1. A magnetotelluric multi-scale and multi-period detection method is characterized in that: the detection method adopts different observation durations to acquire magnetotelluric field data according to different measuring point intervals; and for the measuring points with small point intervals, short observation time is adopted, and for the measuring points with large point intervals, long observation time is adopted, so that the target of obtaining the detection result equivalent to that obtained by the traditional observation mode under the condition of less total observation time is realized.

2. A magnetotelluric multi-scale, multi-period detection method as defined in claim 1, wherein: the detection method comprises the following steps:

s1, designing the measuring points of the detection area according to the geological background, the topographic environment and the existing measuring point distribution of the detection area to obtain the position distribution of the measuring points of the whole area meeting the task requirement;

s2, dividing the measuring points into a plurality of groups according to the measuring point spacing, wherein the groups are An, An-1, An-2, … and A1, the group of An represents the measuring point group with the maximum point spacing, the group of A1 represents the measuring point group with the minimum point spacing, and the rest are analogized in sequence;

s3, designing a larger detection depth for the point groups with larger point spacing, so that a lower detection frequency and longer observation time are needed, and designing a smaller detection depth for the point groups with smaller point spacing, so that the detection frequency can be higher, and the observation time can be shorter; that is, the An group is designed to have the maximum detection depth, the minimum detection low-frequency value and the longest observation time, while the A1 group has the minimum detection depth, the maximum detection low-frequency value and the shortest observation time, and the rest are analogized in turn;

s4, the high-frequency parts of all the measuring point groups are completely consistent and do not change due to different point intervals, namely the resolution of the shallow part is not lost;

s5, performing field construction operation according to the grouped design time to obtain a magnetotelluric time sequence of each measuring point, and forming a multi-scale and multi-time-period field acquisition data set;

s6, processing the magnetotelluric time sequence acquired by each measuring point to obtain a magnetotelluric observation response data set of each measuring point, and generally meeting the data design requirements that the highest frequency of all measuring point groups is the same, the effective frequency value of An group can reach the lowest, and the effective frequency value of An 1 group is the highest in the low-frequency direction;

and S7, performing inversion by using the obtained magnetotelluric observation response data set to obtain an inversion result.

3. A magnetotelluric multi-scale, multi-period detection method as defined in claim 2, wherein: according to the observation carried out in the mode, the effective observation frequency of each group of measuring points needs to be determined according to the measuring point spacing and the resistivity value, namely the effective observation frequency needs to be determined according to the skin depth and the measuring point spacing, and the skin depth is generally required to be more than 3 times of the measuring point spacing; when a measuring point group with measuring point spacing of 10km is designed, the lowest effective frequency can be required to reach 1/1000Hz and the observation time is more than 16 hours under the condition that the average earth resistivity is 10 ohm meters; when measuring points are designed at the distance of 5km, the lowest effective frequency can be required to reach 1/100Hz, and the observation time is required to be more than 8 hours.

4. A magnetotelluric multi-scale, multi-period detection method as defined in claim 2, wherein: the magnetotelluric two-dimensional and three-dimensional inversion results obtained in the mode are consistent with the two-dimensional and three-dimensional inversion results acquired by all traditional measuring point data according to the longest time of An group, namely magnetotelluric detection work is carried out by using the technology of the invention, the field observation time is saved, and the detection precision is not lost.

Background

Magnetotelluric (MT) is an important geophysical exploration method for studying the electrical structure of the earth by using a natural electromagnetic field as a field source. The basic principle is as follows: according to the principle that electromagnetic waves with different frequencies have different skin depths in a conductive medium, an earth electromagnetic response sequence from high frequency to low frequency is measured on the earth surface, and an electrical structure of the earth from shallow to deep is obtained through related data processing. It has the advantages of no shielding by high resistance layer, strong resolving power to high conducting layer, strong transverse resolving power, and the like, and also has the defects of rapid weakening of longitudinal resolving power along with the increase of depth, and the like. The method mainly comprises the following steps: data acquisition, data processing and inversion interpretation.

In data acquisition, a magnetotelluric observation instrument is arranged in the field, underground electric field information changing along with time is acquired through orthogonal paired non-polarized electrodes, underground magnetic field information changing along with time is acquired through orthogonal horizontal and vertical magnetic rods, an underground electromagnetic field time sequence is acquired, and data acquisition of a measuring point is completed. According to research requirements, observation of a plurality of measuring points can be designed to be collected along measuring lines, and data collection can also be carried out according to a measuring network, so that underground electromagnetic field information of sections and areas can be obtained.

In the data processing, an electromagnetic field time series acquired by field data is converted into a power spectrum (Spectra) of a frequency domain by processing, and various MT parameters such as Impedance tensor (Z), dip vector (T), Apparent resistivity (R), Phase (P), and two-dimensional characteristic quantity are calculated based on the power spectrum, and distortion analysis, correction, and the like are performed.

In the inversion interpretation, an inversion model is constructed based on the measuring points, data such as impedance tensor, dip vector, apparent resistivity, phase and the like are selected, an inversion algorithm software is used for calculating to obtain the underground electrical structure of the research area, and then the geophysical and geological interpretation is carried out on the research area based on the inversion result and by combining the existing geology and other geophysical data.

The MT method has a volume effect. The volume effect is the data acquired by the measuring point, and includes not only the information of the electrical anomaly body right below the measuring point, but also the information of the electrical anomaly body in the horizontal direction near the measuring point. Thus, for a certain frequency, how deep longitudinally it probes, how far laterally it affects (fig. 1). When the measuring points are distributed densely, if the detection range of a certain frequency is far greater than the distance between two adjacent measuring points, redundant information exists in the detection range of the two measuring points at the frequency. The low-frequency data of the data set is thinned according to a certain measuring point distance, and the low-frequency data form of the whole data set is not influenced (shown in the attached figures 2 and 3).

The prior art has the following problems:

the MT method increases the detection depth and the resolution decreases rapidly with decreasing frequency. In a high-frequency range, because the detection range is small and the resolution is high, the measurement points are densely distributed to obtain effective detection constraint; in a low-frequency range, because the detection range is large, the resolution ratio is reduced rapidly, and excessively dense measuring points have no significance for detection constraint. Because the assumption of the inverted model is different from the actual earth structure, data of excessively dense measuring points in a low-frequency part can be redundant, and in the inversion calculation, the conditions of contradictory measuring points can be greatly generated, because the response of all measuring points is difficult to simulate by the inverted model.

The acquisition of magnetotelluric low-frequency data requires longer observation times. The longer the cycle, the exponentially longer the required data acquisition time to obtain high quality data. Too dense low frequency stations result in a significant increase in observation costs. This is particularly the case when the encrypted detection is performed for some observation areas where some measuring points exist. The conventional uniform observation mode of all the measuring point frequencies can cause a great waste of observation time and expenses.

Disclosure of Invention

The invention aims to solve the problems and provides a magnetotelluric multi-scale and multi-period detection method which is simple to operate and has improved efficiency.

In order to achieve the purpose, the technical scheme of the invention is as follows:

a magnetotelluric multi-scale, multi-time interval detection method, this detection method adopts different measuring point intervals, different observation duration to carry on the field data acquisition of magnetotelluric; for the measuring points with small point spacing, the observation time is short, and for the measuring points with large point spacing, the observation time is long, so that the detection result equivalent to that of the traditional observation mode can be obtained with less observation time.

Further, the detection method comprises the following steps:

s1, carrying out whole-region measuring point distribution design according to the geological background, the topographic environment and the existing measuring point distribution of the detection region to obtain measuring point positions meeting the detection requirements;

s2, dividing the measuring points into a plurality of groups according to the measuring point spacing, wherein the groups are An, An-1, An-2, … and A1, the group of An represents the measuring point group with the maximum point spacing, the group of A1 represents the measuring point group with the minimum point spacing, and the rest are analogized in sequence;

s3, designing a larger detection depth for the point group with larger point distance, thus needing lower detection frequency and longer observation time, and designing a smaller detection depth for the point group with smaller point distance, thus the detection frequency can be higher and the observation time can be shorter. That is, the An group is designed to have the largest detection depth, the lowest required detection frequency and the longest observation time, while the A1 group has the smallest detection depth, the highest required detection frequency and the shortest observation time, and the rest are analogized in sequence;

s4, the highest frequencies of all the measuring point groups are completely consistent, and the variation is avoided due to the difference of the point intervals, namely the resolution of the shallow part is not lost;

s5, performing field construction operation according to the grouped design time to obtain a magnetotelluric time sequence of each measuring point, and forming a multi-scale and multi-time-period field acquisition data set;

s6, processing the magnetotelluric time sequence acquired by each measuring point to obtain a magnetotelluric observation response data set of impedance tensor, dip vector, apparent resistivity, phase and the like of each measuring point, and generally meeting the data design requirements that the highest frequency of all measuring point groups is the same, the effective frequency value of An group can reach the lowest, and the effective frequency value of An A1 group is the highest in the low-frequency direction;

and S7, performing inversion by using the obtained magnetotelluric observation response data set to obtain an inversion result.

Furthermore, the magnetotelluric two-dimensional and three-dimensional inversion results obtained in the mode are consistent with two-dimensional and three-dimensional inversion results acquired by all traditional measuring point data according to the longest time of An group, namely the magnetotelluric detection work is carried out by utilizing the technology of the invention, the field observation time is saved, and the detection precision is not lost.

Compared with the prior art, the invention has the advantages and positive effects that:

aiming at the problems of high cost and low efficiency of the conventional data acquisition method, the invention provides an alternative station arrangement scheme which adopts long and short observation time according to large and small distances between measurement points; in the method, measuring points are divided into different observation groups according to different design intervals, the observation group with large point distance has a large detection range, low data frequency and long observation time; the observation group with small point distance has small detection range, high data frequency and short observation time. The field construction is carried out according to the observation technology, the data acquisition efficiency is improved on the basis of ensuring the accuracy of the inversion result, the data acquisition time is saved, and the utilization rate of an observation instrument is improved.

Drawings

In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly described below, and it is obvious that the drawings in the following description are only some embodiments of the present invention, and for those skilled in the art, other drawings can be obtained according to these drawings without creative efforts.

FIG. 1 is a simplified diagram of detection ranges of different frequencies at a single point;

FIG. 2 is a schematic diagram of comparison of phase fitting data of different measuring point intervals in two-dimensional theoretical response, wherein A is a schematic diagram of a two-dimensional theoretical model; b is a measuring point xy direction phase (Pxy) fitting data profile with the distance of 0.5km and full frequency band (1000 Hz-1/10000 Hz); c is a cross-sectional view of xy direction phase fitting data of measuring points with the distance of 1km, a full frequency band (1000 Hz-1/10000 Hz), the distance of 0.5km and a high frequency band (1000 Hz-1/100 Hz); d is a cross-sectional view of data fitted by a phase (Pyx) in a direction of a measuring point yx at a distance of 0.5km and in a full frequency band (1000 Hz-1/10000 Hz); e is a phase fitting data profile in the direction of a yx of measuring points at a distance of 1km and a full frequency band (1000 Hz-1/10000 Hz) and measuring points at a distance of 0.5km and a high frequency band (1000 Hz-1/100 Hz);

FIG. 3 is a comparison graph of low frequency (0.009 Hz) three-dimensional theoretical response data for phase fitting of different station spacings; wherein A is a schematic top plane projection diagram of the theoretical model; b is a schematic diagram of the top plane projection of the theoretical model 1 omega m abnormal body; c is a section view at the ash line of the theoretical model; d is a plane projection schematic diagram of xy direction phase (Pxy) fitting data 0.009Hz of measuring points at the interval of 2.5 km; e is a plane projection schematic diagram of the data 0.009Hz of Pxy fitting at the measuring points with the distance of 5 km; f is a plane projection schematic diagram of the phase (Pyx) in the yx direction of a measuring point at the distance of 2.5km and fitting data of 0.009 Hz; g is a plane projection schematic diagram of the fitted data of the measuring points Pyx at the distance of 5km at 0.009 Hz;

FIG. 4 is a schematic view of the distribution of the measuring points in a multi-scale and multi-period detection method; a is a measuring point distribution diagram of two-dimensional detection, a square represents a measuring point with a large point interval, the lowest frequency of the measuring point is low, a star represents a measuring point with a small point interval, and the lowest frequency of the measuring point is high; b is a measuring point distribution diagram of three-dimensional detection, a square represents a measuring point with large point spacing, the lowest frequency of the measuring point is lower, a star represents a measuring point with small point spacing, and the lowest frequency of the measuring point is higher;

FIG. 5 is a simplified schematic diagram of the range of influence of the distance between different measuring points; wherein A is a section diagram of a high-frequency detection range of a measuring point at a two-dimensional measuring line section interval L; b is a two-dimensional measuring line section space 2L measuring point low-frequency range detection range section diagram; c is a three-dimensional measuring net spacing L measuring point high-frequency detection range plan; d is a three-dimensional measuring net space 2L measuring point low-frequency detection range plan view;

FIG. 6 is a flow diagram of a multi-scale, multi-session detection technique;

FIG. 7 is a comparison of two-dimensional theoretical response inversion results; wherein A is a schematic diagram of a two-dimensional theoretical model; b is a schematic diagram of a full-frequency inversion result of all measuring points, the distance between the measuring points is 0.5km, and the inversion frequency is 1000 Hz-1/10000 Hz; c is a schematic diagram of the inversion result of the detection data of multiple scales and multiple time periods, the frequency of the measured point data at the interval of 1km is 1000 Hz-1/10000 Hz, and the frequency of the measured point data at the interval of 0.5km is 1000 Hz-1/100 Hz;

FIG. 8 is a comparison of three-dimensional theoretical response inversion results; wherein A is a schematic top plane projection diagram of the theoretical model; b is a schematic diagram of the top plane projection of the theoretical model 1 omega m abnormal body; c is a section view at the ash line of the theoretical model; d is a section diagram of a gray line of a full-measuring-point full-frequency three-dimensional inversion result, the measuring point distance is 2.5km, and the inversion frequency is 1000 Hz-1/10000 Hz; e is a section diagram at the gray line of the inversion result of the multi-scale and multi-time-interval detection data, the frequency of the measuring point data at the interval of 5km is 1000 Hz-1/10000 Hz, and the frequency of the measuring point data at the interval of 2.5km is 1000 Hz-1/10 Hz;

FIG. 9 is a schematic diagram of the layout of two-dimensional long-section measuring points in a certain measuring area;

FIG. 10 is a comparison graph of inversion results of long-profile two-dimensional measured data; a is a schematic diagram of a full-measuring-point full-frequency inversion result, the measuring point distance is 5km, and the inversion frequency is 320 Hz-1/1800 Hz; b is a schematic diagram of the inversion result of the detection data at multiple scales and multiple time periods, the frequency of the data of the measuring points at the distance of 10km is 320 Hz-1/1800 Hz, and the frequency of the data of the measuring points at the distance of 5km is 320 Hz-1/100 Hz.

Detailed Description

The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be derived from the embodiments of the present invention by a person skilled in the art without any creative effort, should be included in the protection scope of the present invention.

The embodiment discloses a magnetotelluric multi-scale and multi-period detection method; the method adopts the layout combining large and small spacing to carry out multi-scale and multi-period detection (figure 4), the measuring points are grouped according to the point spacing, the minimum frequency required by the collected data of the measuring points in different groups is different, thus obtaining observation data sets with different scales, reducing the observation time, improving the observation efficiency and saving the observation cost on the premise of ensuring that the collected data cover the detection area (figure 5). And carrying out inversion by using the observation response data to obtain an inversion result.

Referring to fig. 6, the data acquisition and inversion process of the present invention comprises the following steps:

the method comprises the following steps: and looking up and analyzing geological background of a detection area and research and exploration data of predecessors, and designing the distance between the measuring points and the time length for acquiring data by the measuring points under the condition of knowing underground media so as to ensure that the influence range of the data of the measuring points can cover the surrounding measuring points. If a measuring point group with measuring point spacing of 10km is designed, the lowest effective frequency can be required to reach 1/1000Hz and the observation time can be required to be more than 16 hours under the condition that the average resistivity of the ground is 10 ohm meters; and measuring points are designed at the distance of 5km, the lowest effective frequency can reach 1/100Hz, and the observation time is more than 8 hours. And the like according to the actual requirements. Obviously, the observation mode can greatly improve the working efficiency and save the field working cost.

Step two: after the measuring point layout and the data acquisition time length design are completed, field data acquisition is carried out according to a general flow, and an underground electromagnetic field time sequence data set with different measuring point distances and different observation time lengths is obtained. And obtaining an observation response data set comprising data such as impedance tensor, dip vector, apparent resistivity, phase and the like through data processing, analysis and other operations.

Step three: and performing inversion calculation by using the observation response data set, and explaining by using a final inversion result by combining the geological background of the detection area and the research and exploration data of the predecessor.

The invention is described in further detail below by way of specific examples in conjunction with figures 7 and 8.

Now, a two-dimensional resistivity model (fig. 7A) including local anomalous bodies is constructed, measuring points are uniformly arranged according to the interval of 0.5km, and whether inversion comparison results are consistent or not is respectively carried out by using two different data sets. Firstly, inverting all the measuring points with the frequency ranges of 1000 Hz-1/10000 Hz in a traditional mode, wherein the inversion result is shown in an attached figure 7B; then, according to a multi-scale and multi-time-interval detection method, the measuring points with the point spacing of 1km are set to be 1/100 Hz-1/10000 Hz in frequency range, the measuring points with the point spacing of 5km are set to be 1000 Hz-1/10000 Hz in frequency range, inversion is carried out, and finally the obtained result is shown in the attached figure 7C. Both approaches result in substantial recovery of the original model.

And then constructing a three-dimensional resistivity model (figures 8A, 8B and 8C) containing the local abnormal body, uniformly setting measuring points according to the distance of 5km, and respectively using two different data sets to perform inversion comparison to determine whether the results are consistent. Firstly, according to a traditional mode, the frequencies of all measuring points are consistent from 1000Hz to 1/1000Hz, and the inversion result is shown in an attached figure 8D; then, according to a multi-scale and multi-time-interval detection method, a measuring point frequency range with a point spacing of 5km is set to be 1000 Hz-1/10 Hz, a measuring point frequency range with a point spacing of 10km is set to be 1000 Hz-1/1000 Hz, inversion is carried out, and finally a result model is shown as an attached figure 8E. Both inversion methods basically restore the original model.

Through the test of the two-dimensional and three-dimensional synthetic data, the inversion result difference of the data obtained by the multi-scale and multi-time-period detection method and the data obtained by the traditional method is very small.

And (3) taking two-dimensional long-section inversion of a certain project as an example to verify the reliability of the inversion result of the new technology.

The distance between the measuring points of the two-dimensional profile (figure 9) is 5km, and the effects of the traditional detection method and the multi-scale and multi-time-interval detection method are compared. Firstly, according to a traditional mode, the frequency range of all measuring points is 320 Hz-1/1800 Hz, and inversion is carried out after data selection to obtain a result model (shown in figure 10A); and then according to a multi-scale and multi-time-interval detection method, carrying out inversion by adopting the same data selection condition and inversion parameters to obtain a result model (shown in figure 10B), wherein the frequency range of the measuring point with the point spacing of 5km is 320 Hz-1/100 Hz, and the frequency range of the measuring point with the point spacing of 10km is 320 Hz-1/1800 Hz.

The comparison shows that the difference between the two results is very small, which indicates that the new technology is effective for the measured data.

The specific implementation steps are as follows:

(1) designing measuring point groups with two distances of 10km and 5km

(2) The 10km observation frequency is 320 Hz-1/1800 Hz, and the observation time is more than 16 hours;

(3) 5km observation frequency is 320 Hz-1/100 Hz, and the observation time is more than 8 hours;

(4) according to the scheme provided by the invention, field data acquisition and indoor data inversion are realized.

By adopting the magnetotelluric multi-scale and multi-period detection method, the response of a two-dimensional and three-dimensional theoretical model and the measured data are tested and compared, and the result shows that under the condition of designing the frequency range of the data acquired by different scales, the data acquired by the multi-scale and multi-period detection method are inverted, compared with the data acquired by the traditional detection method, the inversion result has small difference, the original model is recovered, and the inversion result is consistent with the expectation. The invention has simple application, improves the field data acquisition efficiency, reduces the field acquisition cost and meets the application requirement.