CN112859181A - Local anomaly separation method of wide-area electromagnetic method - Google Patents
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Abstract
The invention relates to the field of geophysical exploration, in particular to a local anomaly separation method by a wide-area electromagnetic method, which comprises the following steps: s1, acquiring the wide area apparent resistivity after inversion, and regarding the wide area apparent resistivity as the superposition of the regional abnormal resistivity and the local abnormal resistivity; s2, carrying out multiple iterative cutting on the wide area apparent resistivity by adopting a dynamic improved interpolation cutting operator, and separating the area abnormal resistivity; and S3, calculating the difference between the wide area apparent resistivity and the regional abnormal resistivity, and outputting the difference as the local abnormal resistivity. Compared with the original wide-area electromagnetic apparent resistivity profile, the invention can weaken the fuzzy boundary between the stratums, so that the boundary between the adjacent electric layers is more obvious, and for the area with complicated geological structure, the final interpretation result is more accurate and reliable due to abnormal separation, and the precision of inversion imaging is also supported and improved.
Description
Technical Field
The invention relates to the field of geophysical exploration, in particular to a local anomaly separation method by a wide-area electromagnetic method.
Background
The wide-area electromagnetic method has the advantages of strong anti-interference capability, high working efficiency, high measurement precision and large exploration depth, expands the artificial source observation area, and is very suitable for the fine exploration of shale gas with multiple layers, large area and large depth under the conditions of complex structure and fracture development. However, for complex structures and fracture development zones, resistivity abnormity often changes irregularly, which brings trouble to traditional electrical method depending on resistivity interpretation and seriously restricts fine interpretation of electromagnetic method. The actual observed resistance parameters are often due to topographic relief, inhomogeneity of the subsurface medium, various rocks overlapping each other, fault fractures criss-crossing, or a synthetic reflection with ore bodies filled therein. The actual parameters observed in the field are usually the superposition of local anomaly, regional background and related noise, so how to effectively separate the local anomaly field, the regional background field and the noise improves the result interpretation precision and accuracy, and has important research value. Many documents on the electromagnetic method have proposed various methods for directly suppressing noise in electromagnetic signals or suppressing noise using inversion techniques to improve accuracy (Goubau et al, 1978; Gamble et al, 1979; Egbert, 1997; Garcia and Jones, 2008; Neukirch and Garcia, 2014; Weckmann et al, 2005; High Silent, 2003, 2004; Alumbaugh, 1996; Jing En, 2012; Song Gegen, 1995; Wen Eihua, 2012; Wuxin, 2015), all of which belong to the field of improving imaging accuracy by indirect methods, and there is room for improving accuracy and reliability of interpretation.
Disclosure of Invention
In order to overcome the defects in the prior art, the invention provides a wide-area electromagnetic local anomaly separation method, which is used for analyzing wide-area electromagnetic inversion imaging data and directly separating wide-area electromagnetic local anomaly from a wide-area electromagnetic apparent resistivity value.
In order to achieve the purpose, the invention adopts the technical scheme that:
a local anomaly separation method of a wide-area electromagnetic method comprises the following steps:
s1, acquiring the wide area apparent resistivity after inversion, and regarding the wide area apparent resistivity as the superposition of the area abnormal resistivity and the local abnormal resistivity;
s2, carrying out multiple iterative cutting on the wide area apparent resistivity by adopting a dynamic improved interpolation cutting operator, and separating the area abnormal resistivity;
and S3, calculating the difference between the wide area apparent resistivity and the regional abnormal resistivity, and outputting the difference as the local abnormal resistivity.
Further, the dynamically improved interpolation operator is:
wherein, R (i, j) is the regional abnormal resistivity value at the point (i, j) after iteration, Z (i, j) is the original wide-area apparent resistivity value at the point (i, j), and the R (i, j) calculated by the current iteration is substituted into the Z (i, j) calculated by the next iteration;
nx and Nz are calculation windows along the x-axis andzradius of axis, l is the order number of calculation points within the calculation window, N ═ 2Nx +1) (2Nz +1) is the total number of calculation points of the calculation window,
to calculate the average apparent resistivity within the window, [ Fx(i,j)]Is the x-direction non-linearity; [ F ]z(i,j)]Is the z-direction non-linearity.
Generally speaking, the larger the calculation window is, the larger the range of the regional abnormality reflected by the sliding average value is, the more the iteration times are, the smoother the obtained regional abnormal resistivity is, the more obvious the local abnormal resistivity is, and if the iteration is repeated, the false abnormality caused by fixed interference and random noise can be effectively suppressed, so that the local abnormality is highlighted. The local abnormal apparent resistivity is calculated through the step S3, the reflected abnormal information is extracted and analyzed, and then the low and slow abnormal zone which is difficult to find is extracted, and particularly weak abnormality caused by local fracture, structure and thin layer can be effectively identified.
Further, when b (i, j) is calculated: when Δ BxIf (i, j) is 0, if
Then b (i, j) is 1; if it is
Then b (i, j) is 0;
when calculating c (i, j), when Δ BzIf (i, j) is 0, if
Then c (i, j) is 1; if it is
Then c (i, j) becomes 0.
Furthermore, the boundary area is subjected to edge expanding processing, so that each point (M, N) within the cutting radius range of the boundary can be subjected to interpolation cutting at (i ═ Nx, -Nx +1, …, M + Nx; j ═ Nz, -Nz +1, …, N + Nz). The region extension methods that can be used include region field extension (RFM), cosine extension (cosine method/CM), polynomial fitting (polynomial fitting), minimum curvature extension (MCM).
Further, the iteration exit condition of the multiple iterative cuts is as follows: lim (small)n→∞max|Rn(i,j)- Rn-1(i, j) | is less than or equal to epsilon, wherein Rn-1(i, j) is the regional anomalous resistivity value of the previous iteration.
Further, the method is used for eliminating the influence of the underground regional field on local resistivity anomaly, and highlighting the construction and fracture development of local stratum structures.
Further, the method is used for further tracking and depicting the thin layer extension on the basis of the construction and fracture of the local stratum structure by taking the local abnormal resistivity profile output in the step S3 as a basis.
A computer device comprising a memory, a processor and a computer program stored on the memory and executable on the processor, the processor implementing the method when executing the computer program.
A computer-readable storage medium storing a computer program for performing the method described above.
The invention has the beneficial effects that:
according to the local anomaly separation method of the wide-area electromagnetic method, the dynamic improved interpolation cutting operator is adopted to carry out wide-area electromagnetic inversion resistivity anomaly separation, and compared with an original wide-area electromagnetic apparent resistivity profile, a fuzzy boundary between stratums can be weakened, so that the boundary between adjacent electric layers is more obvious, and the interpretation result is closer to a geoelectric model; the fault display is clearer, and the fault property and position are more reliable and accurate; the 'edge effect' of two sides of the end point of the measuring line is suppressed, so that the explanation result can more truly reflect the actual geophysical model; the deep stratum undulating structure form can be more clearly highlighted.
The local anomaly separation method of the wide-area electromagnetic method provided by the invention is based on effective inversion result data, and further extracts and analyzes local anomaly field information by effectively suppressing the regional background field. For a geological structure complex area, the final interpretation result is more accurate and reliable due to abnormal separation, and meanwhile, the accuracy of inversion imaging is also supported and improved.
The wide-area electromagnetic method local anomaly separation method provided by the invention can be used as a supplementary means for conventional wide-area electromagnetic method data interpretation, can more fully mine the geological information implicit in the wide-area electromagnetic method, can effectively improve the interpretation accuracy and reliability on the structure and fracture, not only gives consideration to the advantages of large depth and high precision of the conventional wide-area electromagnetic method, but also can highlight the electrical anomaly caused by the local geological structure or thin layer on the basis, realize the accurate positioning of the fracture structure and the thin layer, and form a set of method technical system suitable for wide-area electromagnetic exploration and identification of shale strata under the complex geological conditions. The technology has an important supporting function for identifying the shale bed series in geological condition complex structural areas of the south China and even the whole country, and has wide application prospect.
Drawings
FIG. 1 is a flow chart of a local anomaly separation method of a wide-area electromagnetic method according to embodiment 1;
FIG. 2 is a flowchart of data processing of the wide-area electromagnetic method according to embodiment 1;
FIG. 3 is a conventional resistivity inversion profile of a line according to example 1;
FIG. 4 is a local anomalous resistivity inversion profile of example 1;
FIG. 5 is a cross-section for explaining the known seismic results of example 1.
Detailed Description
The present invention will be described in detail below with reference to the accompanying drawings.
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention will be described in further detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.
Example 1
s1, acquiring the wide area apparent resistivity after inversion, and regarding the wide area apparent resistivity as the superposition of the area abnormal resistivity and the local abnormal resistivity;
s2, carrying out multiple iterative cutting on the wide area apparent resistivity by adopting a dynamic improved interpolation cutting operator, and separating the area abnormal resistivity;
and S3, calculating the difference between the wide area apparent resistivity and the regional abnormal resistivity, and outputting the difference as the local abnormal resistivity.
The dynamic modified interpolation operator is as follows:
wherein, R (i, j) is the regional abnormal resistivity value at the point (i, j) after iteration, Z (i, j) is the original wide-area apparent resistivity value at the point (i, j), and the R (i, j) calculated by the current iteration is substituted into the Z (i, j) calculated by the next iteration;
NxandNzalong the x-axis and for the calculation windows, respectivelyzRadius of axis, l is the order number of calculation points within the calculation window, N ═ 2Nx +1) (2Nz +1) is the total number of calculation points of the calculation window,
to calculate the average apparent resistivity within the window, [ Fx(i,j)]Is the x-direction non-linearity; [ F ]z(i,j)]Is the z-direction non-linearity.
Generally speaking, the larger the calculation window is, the larger the range of the regional abnormality reflected by the sliding average value is, the more the iteration times are, the smoother the obtained regional abnormal resistivity is, the more obvious the local abnormal resistivity is, and if the iteration is repeated, the false abnormality caused by fixed interference and random noise can be effectively suppressed, so that the local abnormality is highlighted. The local abnormal apparent resistivity is calculated through the step S3, the reflected abnormal information is extracted and analyzed, and then the low and slow abnormal zone which is difficult to find is extracted, and particularly weak abnormality caused by local fracture, structure and thin layer can be effectively identified.
In practical application, in the process of separating the wide-area electromagnetic inversion resistivity anomaly by adopting a dynamic improved interpolation cutting operator, the known geological data and the inversion resistivity data are sufficiently combined to select proper calculation window parameters and iteration times so as to relatively accurately separate the regional anomaly resistivity and the local anomaly resistivity.
When b (i, j) is calculated: when Δ BxIf (i, j) is 0, if
Then b (i, j) is 1; if it is
Then b (i, j) is 0;
when calculating c (i, j), when Δ BzIf (i, j) is 0, if
Then c (i, j) is 1; if it is
Then c (i, j) becomes 0.
In order to calculate the regional abnormality of all the points (i, j), the boundary region is subjected to edge expansion processing, so that each point (M, N) within the cutting radius range of the boundary can be subjected to interpolation cutting at (i ═ Nx, -Nx +1, …, M + Nx; j ═ Nz, -Nz +1, …, N + Nz). The region extension methods that can be used include region field extension (RFM), cosine extension (cosine method/CM), polynomial fitting (polynomial fitting), minimum curvature extension (MCM).
The iteration exit conditions of the multiple iteration cutting are as follows: lim (small)n→∞max|Rn(i,j)-Rn-1(i, j) | is less than or equal to epsilon, wherein Rn-1(i, j) is the regional anomalous resistivity value of the previous iteration.
Step S3 calculates local anomaly resistivity ρOffice＝Z(i,j)-Rn(i,j)，Rn(i, j) is the regional abnormal resistivity value at point (i, j) after the final iteration.
The method is used for eliminating the influence of the underground regional field on the local resistivity anomaly and highlighting the structure and fracture development condition of the local stratum structure.
The method is used for further tracking and depicting the thin layer extension on the basis of the construction and the fracture of the local stratum structure by taking the local abnormal resistivity profile output in the step S3 as a support.
The following describes a specific research application case adopting the wide-area electromagnetic method local anomaly separation method. The electrical characteristics of the formation in the electrical logging data of the study area are shown in the following table.
Resistivity statistical table for different strata in research area
From the view point of formation petrophysical characteristics, shale has typical low-resistance characteristics relative to limestone, siltstone and dolomite. Wherein, the couchgrass group of the overlying strata, the Cyrtrya group and the pagoda group of the underlying strata of the first section of the target stratum, the quincunx group show obvious high resistance characteristics and are regarded as high resistance mark layers. From the physical property analysis, the target layer and the surrounding rock have obvious electrical property difference, and a good physical property foundation is provided for developing electromagnetic exploration.
Data acquisition
A certain measuring line was arranged, the length of which was 35.4km and the dot pitch was 50 m. In order to reach the expected detection depth and the actual working time schedule, n is 7, that is, 2 with 7 frequencies are transmitted at a timenA series of pseudo-random signals, receiving 7 different depth electrical signals simultaneously. The measurement frequency is 0.011-8192 Hz, the total frequency points are 54, and the method is suitable for detecting a target body ranging from the earth surface to the underground 5 km. The emitting current is 100A, the emitting voltage is 600-900V, the transmitting-receiving distance is about 20km, and 2 emitting sources (A1-B1 and A2-B2) are arranged to cover the L4 line. A horizontal electric dipole source E _ Ex wide-area electromagnetic method measuring device is adopted, wherein the length of the horizontal electric dipole source is changed by 1.5 km-3 km.
Data processing
The data processing flow chart is shown in fig. 2, and mainly comprises the steps of field original data processing, indoor data preprocessing, two-dimensional inversion imaging, local abnormal resistivity separation and the like. The traditional apparent resistivity inversion result is obtained by processing a wide-area electromagnetic data processing module in a 'heavy magnetoelectricity three-dimensional inversion imaging interpretation integrated system (GME3 DI-V6.2)' which is autonomously developed by professor Daishikun of wide-area electromagnetic subject group of Zhongnan university.
The method comprises the steps of firstly, on-site original data processing, namely, format conversion and normalization processing (current normalization and device coefficient normalization) are carried out on the original data, wide area apparent resistivity is calculated, amplitude abnormality and the like of each point are solved, and obvious interference frequency points and distortion point data are removed.
And secondly, indoor data preprocessing, namely qualitatively analyzing a wide area apparent resistivity curve and a frequency-apparent resistivity section by combining a known geoelectrical structure, and performing smooth filtering, flying point or flying section editing, static displacement correction and the like on single frequency point or single channel resistivity data.
And thirdly, two-dimensional inversion imaging, namely performing one-dimensional continuous medium inversion imaging on the processed data to obtain a depth-apparent resistivity inversion profile as an initial model, correcting the initial model by combining known geological data or other earth physical data such as well-seismic data and the like, and finally performing two-dimensional wide-area electromagnetic two-dimensional inversion imaging processing on the basis of the model to obtain a final inversion resistivity profile.
And fourthly, separating local abnormal resistivity, namely performing abnormal separation treatment on the reversed resistivity according to the wide-area electromagnetic abnormal separation method provided by the embodiment, eliminating the influence of the underground region field on the local resistivity abnormality, and highlighting the structure and fracture development condition of the local stratum structure.
And comprehensively explaining that the thin layer extension is further tracked and carved on the basis of the structure and the fracture of the local stratum structure by taking the local abnormal separation section as a support. And finally, synthesizing the abnormal separation result and the conventional resistivity profile to finely explain the comprehensive geology.
Effect analysis
In order to further verify the effectiveness and reliability of the wide-area electromagnetic method anomaly separation method provided by the embodiment, a certain measuring line is taken as an example, the distribution characteristics of geological structures, faults and thin layers disclosed by a traditional apparent resistivity profile and a local anomaly profile after anomaly separation are compared and analyzed, and the comparison and research are carried out with a known seismic interpretation profile. Fig. 3 is a traditional apparent resistivity inversion section of the survey line, fig. 4 is a local anomaly section separated by the anomaly, fig. 5 is a known seismic result explanation section, and the comparative analysis result is as follows:
as can be seen from FIG. 3, the electrical layer has a syncline structure with shallow ends and deep middle, and the change rule of the electrical layer is low-high-low-sub-high-low-medium resistance-high resistance sequentially from top to bottom within 5 km. The stope system overlying stratum couchgrass group (P2m) to Cyrtz group (P2q) are characterized by high resistance, the underlying stratum Ordovician pagoda group (O2b) is characterized by medium resistance, the main exploration stratum stope system (S) to Ordovician quincunx group (O3w) is characterized by low resistance, and the structural form reflected by the general apparent resistivity is basically similar to that of the known seismic result interpretation profile stratum.
Due to the overlapping of the local field and the local anomalous field, it is difficult to extract and finely analyze local information only by interpreting the conventional apparent resistivity. In particular, the fault property and the thin layer are not fine enough, and the deep hidden local geological structure information is difficult to be highlighted. As can be seen from FIG. 4, after the influence of the local apparent resistivity field is eliminated by the anomaly separation technique, the local electrical anomaly characteristics are more obvious, and particularly, the electrical layer positions displayed in the areas 1 and 3 are obviously discontinuous and have obvious electrical layer position dislocation. Due to the influence of the volume effect, local electrical anomaly characteristics of the traditional apparent resistivity inversion profile (shown in figure 3) are distributed in a bulk shape, and fault properties are difficult to accurately describe. And (3) cumulatively deducing 9 reverse faults according to local abnormal features of the graph 4, tracking and depicting the Longyi-Wufeng group stratum, and effectively identifying 1 low-resistance target thin layer. Analyzing and comparing the result chart of fig. 4, the following recognition is obtained:
(1) and (3) low-resistance thin layer extraction, namely, the Longyi-Wufeng group stratum is used as a target layer of the exploration, has the characteristics of deep burial and thin thickness, and simultaneously, due to the influence of a fault, the resistivity distribution near the fault is disordered and irregular, so that great misleading is brought to thin layer division, and the fine explanation is difficult to achieve. The anti-observation anomaly separation local result diagram 4 can track the deep low-resistance thin layer according to local anomaly characteristics no matter in a complex-structured region or a simple region, so that the identification and extraction of the low-resistance thin layer information are more accurate, and the interpretation result is more accurate and reliable finally.
(2) The structural configuration reflected in FIG. 4 is substantially consistent with that disclosed by a three-dimensional earthquake (FIG. 5).
(3) Three-dimensional earthquake (fig. 5) reveals that the areas 1, 2 and 3 are obvious complex structural zones, namely fracture development zones. In the technical result of the backsight anomaly separation, the formation region 3 near the 600 point is influenced by the faults of Rou 7 and Rou 31 to form local ground barrier characteristics, and the local ground barrier characteristics are basically consistent with the three-dimensional seismic result. The stratum area 1 near the point 60 is affected by F1, lamp 2 and lamp 1 faults, the ground surface forms obvious push-over characteristics, meanwhile, the deep part forms obvious ground barrier characteristics, the figure 5 is reflected, the data of the same phase axis of the earthquake result is poor, and the fault lamp 2 and the lamp 1 are displayed weakly. Region 2 is shown not visible in fig. 3, but clearly in fig. 4, presumably as an intralevel hidden fault of interest, substantially coinciding with fig. 5.
The above description is only for the purpose of illustrating the preferred embodiments of the present invention and is not to be construed as limiting the invention, and any modifications, equivalents and improvements made within the spirit and principle of the present invention are intended to be included within the scope of the present invention.
Claims (9)
1. A local anomaly separation method of a wide-area electromagnetic method is characterized by comprising the following steps:
s1, acquiring the wide area apparent resistivity after inversion, and regarding the wide area apparent resistivity as the superposition of the regional abnormal resistivity and the local abnormal resistivity;
s2, carrying out multiple iterative cutting on the wide area apparent resistivity by adopting a dynamic improved interpolation cutting operator, and separating the area abnormal resistivity;
and S3, calculating the difference between the wide area apparent resistivity and the regional abnormal resistivity, and outputting the difference as the local abnormal resistivity.
2. The method of claim 1, wherein: the dynamic modified interpolation operator is as follows:
wherein, R (i, j) is the abnormal resistivity value of the area at the point (i, j) after iteration, Z (i, j) is the original wide-area apparent resistivity value at the point (i, j), and the R (i, j) calculated by the current iteration is substituted into the Z (i, j) calculated by the next iteration;
nx and Nz are radii of the calculation window along the x-axis and z-axis, respectively, l is a calculation point number within the calculation window, N ═ 2Nx +1) (2Nz +1) is a total number of calculation points of the calculation window,
to calculate the average apparent resistivity within the window, [ Fx(i,j)]Is the x-direction non-linearity; [ F ]z(i,j)]Is the z-direction non-linearity.
3. The method of claim 2, wherein: when b (i, j) is calculated: when Δ BxIf (i, j) is 0, if
Then b (i, j) is 1; if it is
Then b (i, j) is 0;
when calculating c (i, j), when Δ BzIf (i, j) is 0, if
Then c (i, j) is 1; if it is
Then c (i, j) becomes 0.
4. The method of claim 2, wherein: and carrying out edge expansion processing on the boundary area, so that each point (M, N) within the cutting radius range in the boundary can carry out interpolation cutting at (i ═ Nx, -Nx +1, …, M + Nx; j ═ Nz, -Nz +1, …, N + Nz).
5. The method of claim 4, wherein: the iteration exit conditions of the multiple iteration cutting are as follows: lim (small)n→∞max|Rn(i,j)-Rn-1(i, j) | is less than or equal to epsilon, wherein Rn-1(i, j) is the regional anomalous resistivity value of the previous iteration.
6. The method according to claims 1-5, characterized in that: the method is used for eliminating the influence of the underground regional field on the local resistivity anomaly and highlighting the construction and fracture development conditions of the local stratum structure.
7. The method according to claims 1-5, characterized in that: the method is used for further tracking and depicting the thin layer extension on the basis of the construction and the fracture of the local stratum structure by taking the local abnormal resistivity profile output in the step S3 as a support.
8. A computer device comprising a memory, a processor and a computer program stored on the memory and executable on the processor, wherein the processor implements the method of any one of claims 1 to 7 when executing the computer program.
9. A computer-readable storage medium, characterized in that the computer-readable storage medium stores a computer program for executing the method of any one of claims 1 to 7.
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