CN101004454A - 3D electromagnetic fast inversion method of minimized target - Google Patents

Abstract

A 3-D electromagnetic fast inversing method of object minimization includes confirming known object region according to known geologic data, setting up background conductivity combination according to known conductivity parameter and 3-D interpolation, calculating background field of said combination, inversing unknown region as per requirement of production, obtaining general field, carrying out 3-D interpolation on phase curve and actually measured conductivity curve, inversing frequency and fitting inversed frequency to obtain underground conductivity set of object.

Description

The 3 D electromagnetic fast inversion method of minimized target

Technical field

The present invention relates to the magneto-electrotelluric exploration data processing technique that geophysics is surveyed, is a kind of 3 D electromagnetic fast inversion method of minimized target.

Background technology

Mt is a kind of geophysical exploration method, and is general by understand the situation of change of underground medium conductivity in the variation of ground observation electromagnetic field.Last century the nineties, develop by magneto-electrotelluric exploration method (be called for short MT) and a kind of new electromagnetic survey method, be called Electromagnetic Continuous array profile method (being called for short CEMP),, develop into multi-channel acquisition method along survey line array by simple mt single-point acquiring method; Simultaneously, because computer technology fast development, two-dimensional finite element method, two-dimentional continuous medium inversion method are grown up, computational accuracy improves greatly, situation that can the actual subsurface conductivity medium of more accurate simulation, thereby, make this method by being main at regional prospecting, being converted at target exploration is the important oil-gas exploration method of a main class, in seismic prospecting difficulty district, obtain exploration effects preferably as mountain front, pyrogenic rock exposure district, top layer gravel district, carbonate rock area, loess tableland areal coverage, remedied the deficiency of seismic prospecting data.

Along with the deepening continuously of oil-gas exploration work, the electromagnetic survey method also faced more complicated exploration problem in recent years, and exploration work area surficial geology condition is complicated more, and geologic objective is more concealed, makes the electromagnetic survey method be faced with unprecedented challenge.More and more higher to mt and the requirement of Electromagnetic Continuous array profile method inversion accuracy, the needs that two-dimensional finite element method that is using at present or two-dimentional continuous medium method are difficult to satisfy geologic interpretation.Thereby electromagnetism 3-d inversion method is rapidly developed.

But electromagnetism 3-d inversion theoretical method research at present is more, the example of practical application is actually rare, reason is: electromagnetism 3-d inversion method is that underground medium is completely handled as unknown conductivity parameter usually, if an area is 10km * 10km, the three-dimensional exploration of survey area depth capacity 10km, in order to carry out inverting subtly, the conductivity parameter combinations can be split into 20m * 20m * 20m unit, unknown conductivity parameter just will have 500 * 500 * 500=125,000,000 unknown conductivity parameter, visible unknown parameter is very many.

Unknown parameter is many more, and equivalence is serious more; The sensitivity matrix of forming in refutation process is just very huge, can take a large amount of calculator memories, and present computing machine is difficult to bear; Calculated amount is very huge in the forward simulation process, expect desirable inversion result, may with one the week or the longer time.

If increase the subdivision unit, unknown parameter has reduced, but clearly the 3-d inversion simulation precision can decrease.Here it is, and present 3-d inversion method is difficult to practical basic reason.

The area of carrying out the three-dimensional work of magnetotelluric sounding method or Electromagnetic Continuous array profile method all is the area of carrying out seismic survey work generally, or even the area of carrying out 3-d seismic exploration work, given data is many but the electromagnetic method Data Processing is not played a role.As well-log information, shallow earthquake data, be underutilized.

Summary of the invention

The present invention seeks at the problem that exists in the background technology, provide a kind of earthquake or drilling well Given information of utilizing as non-homogeneous electrical medium background, it is minimum that the inverting target reaches, and energy is quick, the 3 D electromagnetic fast inversion method of the minimized target of high-precision three-dimensional inverting.

The present invention realizes that by the following technical solutions inversion step is as follows:

1) in the area of carrying out earthquake, geology or drilling well work, actual measurement target exploration operational area apparent resistivity curve and phase curve adopt usual way to carry out three-dimensional interpolation, make it to be uniformly distributed in the grid node of subdivision unit;

2) the electromagnetic survey workspace is divided into three parts: 1. background area that underground medium is known and resistivity value (σ _n), 2. adjacent known region (D _b) and corresponding resistance parameter (σ _b), 3. by the definite target area (D that will carry out inverting of exploration task _a), its corresponding resistivity value parameter (σ _a) determine by adjacent area known electric well-log information, geology outcrop data;

3) adopt common electromagnetism one dimension analytic formula to calculate known background area and resistivity value (σ _n) relevant electric field (E ⁿ) and magnetic field (H ⁿ), adopt formula (7) (8) 3 D electromagnetic integral formula to calculate known region (D _b) relevant electric field E ^DbAnd magnetic field H ^Db:

$E^{D_b}\left(r_j\right)=\underset{D_b}{\∫}{\hat{G}}_E\left(r_j\right|r)\·\left({\mathrm{\Δ\σ}}_{b}E\left(r\right)\right)\mathrm{dv}---\left(7\right)$

$H^{D_b}\left(r_j\right)=\underset{D_b}{\∫}\widehat{G_H}\left(r_j\right|r)\·\left({\mathrm{\Δ\σ}}_{b}E\left(r\right)\right)\mathrm{dv}---\left(8\right)$

E ^DbAnd H ^Db: represent electric field relevant and magnetic field wherein respectively with known region:

With Be respectively the green operator in electric field and magnetic field, Δ σ _b=σ _b-σ _nThe known region resistivity value is poor with corresponding background resistivity value, and E (r) be any any electric field;

4) adopt following (9) (10) 3 D electromagnetic integral formula to calculate target area (D _a) E ^DaAnd magnetic field H ^Da

$E^{D_a}\left(r_j\right)=\underset{D_a}{\∫}{\hat{G}}_E\left(r_j\right|r)\·\left({\mathrm{\Δ\σ}}_{a}E\left(r\right)\right)\mathrm{dv}---\left(9\right)$

$H^{D_a}\left(r_j\right)=\underset{D_a}{\∫}{\hat{G}}_H\left(r_j\right|r)\·\left({\mathrm{\Δ\σ}}_{a}E\left(r\right)\right)\mathrm{dv}---\left(10\right)$

E ^DaAnd H ^Da: represent electric field and the magnetic field relevant respectively with the target area,

With

Be respectively the green operator in electric field and magnetic field, Δ σ _a=σ _a-σ _nTarget area resistivity value and background resistivity value poor, E (r) is the electric field of any arbitrarily;

5) according to above-mentioned 3), 4) the result calculate ground apparent resistivity and phase place corresponding;

6) according to target area size, buried depth, scale and by 5) apparent resistivity curve and the frequency plot curve of the target area correspondence calculated, determine that the apparent resistivity curve of 5～20 frequencies and phase curve carry out inverting;

7) adopt conventional Nonlinear Conjugate Gradient Methods, carry out three-dimensional iterative inversion, obtain resistance parameter at target area resistivity value parameter;

8) according to background resistivity parameter (σ _n), known region resistivity (σ _b) and obtain resistance parameter (σ by 3-d inversion _a) obtain the resistivity distribution situation of whole underground medium, set up electromagnetism 3-d inversion data volume, as the foundation of geologic interpretation.

The present invention also adopts following technical scheme to realize:

Described (σ _a) (σ _b) represent the resistivity value of each unit in the zone, determine its value according to existing drilling data in the depth range of objective area inverting.

Described (σ _a) (σ _b) represent the resistivity value of each unit in the zone with any resistivity value filling.

The apparent resistivity curve of described 5～20 frequencies and phase curve frequency range are necessarily greater than the frequency range of this target area correspondence.

Described non-linear conjugated gradient refutation process is that the first step is revised the resistance parameter of zone of ignorance, calculates anomalous field; The ambient field that the utilization of second step had been calculated and the anomalous field of zone of ignorance calculate apparent resistivity curve and phase curve; The 3rd step was calculated apparent resistivity curve and phase curve error of fitting; Repeat above-mentioned three the step meet the demands up to error of fitting, obtain the resistance parameter of zone of ignorance.

The present invention can make full use of such as earthquake or drilling well Given information, as known non-homogeneous electrical medium background, before whole refutation process, only need just drill calculating once, because the known conductivity parameter does not participate in inverting, it is minimum that the inverting target reaches, thereby reach purpose quick, the high-precision three-dimensional inverting.

Description of drawings

Fig. 1 is the three-dimensional subsurface conductivity parameter combinations signal of the present invention.

Embodiment

Describe the present invention in detail below in conjunction with accompanying drawing.

The first step of the present invention is determined known zone according to given datas such as earthquake, drilling wells, simultaneously non-study area is determined the conductivity parameter according to given data, sets up three-dimensional background conductance rate parameter combinations by three-dimensional interpolation; Adopt integral equation method to calculate ambient field to three-dimensional background conductance rate parameter combinations, whole refutation process only need be calculated once; Require to determine the 3-d inversion zone of ignorance according to actual production, and set up the initial conductivity parameter combinations of goal in research; Adopt integral equation method to calculate anomalous field and ambient field to the initial conductivity parameter combinations, merge and obtain resultant field, and then obtain the surface em field response of whole conductivity parameter combinations and apparent conductivity, phase response; Actual measurement apparent conductivity curve and phase curve are carried out three-dimensional interpolation, make it to be uniformly distributed in conductivity parameter combinations grid node; According to the main inverting frequency of the degree of depth, the size Selection of target conductivity parameter combinations, calculated amount is further reduced; Adopt the Nonlinear Conjugate Gradient Methods iterative inversion to carry out 3-d inversion; Only make amendment in the inverting, only need the main inverting frequency of match studying the initial conductivity parameter combinations; Finally obtain wanting the subsurface conductivity parameter combinations of goal in research body.

The present invention only carries out inverting to goal in research, and as abnormal area, given initial subsurface conductivity parameter combinations utilizes the method for three-dimensional integral equation to calculate anomalous field simultaneously with goal in research.

The present invention carries out the real data body of grid difference foundation corresponding to the conductivity parameter combinations to real data, determines apparent conductivity curve and impedance phase curve frequency band according to the degree of depth of goal in research, calculates error of fitting.

As constructing an objective area, general earthquake superficial reflex data is fine, and the deep layer geologic information is bad, just the shallow-layer data can be handled as given data, shallow-layer 4km data is known, and unknown conductivity number of parameters just becomes original 50% in the subsurface conductivity parameter combinations during meticulous inverting; In like manner, as constructing in the mountain region, general region of no relief seismic reflection data is better, and the mountain seism data is bad, also the region of no relief can be handled as given data, reduces unknown parameter.Maximum characteristics of the present invention are to make full use of given data, dwindle the inverting zone, and are time saving and energy saving, reduced multi-solution, and precision is higher.

Subsurface conductivity parameter combinations of the present invention such as Fig. 1, background subsurface conductivity parameter combinations is three layers, resistivity is respectively σ ₁, σ ₂, σ ₃, and known three-dimensional abnormal area D _b, resistivity is σ _b, make σ _b=σ _n+ Δ σ _b

Wherein: σ _nBe background resistivity value, target area D _a, resistivity is σ _a,

Make σ _a=σ _n+ Δ σ _a

Maxwell equation can be write as:

$\▿\×H={\mathrm{\σ}}_{n}E+j={\mathrm{\σ}}_{n}E+j^{{\mathrm{\Δ\σ}}_b}+j^{\mathrm{\Δ}{\mathrm{\σ}}_a}+j^e---\left(1\right)$

×E＝iωμ ₀H (2)

$j^{\mathrm{\Δ}{\mathrm{\σ}}_a}=\left\{\begin{array}{cc}\mathrm{\Δ}{\mathrm{\σ}}_{a}E,& r\∈D_a\\ 0,& r\∉D_a\end{array}\right.---\left(3\right)$

$j^{{\mathrm{\Δ\σ}}_b}=\left\{\begin{array}{cc}\mathrm{\Δ}{\mathrm{\σ}}_{b}E,& r\∈D_b\\ 0,& r\∉D_b\end{array}\right.---\left(4\right)$

Electromagnetism three-dimensional integral integral equation is

$E\left(r_j\right)=E^n+\underset{D_b}{\∫}{\hat{G}}_E\left(r_j\right|r)\·\left({\mathrm{\Δ\σ}}_{b}E\left(r\right)\right)\mathrm{dv}$

$+\underset{D_a}{\∫}{\hat{G}}_E\left(r_j\right|r)\·\left({\mathrm{\Δ\σ}}_{a}E\left(r\right)\right)\mathrm{dv}---\left(5\right)$

$H\left(r_j\right)=H^n+\underset{D_b}{\∫}{\hat{G}}_H\left(r_j\right|r)\·\left({\mathrm{\Δ\σ}}_{b}E\left(r\right)\right)\mathrm{dv}$

$+\underset{D_a}{\∫}{\hat{G}}_H\left(r_j\right|r)\·\left({\mathrm{\Δ\σ}}_{a}E\left(r\right)\right)\mathrm{dv}---\left(6\right)$

E in formula (5), (6) ⁿ, H ⁿRepresent background electric field, magnetic field respectively, second electric field, the magnetic field of representing known region respectively, the 3rd electric field, the magnetic field of representing abnormal area respectively.

The concrete implementation step of the present invention is:

1) in the area of carrying out earthquake, geology or drilling well work, actual measurement target exploration operational area apparent resistivity curve and phase curve adopt usual way to carry out three-dimensional interpolation, make it to be uniformly distributed in the grid node of subdivision unit;

When actual in the open air mt or continuous array profile method observation, often be subjected to topographic relief, river, Hu Bo, village, power station, colliery, hi-line, expressway influence, measuring point density inequality, in order to reflect and control subsurface anomaly zone conductivity parameter combinations effectively, apparent conductivity curve and phase curve to actual measurement carry out three-dimensional interpolation, make it on the ground evenly regular distribution.

2) the electromagnetic survey workspace is divided into three parts: 1. background area that underground medium is known and resistivity value (σ _n), 2. adjacent known region (D _b) and corresponding resistance parameter (σ _b), 3. by the definite target area (D that will carry out inverting of exploration task _a), its corresponding resistivity value parameter (σ _a) determine by adjacent area electric logging data, geology outcrop data.

MT or CEMP carry out the area of target exploration work, generally all carry out earthquake, geology or drilling well work, and known physical prospecting geologic information is more or less all arranged.In order to make full use of these given datas, these given datas are incorporated in the 3-d inversion process.

(D _b), (D _a) before inverting, all be split into a plurality of little squares, be exactly said unit, each little unit is considered to uniformly, can represent with a resistivity value, above said (σ _a) (σ _b) represent a series of resistivity value, comprise the resistivity value of each unit in the respective regions.As to want the depth range of inverting be the 4-8 km, if drilling data is arranged near the target area, and can be with reference to providing general value.If do not know any reference information, can be directly with a resistivity value filling.The resistivity value in modifying target zone in refutation process makes it to meet objective reality as far as possible.

Then, determine research object, promptly will carry out the region D of inverting _a, such as wanting the inverting degree of depth, determine its scale according to given data greater than the 3km oil gas structure, extend the degree of depth, thereby obtain wanting the survey region of inverting, according to electric logging data, the geology outcrop data of adjacent area, determine abnormal area conductivity value parameter;

3) the three-dimensional subsurface conductivity parameter combinations to background, known region adopts the 3 D electromagnetic integral equation method to calculate corresponding background electromagnetic field;

According to 2) definite background conductance rate parameter, calculate E respectively according to formula (5), (6) ⁿ, H ⁿ, according to 2) and known region and the conductivity parameter determined calculate first integral respectively, obtains corresponding background electromagnetic field.

Electric field E ^DbAnd magnetic field H ^DbCalculate by the 3 D electromagnetic integral formula:

$E^{D_b}\left(r_j\right)=\underset{D_b}{\∫}{\hat{G}}_E\left(r_j\right|r)\·\left({\mathrm{\Δ\σ}}_{b}E\left(r\right)\right)\mathrm{dv}---\left(7\right)$

$H^{D_b}\left(r_j\right)=\underset{D_b}{\∫}{\hat{G}}_H\left(r_j\right|r)\·\left(\mathrm{\Δ}{\mathrm{\σ}}_{b}E\left(r\right)\right)\mathrm{dv}---\left(8\right)$

Wherein:

With

Be respectively the green operator in electric field and magnetic field, Δ σ _b=σ _b-σ _nThe known region resistivity value is poor with corresponding background resistivity value, and E (r) be any any electric field.

4) adopt integral equation method to calculate the electromagnetic field of abnormal area correspondence, with the ambient field that had calculated, merging obtains total electromagnetic field;

According to 2) zone of ignorance determined and conductivity parameter second integral in computing formula (5), (6) respectively, obtain the electromagnetic field of corresponding zone of ignorance.With 3) the background electromagnetic field that calculates merges and obtains electric field (result of formula (5)) and total magnetic field (result of formula (6)).

Calculate target area (D by the 3 D electromagnetic integral formula _a) E ^DaAnd magnetic field H ^Da:

$E^{D_a}\left(r_j\right)=\underset{D_a}{\∫}{\hat{G}}_E\left(r_j\right|r)\·\left({\mathrm{\Δ\σ}}_{a}E\left(r\right)\right)\mathrm{dv}---\left(9\right)$

$H^{D_a}\left(r_j\right)=\underset{D_a}{\∫}{\hat{G}}_H\left(r_j\right|r)\·\left({\mathrm{\Δ\σ}}_{a}E\left(r\right)\right)\mathrm{dv}---\left(10\right)$

With

Be respectively the green operator in electric field and magnetic field, Δ σ _a=σ _a-σ _nTarget area resistivity value and background resistivity value poor, E (r) is the electric field of any arbitrarily.

5) according to above-mentioned 3), 4) result of calculation calculate ground apparent resistivity and phase place corresponding;

6) according to target area size, buried depth, scale and by 5) apparent resistivity curve and the frequency plot curve of the target area correspondence calculated, determine that the apparent resistivity curve of 5～20 frequencies and phase curve carry out inverting;

The frequency range that actual MT and CEMP method are applied to petroleum prospecting is generally 320～0.00055Hz, the frequency band range broad, when the zone of ignorance scale hour, zone of ignorance conductivity parameter is only relevant with the apparent conductivity curve or the phase curve of several or tens frequencies, therefore only needs to select the band limits relevant with zone of ignorance to carry out inverting.

If the target area degree of depth is 4000-8000m, then according to degree of depth experimental formula $D=356\sqrt{\frac{1}{\mathrm{\σf}}}$ (wherein D represents depth of exploration, and σ is the underground medium average resistivity, and f represents frequency) anti-frequency pushing rate scope.If supposing the σ value is 0.1, Dui Ying frequency range 0.079-0.02Hz then, (and actual frequency range is generally 320-0.00055Hz) generally selects 5-10 frequency in this frequency range, less than or prolong 3-5 frequency respectively greater than this frequency range, as the inverting frequency range.

The apparent resistivity curve of 5～20 frequencies and phase curve frequency range are necessarily greater than the frequency range of this target area correspondence.

7) adopt Nonlinear Conjugate Gradient Methods, carry out three-dimensional iterative inversion, obtain the conductivity parameter of zone of ignorance at abnormal area conductivity value parameter;

Nonlinear Conjugate Gradient Methods is present popular a kind of nonlinear inversion method.In refutation process, the first step is revised the conductivity parameter of zone of ignorance, calculates anomalous field; The ambient field that the utilization of second step had been calculated and the anomalous field of zone of ignorance calculate apparent conductivity curve and phase curve; The 3rd step was calculated apparent conductivity curve and phase curve error of fitting; Repeat above-mentioned three the step meet the demands up to error of fitting, obtain the conductivity parameter of zone of ignorance.

8) the subsurface conductivity parameter combinations of the abnormal area that obtains according to background, known region and by 3-d inversion is set up electromagnetism 3 D electromagnetic inverting data volume, as the foundation of geologic interpretation.

According to background conductance rate parameter (σ _n), known region conductivity (σ _b) and obtain conductivity parameter (σ by 3-d inversion _a) obtain entirely descending the medium conductivity distribution situation, set up electromagnetism 3-d inversion data volume, as the foundation of geologic interpretation.

Claims (5)

1, a kind of 3 D electromagnetic fast inversion method of minimized target, its feature adopts following steps:

1) in the area of carrying out earthquake, geology or drilling well work, conductance curve and phase curve are looked in actual measurement target exploration operational area, adopt usual way to carry out three-dimensional interpolation, make it to be uniformly distributed in the grid node of subdivision unit;

2) the electromagnetic survey workspace is divided into three parts: 1. background area that underground medium is known and conductivity values (σ _n), 2. adjacent known region (D _b) and corresponding conductance parameter (σ _b), 3. by the definite target area (D that will carry out inverting of exploration task _a), its corresponding conductivity values parameter (σ _a) determine by adjacent area known electric well-log information, geology outcrop data;

3) adopt common electromagnetism one dimension analytic formula to calculate known background area and conductivity values (σ _n) relevant electric field (E ⁿ) and magnetic field (H ⁿ), adopt formula (7) (8) 3 D electromagnetic integral formula to calculate known region (D _b) relevant electric field E ^DbAnd magnetic field H ^Db:

$E^{D_b}\left(r_j\right)=\underset{D_b}{\∫}{\hat{G}}_E\left(r_j\right|r)\·\left(\mathrm{\Δ}{\mathrm{\σ}}_{b}E\left(r\right)\right)\mathrm{dv}---\left(7\right)$

$H^{D_b}\left(r_j\right)=\underset{D_b}{\∫}{\hat{G}}_H\left(r_j\right|r)\·\left(\mathrm{\Δ}{\mathrm{\σ}}_{b}E\left(r\right)\right)\mathrm{dv}---\left(8\right)$

E ^DbAnd H ^Db: represent electric field relevant and magnetic field wherein respectively with known region:

With

Be respectively the green operator in electric field and magnetic field, Δ σ _b=σ _b-σ _nThe known region conductivity values is poor with corresponding background conductivity values, and E (r) be any any electric field;

4) adopt following 3 D electromagnetic integral formula to calculate target area (D _a) E ^DaAnd magnetic field H ^Da

$E^{D_a}\left(r_j\right)=\underset{D_a}{\∫}{\hat{G}}_E\left(r_j\right|r)\·\left(\mathrm{\Δ}{\mathrm{\σ}}_{a}E\left(r\right)\right)\mathrm{dv}---\left(9\right)$

$H^{D_a}\left(r_j\right)=\underset{D_a}{\∫}{\hat{G}}_H\left(r_j\right|r)\·\left(\mathrm{\Δ}{\mathrm{\σ}}_{a}E\left(r\right)\right)\mathrm{dv}---\left(10\right)$

E ^DaAnd H ^Da: represent electric field and the magnetic field relevant respectively with the target area,

With

Be respectively the green operator in electric field and magnetic field, Δ σ _a=σ _a-σ _nTarget area conductivity values and background conductivity values poor, E (r) is the electric field of any arbitrarily;

5) according to above-mentioned 3), 4) result of calculation calculate and groundly to look conductance and phase place is corresponding;

6) according to the target area correspondence of target area size, buried depth, scale and calculating look conductance curve and frequency plot curve, determine 5～20 frequencies look the conductance curve and phase curve carries out inverting;

7) adopt conventional Nonlinear Conjugate Gradient Methods, carry out three-dimensional iterative inversion, obtain the conductance parameter at target area conductivity values parameter;

8) according to background conductance parameter (σ _n), known region conductance (σ _b) and obtain conductance parameter (σ by 3-d inversion _a) obtain the conductivity distribution situation of whole underground medium, set up electromagnetism 3-d inversion data volume, form the foundation of geologic interpretation.

2, the 3 D electromagnetic fast inversion method of minimized target according to claim 1 is characterized in that: described (σ _a) (σ _b) represent the conductivity values of each unit in the zone, determine its value according to existing drilling data in the depth range of objective area inverting.

3, the 3 D electromagnetic fast inversion method of minimized target according to claim 1 and 2 is characterized in that: described (σ _a) (σ _b) represent the conductivity values of each unit in the zone with any conductivity values filling.

4, the 3 D electromagnetic fast inversion method of minimized target according to claim 1 is characterized in that: described 5～20 frequencies look conductance curve and phase curve frequency range necessarily greater than the frequency range of this target area correspondence.

5, the 3 D electromagnetic fast inversion method of minimized target according to claim 1 is characterized in that: described non-linear conjugated gradient refutation process is that the first step is revised the conductance parameter of zone of ignorance, calculates anomalous field; The ambient field that the utilization of second step had been calculated and the anomalous field of zone of ignorance calculate and look conductance curve and phase curve; The 3rd step calculated and looks conductance curve and phase curve error of fitting; Repeat above-mentioned three the step meet the demands up to error of fitting, obtain the conductance parameter of zone of ignorance.

Images