CN105572748A - Transient electromagnetic detection time window determination method and device - Google Patents

Abstract

An embodiment of the invention provides a transient electromagnetic detection time window determination method and device and relates to the field of geophysical exploration. The method comprises the following steps: obtaining resistivity of each stratum in a detection region and thickness information of each stratum; calculating comprehensive resistivity of the stratums in the detection region according to the resistivity and thickness information of the stratums in the detection region; calculating time required for propagating a transient electromagnetic field to a target body according to the comprehensive resistivity of the stratums in the detection region; and determining window range of the required time in detection according to the time required for propagating the transient electromagnetic field to the target body. The method and device can determine the time window of transient electromagnetic detection on the basis of field investigation and regional geological data analysis.

Description

A kind of determination method and apparatus of transient electromagnetic detecting time window

Technical field

The present invention relates to field of geophysical exploration, be specifically related to a kind of determination method and apparatus of transient electromagnetic detecting time window.

Background technology

The North China type coalfield of China, most of earth's surface is all covered by the Quaternary system sedimentary deposit of low-resistance, and deposit thickness is comparatively large, and generally at 100m, some area can reach 400 ~ 600m.In recent years, the application in field such as to detect in coal exploration, goaf more and more extensive for transient electromagnetic method.One of remarkable advantage of transient electromagnetic method to penetrate shield layer with high resistivity layer, to shield layer with high resistivity layer underlying formation or be configured with good resolution characteristic.But have low resistivity zone particularly earth's surface be low-resistance tectal area construction time, due to stronger signal can be observed on the ground, often ignore the impact that low resistivity zone is explored transient electromagnetic.

The transient electromagnetic detecting degree of depth is determined jointly by formation resistivity and observation time, if layer resistivity is equably, then the diffusion depth of a certain moment transient electromagnetic field is:

${\mathrm{\δ}}_{TD}=\sqrt{\frac{t\mathrm{\ρ}}{2{\mathrm{\μ}}_0}}$

In formula, δ _tDfor diffusion depth, t is time delay, and ρ represents resistivity, μ ₀represent the magnetic permeability of underground medium, the velocity of propagation of transient electromagnetic field is:

$v={\mathrm{\δ}}_{TD}/t=\sqrt{\frac{t\mathrm{\ρ}}{2{\mathrm{\μ}}_0}}/t=\sqrt{\frac{\mathrm{\ρ}}{2{\mathrm{t\μ}}_0}}$

From above formula, formation resistivity ρ is less, and transient electromagnetic field velocity of propagation is in this layer slower.

Utilize Simulation formation resistivity on the impact of transient electromagnetic field velocity of propagation.For the analysis of transient electromagnetic field response feature, mostly adopt the mode result of frequency field being transformed into time domain through Fourier transform.Although also can directly analyze in the form of time domain integral equation, the method only carries out physical interpretation according to ground response to transient electromagnetic field, and its investigation depth, detection accuracy, resolution etc. are inadequate.And in time domain, the method for finite difference is more convenient.

First by passive Maxwell equation derive quasi-static electric field diffusion equation:

${\▿}^{2}E(r,t)-{\mathrm{\μ}}_0\mathrm{\σ}\left(r\right)\frac{\∂E(r,t)}{\∂t}=0$

In formula, E represents electric field intensity, and σ (r) represents the conductivity of medium, and r represents the distance that source point is shown up a little.

Be reduced to the scalar diffusion equation of on the strike direction electric field further:

$\frac{{\∂}^{2}E}{\∂x^2}+\frac{{\∂}^{2}E}{\∂z^2}={\mathrm{\μ}}_0\mathrm{\σ}\left(r\right)\frac{\∂E}{\∂t}$

Under rectangular coordinate system, solution room is split into rectangular node, forms five-point difference scheme, push away to obtain discrete differential equation further:

$\begin{array}{c}\mathrm{\μ}{\stackrel{\‾}{\mathrm{\σ}}}_{i,j}{E}_{i,j}^n=\frac{1}{{\mathrm{\Δz}}_j{\mathrm{\Δz}}_{j+1}}\[\frac{2{\mathrm{\Δz}}_{j+1}}{{\mathrm{\Δz}}_j+{\mathrm{\Δz}}_{j+1}}{E}_{i,j-1}^n+\frac{2{\mathrm{\Δz}}_j}{{\mathrm{\Δz}}_j+{\mathrm{\Δz}}_{j+1}}{E}_{i,j+1}^n-2E_{i,j}^n\]+\\ \frac{1}{{\mathrm{\Δx}}_i{\mathrm{\Δx}}_{i+1}}\[\frac{2{\mathrm{\Δx}}_{i+1}}{{\mathrm{\Δx}}_i+{\mathrm{\Δx}}_{i+1}}{E}_{i-1,j}^n+\frac{2{\mathrm{\Δx}}_i}{{\mathrm{\Δx}}_i+{\mathrm{\Δx}}_{i+1}}{E}_{i+1,j}^n-2E_{i,j}^n\]\end{array}---\left(5\right)$

Wherein, upper table t=n Δ t, e _{i, j}the weighted mean value of four grid conductivity areas around:

${\stackrel{\‾}{\mathrm{\σ}}}_{i,j}=\frac{{\mathrm{\σ}}_{i-1,j-1}{\mathrm{\Δz}}_j{\mathrm{\Δx}}_i+{\mathrm{\σ}}_{i-1,j}{\mathrm{\Δz}}_{j+1}{\mathrm{\Δx}}_i+{\mathrm{\σ}}_{i,j-1}{\mathrm{\Δz}}_j{\mathrm{\Δx}}_{i+1}+{\mathrm{\σ}}_{i,j}{\mathrm{\Δz}}_{j+1}{\mathrm{\Δx}}_{i+1}}{\[{\mathrm{\Δz}}_j+{\mathrm{\Δz}}_{j+1}\]\[{\mathrm{\Δx}}_i+{\mathrm{\Δx}}_{i+1}\]}$

Prior art by providing boundary condition, and then carries out finite difference computing.In the calculation, source is introduced as starting condition, in order to process the boundary condition on earth's surface, has also used quick Fourier transformation and cubic spline interpolation.

As shown in Figure 1, in order to study the travel-time of electromagnetic field in different medium, method of finite difference is utilized to carry out the simulation of transient electromagnetic field velocity of propagation, the homogeneous half space model resistivity of this numerical simulation is 50 Ω .m, the wide 30m of vertical anomalous body, high 120m, top buried depth 400m, resistivity is 5 Ω .m, as shown in Figure 1.In figure ,+I and-I represents the positive source of endless current emission wire frame and negative source respectively.The isoline that Fig. 1 (a) is observation time when being 3ms, now electromagnetic field has been diffused into underground below 1200m.Fig. 1 (b) be on the homogeneous half space of Fig. 1 (a), insert thick 200m, resistivity is after the overlayer of 20 Ω .m, observation time is still the isoline of 3ms, and now electromagnetic field has just been diffused into the degree of depth of underground 600m.

For identical observation time, when there being low resistivity zone to cover, the propagation degree of depth of transient electromagnetic field is less than propagation degree of depth when covering without low resistivity zone.Therefore, in transient electromagnetic work progress, the selection of time window not only will consider objective body depth of burial, also will consider tectal resistivity above objective body.But during conventional transient electromagnetic method construction, often ignore the tectal impact of low-resistance, operating personnel are just according to personal experience's select time window, and randomness is larger.If time window is long, can redundant data be produced, affect the precision of data interpretation; Time window is too short, and investigation depth can be made not enough, does not reach predetermined detection object.

Summary of the invention

The technical problem to be solved in the present invention how to determine the time window of transient electromagnetic detecting, the invention provides a kind of determination method and apparatus of transient electromagnetic detecting time window, in transient electromagnetic construction, consider the impact of formation resistivity on field velocity of propagation, realize the time window of science determination transient electromagnetic detecting.

In order to realize foregoing invention object, the technical scheme that the present invention takes is as follows:

A defining method for transient electromagnetic detecting time window, comprising:

Obtain the thickness information surveying each formation resistivity and each stratum in district;

According to each formation resistivity and thickness information in survey district, calculate and survey stratum, district aggregate resistance rate;

According to survey stratum, district aggregate resistance rate, calculating transient electromagnetic field propagates into the time needed for objective body;

The time needed for objective body is propagated into, required time window ranges when determining to detect according to transient electromagnetic field.

Preferably, obtain the thickness information surveying each formation resistivity and each stratum in district to comprise:

Borehole data according to surveying district obtains the thickness information surveying each formation resistivity in district and each stratum.

Preferably, comprise before calculating the time that transient electromagnetic field propagates into needed for objective body:

Determine described objective body depth of burial.

Preferably, calculate survey stratum, district aggregate resistance rate to comprise:

The thickness of each formation resistivity with corresponding each stratum is multiplied;

Described product is surveyed stratum, district aggregate resistance rate divided by the thickness sum acquisition on described each stratum.

Preferably, the time that calculating transient electromagnetic field propagates into needed for objective body comprises:

Calculate transient electromagnetic field respectively and propagate into interface and the time needed for lower interface on objective body.

Preferably, utilize following formula to calculate transient electromagnetic field respectively and propagate into interface and the time needed for lower interface on objective body:

Wherein, t ₁for transient electromagnetic field propagates into the time on objective body needed for interface, t ₂for transient electromagnetic field propagates into the time under objective body needed for interface, d ₁for the depth of burial at interface on objective body, d ₂for the depth of burial at interface under objective body, ρ _combinerepresent and survey stratum, district aggregate resistance rate, μ ₀represent the magnetic permeability of underground medium.

Preferably, propagate into the time needed for objective body according to transient electromagnetic field, when determining to detect, required time window ranges comprises:

During detection, required time window ranges is more than or equal to interval (t ₁, t ₂).

Preferably, propagate into the time needed for objective body according to transient electromagnetic field, when determining to detect, required time window ranges comprises:

During detection, the minimal instant of required time window is less than or equal to t ₁, during detection, the maximum moment of required time window is greater than or equal to t ₂.

For solving the problems of the technologies described above, the invention provides a kind of determining device of transient electromagnetic detecting time window, comprising:

Collection module, for obtaining the thickness information surveying each formation resistivity and each stratum in district;

Resistivity module, for according to each formation resistivity and thickness information in survey district, calculates and surveys stratum, district aggregate resistance rate;

Time module, for according to survey stratum, district aggregate resistance rate, calculates transient electromagnetic field and propagates into time needed for objective body;

Window module, for propagating into the time needed for objective body according to transient electromagnetic field, required time window ranges when determining to detect.

Preferably, the thickness information that each formation resistivity and each stratum in district are surveyed in described collection module acquisition comprises:

Borehole data according to surveying district obtains the thickness information surveying each formation resistivity in district and each stratum.

Preferably, described device also comprises: depth module, for determining described objective body depth of burial.

Preferably, described resistivity module calculating survey stratum, district aggregate resistance rate comprises:

The thickness of each formation resistivity with corresponding each stratum is multiplied;

Described product is surveyed stratum, district aggregate resistance rate divided by the thickness sum acquisition on described each stratum.

Preferably, the time that described time module calculating transient electromagnetic field propagates into needed for objective body comprises:

Calculate transient electromagnetic field respectively and propagate into interface and the time needed for lower interface on objective body.

Preferably, described time module utilizes following formula to calculate transient electromagnetic field respectively and propagates into interface and the time needed for lower interface on objective body:

Wherein, t ₁for transient electromagnetic field propagates into the time on objective body needed for interface, t ₂for transient electromagnetic field propagates into the time under objective body needed for interface, d ₁for the depth of burial at interface on objective body, d ₂for the depth of burial at interface under objective body, ρ _combinerepresent and survey stratum, district aggregate resistance rate, μ ₀represent the magnetic permeability of underground medium.

Preferably, described window module propagates into the time needed for objective body according to transient electromagnetic field, and when determining to detect, required time window ranges comprises:

During detection, required time window ranges is more than or equal to interval (t ₁, d ₂).

Preferably, described window module propagates into the time needed for objective body according to transient electromagnetic field, and when determining to detect, required time window ranges comprises:

During detection, the minimal instant of required time window is less than or equal to t ₁, during detection, the maximum moment of required time window is greater than or equal to t ₂.

Compared to the prior art the present invention, has following beneficial effect:

Method and apparatus of the present invention, consider that formation resistivity is on the impact of field velocity of propagation, on the basis of on-site inspection and areal geology analysis, determine the time window of transient electromagnetic detecting, during real work, suitable time window according to determining detects, and can arrive optimal detection object.

Accompanying drawing explanation

Fig. 1 is the isogram of prior art simulation transient electromagnetic field;

Fig. 2 is the process flow diagram of the defining method of a kind of transient electromagnetic detecting time window of the embodiment of the present invention;

Fig. 3 is the structural representation of the determining device of a kind of transient electromagnetic detecting time window of the embodiment of the present invention;

Fig. 4 is the strata division information schematic diagram of the embodiment of the present invention 1;

Fig. 5 is the transient electromagnetic detecting result schematic diagram of the embodiment of the present invention 2.

Embodiment

Clearly understand for making goal of the invention of the present invention, technical scheme and beneficial effect, below in conjunction with accompanying drawing, embodiments of the invention are described, it should be noted that, when not conflicting, the embodiment in the application and the feature in embodiment can combination in any mutually.

As shown in Figure 2, the embodiment of the present invention provides a kind of defining method of transient electromagnetic detecting time window, comprising:

Obtain the thickness information surveying each formation resistivity and each stratum in district;

According to each formation resistivity and thickness information in survey district, calculate and survey stratum, district aggregate resistance rate;

According to survey stratum, district aggregate resistance rate, calculating transient electromagnetic field propagates into the time needed for objective body;

The time needed for objective body is propagated into, required time window ranges when determining to detect according to transient electromagnetic field.

Wherein, obtain the thickness information surveying each formation resistivity and each stratum in district to comprise:

Borehole data according to surveying district obtains the thickness information surveying each formation resistivity in district and each stratum.

Wherein, comprise before calculating the time that transient electromagnetic field propagates into needed for objective body:

Determine described objective body depth of burial.

Wherein, calculate survey stratum, district aggregate resistance rate to comprise:

The thickness of each formation resistivity with corresponding each stratum is multiplied;

Described product is surveyed stratum, district aggregate resistance rate divided by the thickness sum acquisition on described each stratum.

Wherein, the time that calculating transient electromagnetic field propagates into needed for objective body comprises:

Calculate transient electromagnetic field respectively and propagate into interface and the time needed for lower interface on objective body.

Wherein, utilize following formula to calculate transient electromagnetic field respectively and propagate into interface and the time needed for lower interface on objective body:

Wherein, t ₁for transient electromagnetic field propagates into the time on objective body needed for interface, t ₂for transient electromagnetic field propagates into the time under objective body needed for interface, d ₁for the depth of burial at interface on objective body, d ₂for the depth of burial at interface under objective body, ρ _combinerepresent and survey stratum, district aggregate resistance rate, μ ₀represent the magnetic permeability of underground medium.

Propagate into the time needed for objective body according to transient electromagnetic field, when determining to detect, required time window ranges comprises:

During detection, required time window ranges is more than or equal to interval (t ₁, t ₂).

Propagate into the time needed for objective body according to transient electromagnetic field, when determining to detect, required time window ranges comprises:

During detection, the minimal instant of required time window is less than or equal to t ₁, during detection, the maximum moment of required time window is greater than or equal to t ₂.

As shown in Figure 3, the embodiment of the present invention provides a kind of determining device of transient electromagnetic detecting time window, comprising:

Collection module, for obtaining the thickness information surveying each formation resistivity and each stratum in district;

Resistivity module, for according to each formation resistivity and thickness information in survey district, calculates and surveys stratum, district aggregate resistance rate;

Time module, for according to survey stratum, district aggregate resistance rate, calculates transient electromagnetic field and propagates into time needed for objective body;

Window module, for propagating into the time needed for objective body according to transient electromagnetic field, required time window ranges when determining to detect.

Described collection module obtains the thickness information surveying each formation resistivity and each stratum in district and comprises:

Borehole data according to surveying district obtains the thickness information surveying each formation resistivity in district and each stratum.

Described device also comprises: depth module, for determining described objective body depth of burial.

Described resistivity module calculates survey stratum, district aggregate resistance rate and comprises:

The thickness of each formation resistivity with corresponding each stratum is multiplied;

Described product is surveyed stratum, district aggregate resistance rate divided by the thickness sum acquisition on described each stratum.

The time that described time module calculating transient electromagnetic field propagates into needed for objective body comprises:

Calculate transient electromagnetic field respectively and propagate into interface and the time needed for lower interface on objective body.

Described time module utilizes following formula to calculate transient electromagnetic field respectively and propagates into interface and the time needed for lower interface on objective body:

Wherein, t ₁for transient electromagnetic field propagates into the time on objective body needed for interface, t ₂for transient electromagnetic field propagates into the time under objective body needed for interface, d ₁for the depth of burial at interface on objective body, d ₂for the depth of burial at interface under objective body, ρ _combinerepresent and survey stratum, district aggregate resistance rate, μ ₀represent the magnetic permeability of underground medium.

Described window module propagates into the time needed for objective body according to transient electromagnetic field, and when determining to detect, required time window ranges comprises:

During detection, required time window ranges is more than or equal to interval (t ₁, d ₂).

Described window module propagates into the time needed for objective body according to transient electromagnetic field, and when determining to detect, required time window ranges comprises:

During detection, the minimal instant of required time window is less than or equal to t ₁, during detection, the maximum moment of required time window is greater than or equal to t ₂.

Embodiment 1

Before carrying out transient electromagnetic detecting, collection analysis is carried out to survey district geologic information, obtains and survey stratum, district roughly distribution situation.The information that must obtain comprises: (1) surveys the thickness of district's strata division number and every layer; (2) resistivity information on every layer of stratum; (3) objective body depth of burial.

As shown in Figure 4, survey stratum, district and be divided into n floor.Every layer thickness is respectively h ₁, h ₂..., h _n, resistivity is respectively ρ ₁, ρ ₂..., ρ _n.

Calculate according to the following formula and survey stratum, district aggregate resistance rate:

Obtaining interface depth of burial on objective body is d ₁, lower interface depth of burial is d ₂.With aggregate resistance rate ρ _combinefor reference, transient electromagnetic field can be obtained and propagate into interface and lower interface required time on objective body and be respectively,

The diffusion depth of transient electromagnetic field was directly proportional to time delay, and therefore, to enable measurement data reflect target volume information completely, then adopted time window scope should be more than or equal to interval (t ₁, t ₂).That is, for the time window chosen, Ying You

t _min≤t ₁，t _max≥t ₂

T in formula _minfor the minimal instant of time window, t _maxfor the time window maximum moment.

Meanwhile, consider generally aggregate resistance rate and each stratum actual resistivity unequal, for ensureing that investigation depth is enough, time window scope should be greater than interval (t ₁, t ₂), that is:

t _min＜t ₁，t _max＞t ₂

Transient electromagnetic is visited can Self defined time window, and during real work, the principle that the method and apparatus provided according to the embodiment of the present invention is determined defines suitable time window and detects, and can arrive optimal detection object.

Embodiment 2

According to this mining area geology in the past, well-log information, infer that this survey district strata division situation is roughly as shown in table 1:

District's formation information table surveyed by table 1

Note: because the objective body degree of depth is roughly 180 ~ 250m, therefore only gives six layers of formation information within 270m in table.

Calculate and survey stratum, district aggregate resistance rate,

Calculate time when transient electromagnetic field propagates into objective body upper and lower interphase

$t_1=\frac{2\×4\mathrm{\π}\×{10}^{-7}\×{180}^2}{52}=1.57ms,$

$t_2=\frac{2\×4\mathrm{\π}\×{10}^{-7}\×{250}^2}{52}=3.02ms$

Determine time window, because adopted data should reflect superficial part information as much as possible, therefore t _minselection will as much as possible close to the turn-off time.Be combined instrument performance, definition time window is 0.5 ~ 3.5ms.In practical application time window also with use the kind of instrument, use the real time window of instrument relevant, comprehensively determine to select correct transmission frequency, correct launching phase.

Carry out apparent resistivity to the data gathered to ask for, and computation of apparent resistivity result is become figure by Surfer, result as shown in Figure 5, can be found out, result of detection gives the location of anomalous body clearly, and upper and lower interface also can clearly be differentiated.

Although disclosed embodiment as above, the embodiment that its content just adopts for the ease of understanding technical scheme of the present invention, is not intended to limit the present invention.Technician in any the technical field of the invention; under the prerequisite not departing from disclosed core technology scheme; any amendment and change can be made in the form implemented and details; but the protection domain that the present invention limits, the scope that still must limit with appending claims is as the criterion.

Claims (10)

1. a defining method for transient electromagnetic detecting time window, is characterized in that, comprising:

Obtain the thickness information surveying each formation resistivity and each stratum in district;

According to each formation resistivity and thickness information in survey district, calculate and survey stratum, district aggregate resistance rate;

According to survey stratum, district aggregate resistance rate, calculating transient electromagnetic field propagates into the time needed for objective body;

The time needed for objective body is propagated into, required time window ranges when determining to detect according to transient electromagnetic field.

2. the method for claim 1, is characterized in that: obtain the thickness information surveying each formation resistivity and each stratum in district and comprise:

Borehole data according to surveying district obtains the thickness information surveying each formation resistivity in district and each stratum.

3. the method for claim 1, is characterized in that, calculates survey stratum, district aggregate resistance rate and comprises:

The thickness of each formation resistivity with corresponding each stratum is multiplied;

Described product is surveyed stratum, district aggregate resistance rate divided by the thickness sum acquisition on described each stratum.

4. method as claimed in claim 3, is characterized in that: utilize following formula to calculate transient electromagnetic field respectively and propagate into interface and the time needed for lower interface on objective body:

Wherein, t ₁for transient electromagnetic field propagates into the time on objective body needed for interface, t ₂for transient electromagnetic field propagates into the time under objective body needed for interface, d ₁for the depth of burial at interface on objective body, d ₂for the depth of burial at interface under objective body, ρ _combinerepresent and survey stratum, district aggregate resistance rate, μ ₀represent the magnetic permeability of underground medium.

5. method as claimed in claim 4, is characterized in that: propagate into the time needed for objective body according to transient electromagnetic field, and when determining to detect, required time window ranges comprises:

During detection, required time window ranges is more than or equal to interval (t ₁, t ₂).

6. a determining device for transient electromagnetic detecting time window, is characterized in that, comprising:

Collection module, for obtaining the thickness information surveying each formation resistivity and each stratum in district;

Resistivity module, for according to each formation resistivity and thickness information in survey district, calculates and surveys stratum, district aggregate resistance rate;

Time module, for according to survey stratum, district aggregate resistance rate, calculates transient electromagnetic field and propagates into time needed for objective body;

Window module, for propagating into the time needed for objective body according to transient electromagnetic field, required time window ranges when determining to detect.

7. device as claimed in claim 6, is characterized in that: described collection module obtains the thickness information surveying each formation resistivity and each stratum in district and comprises:

Borehole data according to surveying district obtains the thickness information surveying each formation resistivity in district and each stratum.

8. device as claimed in claim 6, is characterized in that, described resistivity module calculates survey stratum, district aggregate resistance rate and comprises:

The thickness of each formation resistivity with corresponding each stratum is multiplied;

Described product is surveyed stratum, district aggregate resistance rate divided by the thickness sum acquisition on described each stratum.

9. device as claimed in claim 8, is characterized in that: described time module utilizes following formula to calculate transient electromagnetic field respectively and propagates into interface and the time needed for lower interface on objective body:

Wherein, t ₁for transient electromagnetic field propagates into the time on objective body needed for interface, t ₂for transient electromagnetic field propagates into the time under objective body needed for interface, d ₁for the depth of burial at interface on objective body, d ₂for the depth of burial at interface under objective body, ρ _combinerepresent and survey stratum, district aggregate resistance rate, μ ₀represent the magnetic permeability of underground medium.

10. device as claimed in claim 9, is characterized in that: described window module propagates into the time needed for objective body according to transient electromagnetic field, and when determining to detect, required time window ranges comprises:

During detection, required time window ranges is more than or equal to interval (t ₁, t ₂).