CN112859181B - 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 of a wide-area electromagnetic method, which comprises the following steps: s1, acquiring inverted wide area apparent resistivity, and regarding the wide area apparent resistivity as superposition of regional abnormal resistivity and local abnormal resistivity; s2, performing repeated iterative cutting on the wide area apparent resistivity by adopting a dynamic improved interpolation cutting operator, and separating out the regional 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 method can weaken the fuzzy boundary between strata, so that the boundary between adjacent electrical strata is more obvious, and for a geological complex region, the final interpretation result is more accurate and reliable due to abnormal separation, and meanwhile, the inversion imaging precision is supported and improved.
Description
Technical Field
The invention relates to the field of geophysical exploration, in particular to a local anomaly separation method of 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 an artificial source observation area, and is very suitable for multi-layer, large-area and large-depth shale gas fine exploration under complex construction and fracture development conditions. However, for complex structures and fracture development zones, the resistivity abnormality often changes irregularly, which brings trouble to the traditional electric method depending on resistivity interpretation and severely restricts the fine interpretation of the electromagnetic method. The actual observed resistance parameters are often due to topography fluctuations, non-uniformity of the underground medium, overlapping of various rocks, crisscross fault fissures, or comprehensive reflections of ore bodies filling them. The field observation actual parameters are usually superposition of local abnormality, regional background and related noise, so that how to effectively separate the local abnormality field, regional background field and noise, and the accuracy and precision of result interpretation are improved, and the method has important research value. Various methods have been proposed 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 etal, 2005; gao Jinghuai, 2003,2004; alumbiugh, 1996; jing Jianen, 2012, song Shougen, 1995; weng Ai, 2012; wu Xin, 2015), which all belong to improving electromagnetic imaging accuracy using indirect methods, there is room for improvement in both accuracy and reliability of result interpretation.
Disclosure of Invention
Aiming at the defects of the prior art scheme, the invention provides a local anomaly separation method for a wide-area electromagnetic method, which is used for analyzing inversion imaging data of the wide-area electromagnetic method and directly separating local anomalies of the wide-area electromagnetic from apparent resistivity values of the wide-area electromagnetic method.
In order to achieve the above purpose, the technical scheme adopted by the invention is as follows:
a local anomaly separation method of a wide-area electromagnetic method comprises the following steps:
s1, acquiring inverted wide area apparent resistivity, and regarding the wide area apparent resistivity as superposition of regional abnormal resistivity and local abnormal resistivity;
s2, performing repeated iterative cutting on the wide area apparent resistivity by adopting a dynamic improved interpolation cutting operator, and separating out the regional 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 dynamic improved interpolation operator is:
wherein R (i, j) is the abnormal resistivity value of the region at the point (i, j) after iteration, Z (i, j) is the initial wide area apparent resistivity value at the point (i, j), and R (i, j) calculated by the current iteration is substituted into Z (i, j) calculated by the next iteration;
Nx and Nz the calculation windows are along the x-axis and along the x-axis respectively z The radius of the axis, i is the number of the calculation points in the calculation window, n= (2nx+1) (2nz+1) is the total number of calculation points in the calculation window,to calculate the apparent resistivity average within the window, [ F x (i,j)]Is the x-direction nonlinearity; [ F z (i,j)]Is the z-direction nonlinearity.
Generally, 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 the local abnormal resistivity is obvious, and if the iteration is repeated, the false abnormality caused by fixed interference and random noise can be effectively suppressed, and the local abnormality is highlighted. The local abnormal apparent resistivity is obtained through calculation in the step S3, reflected abnormal information is extracted and analyzed, and then a low-speed abnormal zone which is difficult to find is extracted, so that weak abnormal caused by local fracture, structure and thin layer can be effectively identified.
Further, when b (i, j) is calculated: when DeltaB x When (i, j) =0, ifB (i, j) =1; if it isB (i, j) =0;
when c (i, j) is calculated, when ΔB z When (i, j) =0, ifC (i, j) =1; if->C (i, j) =0.
Further, edge expansion processing is performed on the boundary area, so that each point (M, N) within the boundary within the inner radius range can be subjected to interpolation cutting at the positions (i= -Nx, -nx+1, …, M+nx; j= -Nz, -nz+1, …, N+nz). The area edge-enlarging method includes area field edge-enlarging (regional field method/RFM), cosine edge-enlarging (cosine method/CM), polynomial fitting (polynomial fitting) and minimum curvature edge-enlarging (minimum curvature method/MCM).
Further, the iteration exit condition of the multiple iteration cutting is as follows: lim n→∞ max|R n (i,j)- R n-1 (i, j) ε, wherein R n-1 (i, j) is the region anomaly resistivity value of the previous iteration.
Further, the method is used for eliminating the influence of the underground area field on the local resistivity abnormality, and highlighting the structure and fracture development condition of the local stratum structure.
Furthermore, the method is used for further tracking and describing the thin layer extension based on the construction and fracture of the local stratum structure on the basis of the local abnormal resistivity profile output in the step S3.
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 described above when executing the computer program.
A computer readable storage medium storing a computer program for executing the above method.
The invention has the beneficial effects that:
according to the local anomaly separation method of the wide-area electromagnetic method, dynamic improved interpolation cutting operators are adopted to conduct anomaly separation of wide-area electromagnetic inversion resistivity, compared with an original wide-area electromagnetic apparent resistivity profile, fuzzy boundaries between strata can be weakened, boundaries between adjacent electrical layers are obvious, and interpretation results are closer to a ground model; the fault display is clearer, and the fault property and the position are more reliable and accurate; the edge effect on two sides of the end point of the measuring line is suppressed, so that the interpretation result can more truly reflect the actual geophysical model; the deep stratum relief structure form can be more clearly highlighted.
The local anomaly separation method of the wide-area electromagnetic method is based on effective inversion result data, and the local anomaly field information is further extracted and analyzed by effectively compressing the regional background field. For a geological structure complex region, the final interpretation result is more accurate and reliable due to abnormal separation, and meanwhile, the inversion imaging precision is supported and improved.
The local anomaly separation method of the wide-area electromagnetic method can be used as a supplementary means for interpretation of the conventional wide-area electromagnetic method data, can more fully mine implicit geological information in the information, can effectively improve interpretation accuracy and reliability of construction 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 electrical anomalies caused by local geological structures or thin layers on the basis, realizes accurate positioning of fracture structures and thin layers, and forms a set of method and technical system suitable for identifying shale layers in wide-area electromagnetic exploration under complex geological conditions. The technique has important supporting function for identifying shale layers in regions with complicated geological conditions in Chuan nan area and even nationwide, and has wide application prospect.
Drawings
FIG. 1 is a flow chart of a method for local anomaly isolation for wide area electromagnetic method of example 1;
FIG. 2 is a flowchart of the wide area electromagnetic method data processing according to embodiment 1;
FIG. 3 is a conventional resistivity inversion section of a line of example 1;
FIG. 4 is a local anomaly resistivity inversion profile for example 1;
FIG. 5 is a cross-section of a known seismic result interpretation of example 1.
Detailed Description
The present invention will be described in detail with reference to the accompanying drawings.
The present invention will be described in further detail with reference to the drawings and examples, in order to make the objects, technical solutions and advantages of the present invention more apparent. It should be understood that the specific embodiments described herein are for purposes of illustration only and are not intended to limit the scope of the invention.
Example 1
Embodiment 1 provides a local anomaly separation method of a wide area electromagnetic method, as shown in fig. 1, including:
s1, acquiring inverted wide area apparent resistivity, and regarding the wide area apparent resistivity as superposition of regional abnormal resistivity and local abnormal resistivity;
s2, performing repeated iterative cutting on the wide area apparent resistivity by adopting a dynamic improved interpolation cutting operator, and separating out the regional 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 improved interpolation operator is as follows:
wherein R (i, j) is the abnormal resistivity value of the region at the point (i, j) after iteration, Z (i, j) is the initial wide area apparent resistivity value at the point (i, j), and R (i, j) calculated by the current iteration is substituted into Z (i, j) calculated by the next iteration;
Nx and Nz the calculation windows are along the x-axis and along the x-axis respectively z The radius of the axis, i is the number of the calculation points in the calculation window, n= (2nx+1) (2nz+1) is the total number of calculation points in the calculation window,to calculate the apparent resistivity average within the window, [ F x (i,j)]Is the x-direction nonlinearity; [ F z (i,j)]Is the z-direction nonlinearity.
Generally, 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 the local abnormal resistivity is obvious, and if the iteration is repeated, the false abnormality caused by fixed interference and random noise can be effectively suppressed, and the local abnormality is highlighted. The local abnormal apparent resistivity is obtained through calculation in the step S3, reflected abnormal information is extracted and analyzed, and then a low-speed abnormal zone which is difficult to find is extracted, so that weak abnormal caused by local fracture, structure and thin layer can be effectively identified.
In practical application, in the process of carrying out wide-area electromagnetic inversion resistivity anomaly separation by adopting a dynamic improved interpolation cutting operator, proper calculation window parameters and iteration times are required to be selected by fully combining known geological data and inversion resistivity data so as to achieve relatively accurate separation of regional anomaly resistivity and local anomaly resistivity.
When calculating b (i, j): when DeltaB x When (i, j) =0, ifB (i, j) =1; if->B (i, j) =0;
when c (i, j) is calculated, when ΔB z When (i, j) =0, ifC (i, j) =1; if->C (i, j) =0.
In order to calculate the region anomalies of all points (i, j), the edge-enlarging process is performed on the boundary area, so that each point (M, N) within the boundary within the inner radius range can be subjected to interpolation cutting at the positions (i= -Nx, -nx+1, …, M+nx; j= -Nz, -nz+1, …, N+nz). The area edge-enlarging method includes area field edge-enlarging (regional field method/RFM), cosine edge-enlarging (cosine method/CM), polynomial fitting (polynomial fitting) and minimum curvature edge-enlarging (minimum curvature method/MCM).
The iteration exit condition of the multiple iteration cutting is as follows: lim n→∞ max|R n (i,j)-R n-1 (i, j) ε, wherein R n-1 (i, j) is the region anomaly resistivity value of the previous iteration.
Step S3 calculates the local abnormal resistivity ρ Office (bureau) ＝Z(i,j)-R n (i,j)，R n (i, j) is the region anomaly resistivity value at point (i, j) after the final iteration.
The method is used for eliminating the influence of the underground area field on the local resistivity abnormality, and highlighting the structure and fracture development condition of the local stratum structure.
The method is used for further tracking and describing the thin layer extension based on the construction and fracture of the local stratum structure by taking the local abnormal resistivity profile output in the step S3 as a support.
The specific research application cases of the local anomaly separation method adopting the wide-area electromagnetic method are described below. The formation electrical characteristics of the electrical logging data of the investigation region are shown in the following table.
Statistical table for resistivity of different stratum in research area
Shale has typical low resistance characteristics relative to limestone, siltstone and dolomite from the physical properties of the formation rock. Wherein, the upper stratum cogongrass mouth group-sauvignon group and the lower stratum pagoda group of the target layer dragon one-five peak group show obvious high resistance characteristics and are regarded as high resistance mark layers. According to the physical property analysis, the target layer and the surrounding rock have obvious electrical property difference, and a good physical property basis is provided for carrying out electromagnetic method exploration work.
Data acquisition
A certain line is arranged, the length of which is 35.4km, and the point distance is 50m. In order to achieve the desired detection depth and the requirements of the actual operating schedule, n=7 is adopted, i.e. 2 with 7 frequencies is emitted at one time n A series of pseudo-random signals are received simultaneously with 7 different depth electrical signals. The measurement frequency is 0.011-8192 Hz, 54 frequency points are used, and the method is suitable for detecting a target body ranging from the earth surface to the underground 5 km. The emission current is 100A, the emission voltage is 600-900V, the receiving and transmitting distance is about 20km, and 2 emission sources (A1-B1 and A2-B2) are distributed to cover the L4 line. The measuring device of the horizontal electric dipole source E_Ex wide area electromagnetic method 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 processed by adopting a wide area electromagnetic data processing module in a heavy-magnetic-electricity three-dimensional inversion imaging interpretation integrated system (GME 3 DI-V6.2) which is taught by a wide area electromagnetic subject group Dai Shikun of the university of south China.
(1) And (3) processing the original data in the field, namely performing format conversion, normalization processing (current normalization and device coefficient normalization) on the original data, calculating the wide area apparent resistivity, solving amplitude anomalies of each point and the like, and removing obvious interference frequency point and distortion point data.
(2) Indoor data preprocessing, namely qualitatively analyzing a wide area apparent resistivity curve and a frequency-apparent resistivity section by combining a known electrical structure, and performing smooth filtering, flying spot or flying segment editing, static displacement correction and other processing on single-frequency point or single-channel resistivity data.
(3) And performing two-dimensional inversion imaging, namely performing one-dimensional continuous medium inversion imaging on the processed data, taking the obtained 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-earthquake and the like, and finally performing two-dimensional wide area electromagnetic two-dimensional inversion imaging processing based on the model to obtain a final inversion resistivity profile.
(4) And (3) separating local abnormal resistivity, namely performing abnormal separation treatment on the reverse resistivity according to the wide-area electromagnetic abnormal separation method provided by the embodiment, eliminating the influence of the underground area field on the local resistivity abnormality, and highlighting the structure and fracture development condition of the local stratum structure.
(5) And comprehensively explaining, namely further tracking and describing the thin layer extension based on the construction and fracture of the local stratum structure by taking the local abnormal separation section as a support. And finally, combining abnormal separation results and a conventional resistivity section to make fine explanation on the combined 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, taking a certain measuring line as an example, the distribution characteristics of a geological structure, faults and thin layers revealed by a traditional apparent resistivity section and a local anomaly section after anomaly separation are compared and analyzed, and compared with a known seismic interpretation section for research. Fig. 3 is a conventional apparent resistivity inversion section of the test line, fig. 4 is a local anomaly section after the anomaly separation, fig. 5 is an interpretation section of known seismic achievements, and the comparison analysis results are as follows:
as can be seen from FIG. 3, the electrical layer has a syncline structure characteristic with shallow ends and deep middle, and the electrical layer within 5km has a low-high-low-secondary high-low-middle-resistance-high-resistance change rule from top to bottom. The morphology of the general visual resistivity reflection is basically similar to that of the known seismic achievement interpretation section stratum.
Because of the overlapping of the regional field and the local anomaly field, it is difficult to extract and finely analyze the local information only by interpretation according to the conventional apparent resistivity. Especially fault properties and thin layers are not fine enough, and deep hidden local geological structure information is difficult to be highlighted. As can be seen from FIG. 4, after the influence of the region apparent resistivity field is removed by adopting the anomaly separation technology, the local electrical anomaly characteristics are more obvious, and particularly, the electrical horizons displayed in the region 1 and the region 3 are obviously discontinuous and have obvious electrical horizon dislocation. Due to the influence of volume effect, the local electrical abnormal characteristics of the conventional apparent resistivity inversion section (figure 3) are distributed in a bulk form, and the fault properties are difficult to accurately describe. According to the local abnormal characteristic accumulation deduction reverse faults 9 of the figure 4, the stratum of the Dragon-five peak group is tracked and described, and 1 low-resistance target thin layer is effectively identified. Analysis of the result graph of fig. 4 gave the following recognition:
(1) And (3) extracting a low-resistance thin layer, namely taking a Dragon-five peak group stratum as a target layer of the exploration, wherein the Dragon-five peak group stratum has the characteristics of deep burial and thin thickness, and meanwhile, the resistivity distribution around the fault is disordered and irregular due to the influence of the fault, so that larger misleading is brought to the thin layer division, and fine interpretation is difficult to achieve. The inverse abnormal separation local achievement map 4, whether the structure is a complex region or a simple region, can track the deep low-resistance thin layer according to the local abnormal characteristics, is more accurate in identifying and extracting the low-resistance thin layer information, and finally enables the interpretation achievement to be more accurate and reliable.
(2) The structural morphology reflected in fig. 4 is substantially identical to that revealed by the three-dimensional earthquake (fig. 5).
(3) Fault distribution three-dimensional earthquake (figure 5) reveals that region 1, region 2 and region 3 are distinct complex structural zones, namely fracture development zones. And (3) reversely observing abnormal separation technical results, wherein the stratum area 3 near 600 points is influenced by the faults of the Row 7 and the Row 31, so that local barrier characteristics are formed, and the local barrier characteristics are basically consistent with three-dimensional earthquake results. The stratum area 1 near the 60 points is affected by faults of F1, lamps 2 and lamps 1, obvious pushing and covering features are formed on the earth surface, obvious barrier features are formed in the deep part, the earthquake achievement phase axis data of the anti-observation figure 5 are poor, and the faults of lamps 2 and 1 are weak. Region 2 is shown not in fig. 3, but clearly in fig. 4, presumably as a hidden fault within the layer of interest, substantially coincident with fig. 5.
The foregoing description of the preferred embodiments of the invention is not intended to be limiting, but rather to cover all modifications, equivalents, and alternatives falling within the spirit and principles of the invention.
Claims (8)
1. The local anomaly separation method of the wide-area electromagnetic method is characterized by comprising the following steps of:
s1, acquiring inverted wide area apparent resistivity, and regarding the wide area apparent resistivity as superposition of regional abnormal resistivity and local abnormal resistivity;
s2, performing repeated iterative cutting on the wide area apparent resistivity by adopting a dynamic improved interpolation cutting operator, and separating out the regional abnormal resistivity;
s3, calculating the difference between the wide area apparent resistivity and the regional abnormal resistivity to be used as a local abnormal resistivity output;
the dynamic improved interpolation cutting operator is as follows:
a(i,j)＝b(i,j)+c(i,j)，
ΔB x (i,j)＝Z(i+Nx,j)-Z(i-Nx,j)，
ΔB z (i,j)＝Z(i,j+Nz)-Z(i,j-Nz)；
wherein R (i, j) is the abnormal resistivity value of the region at the point (i, j) after iteration, Z (i, j) is the original wide area apparent resistivity value at the point (i, j), and R (i, j) calculated by the current iteration is substituted into Z (i, j) calculated by the next iteration;
nx and Nz are the radii of the calculation window along the x-axis and z-axis, respectively, l is the calculation point number within the calculation window, n= (2nx+1) (2nz+1) is the total number of calculation points of the calculation window,to calculate the apparent resistivity average within the window, [ F x (i,j)]Is the x-direction nonlinearity; [ F z (i,j)]Is the z-direction nonlinearity.
2. The method according to claim 1, characterized in that: when calculating b (i, j): when DeltaB x When (i, j) =0, ifB (i, j) =1; if->B (i, j) =0;
when c (i, j) is calculated, when ΔB z When (i, j) =0, ifC (i, j) =1; if->C (i, j) =0.
3. The method according to claim 1, characterized in that: the boundary region is subjected to an edge-enlarging process, so that points within the boundary within the range of the radius (M, N) at (i= -Nx, -Nx +1, the M is equal to Nx; j= -Nz, -nz+1, ·, n+nz) can be interpolated cut.
4. A method according to claim 3, characterized in that: the iteration exit condition of the multiple iteration cutting is as follows:wherein R is n-1 (i, j) is the region anomaly resistivity value of the previous iteration.
5. The method according to any one of claims 1-4, wherein: the method is used for eliminating the influence of the underground area field on the local resistivity abnormality and highlighting the structure and fracture development condition of the local stratum structure.
6. The method according to any one of claims 1-4, wherein: the method is used for further tracking and describing the thin layer extension based on the construction and fracture of the local stratum structure by taking the local abnormal resistivity profile output in the step S3 as a support.
7. A computer device comprising a memory, a processor and a computer program stored on the memory and executable on the processor, characterized in that the processor implements the method of any of claims 1 to 6 when executing the computer program.
8. 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 6.
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