of hydrocarbons (for example, nickel, copper), and reduced vanadium contents. Therefore, when applying the proposed method, the location of such arrays is characterized by increased covariance values for the magnetic and gravitational fields, reduced covariance values for the vanadium field, which distinguishes them from hydrocarbon deposits. Most sulfide ore bodies, relative to the enclosing rocks, are characterized by increased magnetization, increased density and increased contents of chemical elements characteristic of these ores. Correspondingly, the location of ore bodies is recorded in the earth by local complex increases in the covariance values for the magnetic, gravitational and distribution of chemical element concentrations characteristic of these ores (for example, polymetallic ores of lead and zinc). Archaeological sites are characterized by increased magnetization and elevated contents of a number of chemical elements, such as copper and zinc. Accordingly, their spatial position is determined by increased values of the covariance for the magnetic field and the fields of copper and zinc.

The testing of the method was carried out in a number of areas.

In the first section (Fig. 1), geophysical (magnetic exploration and gravity exploration) and geoelectrochemical studies of TMMM were carried out along the given surface along specified parallel profiles with an observation step of 0.5 km and a distance between profiles of 1 km. Geoelectrochemical studies included the selection of soil samples, extraction of the corresponding forms of finding elements, followed by determination in the extracts of indicator elements of hydrocarbons (manganese, nickel, vanadium and others). Based on the results of all these works, maps of the distribution over the studied geophysical area (amplitudes of the full vector of the magnetic field strength ΔТ, distribution of the local component of the gravitational field Δg) and geochemical (distribution of the contents of various elements: manganese, nickel, vanadium, determined TMHM in soil samples) were constructed fields.

At a depth of 0.5 km in the space under the investigated area, the first point was chosen. In this case, the depth of the selected point corresponded to the distance between the observation points.

Calculations of magnetic, gravitational and geochemical anomalies from sources placed at the selected point were carried out. Each anomaly is calculated according to the equations of the corresponding field. For a magnetic anomaly, an expression of the magnetic field from a point source was used, taking into account the direction of magnetization (Logachev A.A., Zakharov V.P. Magnetic prospecting. L .: Nedra, 1979, p.129-133), for a gravitational anomaly, an expression of the gravitational field from a point source (Mironov BC Gravity prospecting course. L .: Nedra, 1980, p. 333-343), for a geochemical anomaly, an expression for the distribution of element concentrations from a point source (Shtokalenko MB, Putikov OF, Alekseev S. G. et al. Evaluation of the parameters of jet migration along the width of the halo a second time captive forms of chemical elements // Geophysics, 2006, No. 4, p. 55-60). In a particular case, when solving a two-dimensional problem, one can use expressions for physical and geochemical fields from sources in the form of a horizontal circular cylinder lying in the cross of the profile under study.

Band pass filtrations of the observed physical and geochemical fields were performed. Such filtering was carried out by subtracting at each observation point from the observed initial field value the arithmetic mean of the field in a circle of a certain radius, selected taking into account the depth of the selected point (in this case, the depth is 0.5 km). Using model examples, it was found that the radius in which averaging is carried out should be equal to the distance at which the amplitude of the calculated anomaly from the selected point for the corresponding field decreases by an order of magnitude compared to the extremum above the source. For a depth of 0.5 km, the radius of the averaging window for a magnetic field was 0.35 km, for a gravitational field - 1.5 km, for a geochemical field - 15 km.

For each of the fields, the covariance values of the calculated anomalies from the selected point and the components of the corresponding observed fields obtained after filtration were obtained.

At a depth of 0.5 km, a second point was chosen at a distance of 0.5 km from the first. In this case, for the convenience of calculations and the subsequent construction of maps, the calculation points in the lower half-space were placed with the same step as the step of measuring the fields on the day surface. For the second point, the covariance of the fields of the source placed at this point and the corresponding observed fields after filtration was also calculated. Then, at a depth of 0.5 km, a third, fourth, fifth, etc., was chosen. points in increments of 0.5 km. For a depth of 0.5 km in the lower half-space, the covariance values over a network of 0.5 × 0.5 km were obtained, and a map of the distribution of the obtained covariance values at this depth was constructed.

Then a point was selected at a depth of 1.0 km, and physical and geochemical fields from this point were calculated. Filtration of each of the observed fields with a radius corresponding to this depth was carried out. For the magnetic field, the averaging radius was 0.7 km, for the gravitational field - 3 km, for the geochemical field - 21 km. The covariance calculations of the calculated fields of point sources and the corresponding observed fields after filtration were performed. Then the second point was chosen at a distance of 0.5 km from the first, also at a depth of 1.0 km, and for it the covariance of the calculated and corresponding observed fields after filtration was also calculated. Similarly, at a depth of 0.5 km, covariance values in the lower half-space were obtained over a network of 0.5 × 0.5 km and for a depth of 1 km.

Similar operations for calculating covariance over a 0.5 × 0.5 km network were performed for depths of 1.5; 2; 2.5; 3.0; 3.5; 4.0; 4.5 and 5.0 km. In this case, the filtering of the observed fields was carried out for each depth of the selected points in the window with a radius depending on this depth for each of the calculated fields.

Spatial diagrams of the distribution of covariance values in the lower half-space and maps of changes in these values in sections along the profiles were constructed. The obtained covariance values corresponded to the intensities of the sources of the corresponding fields placed at a given point in the lower half-space. To control the quality of the construction of sections, decisions were made of the so-called “direct tasks”, i.e. field calculations from sources whose intensity is directly proportional to covariances. Each calculated field obtained was compared with the corresponding measured one. In all cases, a satisfactory agreement between the indicated fields was observed.

In the geological interpretation of the data, it was taken into account that oil deposits are characterized by a decrease in the density and magnetization of rocks, on the one hand, and increased contents of indicator elements of hydrocarbons, in particular nickel, bromine, and a number of others, recorded by the thermomagnetic geochemical method (TMHM). Figure 1 shows the contour lines of the obtained covariance values for the magnetic, gravitational fields and TMHM nickel contents along one of the profiles. Two regions are distinguished by this profile, characterized by the minimum covariance of the magnetic and gravitational fields at intervals of 10–20 (northern flank of the profile) and 35–45 km (southern flank) of the profile. The region in the range of 10–20 km of the profile is also characterized by increased covariance values for the distribution field of nickel contents. This area, located at a depth of 2-4 km and outlined in Fig. 1 by a thick line, is the most promising for the discovery of hydrocarbons. The section on the interval of 35-45 km, which was not marked by increased values of the covariance of the geochemical field, was not recommended as promising for hydrocarbons. The results were confirmed by seismic surveys.

On another site (figure 2), with a well-studied geological section and the presence of an oil deposit, magnetic exploration, gravity exploration, and geoelectrochemical work were carried out on individual profiles with a step of 0.25 km. Similarly to the previous case, the covariances were calculated for each of the observed fields, starting from a depth of 0.25 km and then through 0.25 km to 3 km, over a network of 0.25 × 0.25 km. According to the profile, a section was constructed of changes in covariance values, starting from a depth of 0.75 km (Fig. 2). The oil reservoir at an interval of 32-34 km and a depth of 2.1 km, according to the obtained section, is confined to the region of a local decrease in the covariance values for the magnetic field on a general, linearly decreasing background from 0 to -0.5 minimum covariance values for the gravitational field and increased covariance values for the distribution field of the content of the indicator element of the hydrocarbon bromine TMGM. Over the interval 35–36 km and a depth of 0.5–1 km, a decrease in covariance values only for the magnetic field was observed in the obtained section, which can be explained by the presence of less magnetization in this interval of rocks. From a geological point of view, this interval with respect to oil and gas potential was considered unpromising.

The above examples show that the proposed method for the first time allows, conducting observations only on the surface, to unambiguously establish the depth distribution and material composition of sources of geophysical and geochemical fields.

Claims (1)

A method for studying the spatial distribution of sources of geophysical and geochemical fields, including measuring geophysical fields with the construction of maps of the distribution of their intensity, highlighting the anomalous components of these fields by which the sources of anomalies are selected and the fields of these sources are calculated, and the correctness of their choice is judged by the proximity of the observed and calculated fields, characterized in that on the same area the spatial distribution of the concentrations of chemical elements and gases in mobile forms is studied location, receiving geochemical fields, select arbitrary points in the space under the investigated area, perform band-pass filtering of the obtained geophysical and geochemical fields taking into account the depth of the selected points, calculate the covariances of the calculated anomalies from the sources of the fields placed at the selected points, with the components of the observed fields highlighted by filtration , build maps of the distribution of covariances in the earth and by the spatial coincidence of sections of local extreme values of the covariance of geophysical and geochemistry fields will receive a position in the earth of field sources.

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