CN105929456B - Transient electromagnetic and nmr integration detection method based on new transmitted waveform - Google Patents

Abstract

The invention discloses a transient electromagnetic and nuclear magnetic resonance integrated detection method based on a new emission waveform. The steps are as follows: S1: launch a new waveform that satisfies the excitation conditions of transient electromagnetic and nuclear magnetic resonance at the same time, and then simultaneously perform transient Observation of electromagnetic and nuclear magnetic resonance; S2: use transient electromagnetic detection technology to obtain underground electrical distribution, and then perform nuclear magnetic resonance inversion on this basis to obtain the water content distribution of groundwater; S3: according to the set research scale, the research space It is discretized into grid units, and the internal physical properties of each grid are uniformly distributed, and the calculation formula of the initial amplitude of nuclear magnetic resonance is obtained. The method realizes the integrated detection of transient electromagnetic and nuclear magnetic resonance by providing a waveform that satisfies the excitation conditions of both transient electromagnetic and nuclear magnetic resonance at the same time. It can greatly improve the observation efficiency and save the cost.

Description

Transient electromagnetic and nuclear magnetic resonance integrated detection method based on new emission waveform

technical field

The invention relates to transient electromagnetic detection technology, in particular to a transient electromagnetic and nuclear magnetic resonance integrated detection method based on a new emission waveform.

Background technique

In recent years, the development of transient electromagnetic method is very rapid, including ground transient electromagnetic method, aviation transient electromagnetic method, mine transient electromagnetic method and so on. The pure secondary field can be observed by transient electromagnetic method, which can be measured in the near area; the working devices are flexible and diverse, including overlapping loops, center loops, separated loops, large loop sources, wells and other working devices; it has low resistance It has the advantages of sensitive volume response, small volume effect, high vertical and horizontal resolution, convenient construction and high efficiency. Therefore, this method is widely used in metal ore exploration, coal mine hydrogeological investigation and engineering exploration and other fields. In addition, TEM can also be applied to the observation of the tunnel face. Tunnel construction may face many geological disasters, such as water and mud inrush, rock burst, landslide, gas outburst, etc. Among them, sudden (surge) water and sudden (surge) mud disasters are one of the main geological disasters in tunnel construction. It has become a bottleneck problem restricting the construction of tunnels and other underground projects, which has brought serious safety problems and economic losses to tunnel construction, and is very easy to induce secondary geological disasters and ecological environment problems. Therefore, the advanced detection of water-bearing geological structures in front of the tunnel face has become an important engineering and technological problem that needs to be studied and solved urgently.

At present, the nuclear magnetic resonance method is a geophysical method that can directly detect water bodies. It uses artificially excited electromagnetic fields to make the macroscopic magnetic moments of hydrogen atoms in the ground chamfer. Under the precession, the frequency is unique to the hydrogen nucleus. The presence of water can be detected by receiving the precession signal generated by the macroscopic magnetic moment with a coil laid on the ground. However, nuclear magnetic resonance can only give the spatial variation of water content, and it is difficult to accurately describe the resistivity distribution of the existing medium. This is because nuclear magnetic resonance emits single-frequency electromagnetic waves, and the received signal is hydrogen in water. The electromagnetic waves released during the atomic precession attenuation process are not the electromagnetic signals released during the electromagnetic induction attenuation process, and the distribution of the electrical medium will directly affect the propagation and distribution of electromagnetic waves in the frequency domain. Therefore, the degree of understanding of the distribution of the electrical medium directly affects the interpretation accuracy of NMR detection.

TEM and NMR detection have many advantages and are used in many ways. However, nowadays transient electromagnetic and nuclear magnetic resonance are observed separately. If you want to use the data of transient electromagnetic and nuclear magnetic resonance at the same time for comprehensive interpretation, you must separately excite transient electromagnetic and nuclear magnetic resonance, and then collect the transient data separately. EM and NMR data.

Contents of the invention

The purpose of the present invention is to provide a transient electromagnetic and nuclear magnetic resonance integrated detection method based on a new emission waveform, which can greatly improve observation efficiency and save costs.

In order to achieve the above tasks, the present invention is achieved through the following technical solutions:

A transient electromagnetic and nuclear magnetic resonance integrated detection method based on a new emission waveform is characterized in that it includes the following steps:

S1: launch a new waveform that satisfies both transient electromagnetic and nuclear magnetic resonance excitation conditions, and then simultaneously conduct transient electromagnetic and nuclear magnetic resonance observations;

S2: Use transient electromagnetic detection technology to obtain underground electrical distribution, and then perform NMR inversion on this basis to obtain the water content distribution of groundwater;

S3: According to the set research scale, the research space is discretized into grid units, and the internal physical properties of each grid are uniformly distributed, and the calculation formula of the initial amplitude of NMR is obtained:

Among them, N is the number of subdivision units, K _j is the initial amplitude generated by the water contained in the jth unit, and n _j is the water content of the jth unit;

Under the excitation of multiple pulse distances, the above formula (1) can be expressed in matrix form:

Kn=E (2)

in, K _ij is the initial amplitude generated by the water volume in the jth unit under the excitation of the ith pulse distance;

The moisture content of each unit can be obtained by solving the linear equation group (2).

In the S1 step:

S11: The new waveform described is: the rising edge of the trapezoidal wave (0-t ₁ segment), the cosine wave of the Larmor frequency (t ₁ -t ₂ segment) and the trapezoidal wave falling edge (t ₂ -t ₃ segment) Combination; DC component I ₀ is 5-50A, cosine wave amplitude I ₁ is 0.5-5A, duration Δt ₁ (ie t ₂ -t ₁ ) is 100-1000ms, off time Δt ₂ (ie t ₃ -t ₂ ) is 10 ~ 500us.

S12. Simultaneously excite transient electromagnetic and nuclear magnetic resonance responses through the new waveform, and simultaneously collect transient electromagnetic and nuclear magnetic resonance data in one observation.

The steps of NMR inversion in the S2 step are:

S21. In an external magnetic field perpendicular to the direction of the earth's magnetic field, the interaction between the macroscopic magnetic moment and the applied magnetic field produces a torque, so that the macroscopic magnetic moment is no longer parallel to the earth's magnetic field, but produces a reversing angle with the earth's magnetic field;

S22. After the external magnetic field is removed, the macroscopic magnetic moment will interact with the earth's magnetic field so that the macroscopic magnetic moment gradually returns to the state before the magnetic field is applied. During the process, a free attenuation signal equal to the Larmor frequency is released;

S23. Deduce the distribution and water content of the underground water-bearing body according to the initial amplitude of the free decay signal.

The initial amplitude formula of the free decay signal is:

where E ₀ is the initial amplitude of the free decay signal, ω ₀ is the local Larmor frequency, M ₀ is the proton magnetic moment, I is the emission current, γ is the proton magnetic gyro ratio, and n is the water content at point r , q is the transmitted pulse distance, B _⊥ is the component of the external magnetic field perpendicular to the geomagnetic field.

The calculation method of the initial amplitude of the free attenuation signal is: use the finite element method to obtain the distribution of the external magnetic field; use the upright hexahedron subdivision and linear interpolation to obtain the overall matrix, and then solve the left end containing electrical distribution information and the right end containing source item information The distribution of magnetic field values can be obtained by the equations; the right-hand term is formed by back-calculating the source term through the half-space field value, and then the NMR response of the model can be obtained according to the magnetic field value calculated by the finite element method.

In the S2 step, in the transient electromagnetic method, the signal receiving device is a coil or a probe wound by a multi-turn wire, and what is received is the rate of change of the secondary magnetic field with time, according to the equivalent conductive plane theory:

Among them, S is the longitudinal conductance, K is a constant related to transmission and reception, I is the transmission current, a is the radius of the transmission loop, F is a function related to both time and conductance, and the expression of F is:

Among them, h is the depth of the thin plate, and t is the receiving time;

The longitudinal conductance S can be calculated according to the following formula:

The formula (6) is a nonlinear equation about the longitudinal conductance S, and the longitudinal conductance S can be obtained by solving this equation.

According to the definition of longitudinal conductance, there is a resistivity value ρ _τ (h)=dh/dS, and the corresponding depth is:

The integrated detection method of transient electromagnetic and nuclear magnetic resonance based on the new emission waveform of the present invention realizes the integrated detection of transient electromagnetic and nuclear magnetic resonance through the given new waveform, improves work efficiency and saves costs.

Description of drawings

Figure 1 is a schematic diagram of the new waveform transmitted.

Detailed ways

In order to make the objects and advantages of the present invention clearer, the present invention will be further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described here are only used to explain the present invention, not to limit the present invention.

This specific implementation provides an integrated detection method of transient electromagnetic and nuclear magnetic resonance based on a new emission waveform. This method first provides a new waveform that can simultaneously satisfy the excitation conditions of both transient electromagnetic and nuclear magnetic resonance detection. On this basis, the data of transient electromagnetic and nuclear magnetic resonance are collected at the same time through one observation; the data of transient electromagnetic and nuclear magnetic resonance are processed separately; finally, the comprehensive analysis of the detection results is comprehensively interpreted according to the data of transient electromagnetic and nuclear magnetic resonance.

Specifically follow the steps below:

S1. Launch a new waveform that satisfies the excitation conditions of transient electromagnetic and nuclear magnetic resonance at the same time, and then observe transient electromagnetic and nuclear magnetic resonance at the same time.

S2. Use transient electromagnetic detection technology to obtain underground electrical distribution, and perform nuclear magnetic resonance inversion on this basis;

The new waveform in step S1 is shown in Figure 1. The new waveform is the combination shape of the rising edge of the trapezoidal wave + the cosine wave of the Larmor frequency [I ₁ COS(wot)+I ₀ ] + the falling edge of the trapezoidal wave.

Among them, the 0~ _t1 section is the rising edge of the trapezoidal wave, the _t1 ~ _t2 section is the cosine wave of Larmor frequency, the _t2 ~ _t3 section is the falling edge of the trapezoidal wave; the DC component I ₀ is 5~50A, The amplitude I ₁ of the cosine wave is 0.5-5A, the duration Δt ₁ (ie t ₂ -t ₁ ) is 100-1000ms, and the turn-off time Δt ₂ (ie t ₃ -t ₂ ) is 10-500us.

Through the new waveform, the transient electromagnetic and nuclear magnetic resonance responses are excited at the same time, and the transient electromagnetic and nuclear magnetic resonance data are collected at the same time in one observation.

The steps of NMR inversion in the S2 step are:

S21. In an external magnetic field perpendicular to the direction of the earth's magnetic field, the interaction between the macroscopic magnetic moment and the applied magnetic field produces a torque, so that the macroscopic magnetic moment is no longer parallel to the earth's magnetic field, but produces a reversing angle with the earth's magnetic field;

S22. After the external magnetic field is removed, the macroscopic magnetic moment will interact with the earth's magnetic field so that the macroscopic magnetic moment gradually returns to the state before the magnetic field is applied. During the process, a free attenuation signal equal to the Larmor frequency is released;

S23. Perform nuclear magnetic resonance inversion according to the initial amplitude of the free decay signal.

The initial amplitude formula of the free decay signal is:

where E ₀ is the initial amplitude of the free decay signal, ω ₀ is the local Larmor frequency, M ₀ is the proton magnetic moment, I is the emission current, γ is the proton magnetic gyro ratio, and n is the water content at point r , q is the transmitted pulse distance, B _⊥ is the component of the external magnetic field perpendicular to the geomagnetic field.

The calculation method of the initial amplitude of the free attenuation signal is: use the finite element method to obtain the distribution of the external magnetic field; use the upright hexahedron subdivision and linear interpolation to obtain the overall matrix, and then solve the left end containing electrical distribution information and the right end containing source item information The distribution of magnetic field values can be obtained by the equations; the right-hand term is formed by back-calculating the source term through the half-space field value, and then the NMR response of the model can be obtained according to the magnetic field value calculated by the finite element method.

In the S2 step, in the transient electromagnetic method, the signal receiving device is a coil or a probe wound by a multi-turn wire, and what is received is the rate of change of the secondary magnetic field with time, according to the equivalent conductive plane theory:

Among them, S is the longitudinal conductance, K is a constant related to transmission and reception, I is the transmission current, a is the radius of the transmission loop, and F is a function related to both time and conductance;

The expression of F is:

In the formula, h is the depth of the thin plate, and t is the receiving time;

The longitudinal conductance S can be calculated according to the following formula:

Described (6) formula is about the non-linear equation of longitudinal conductance S, solves this equation and obtains longitudinal conductance S;

The above is only a preferred embodiment of the present invention, it should be pointed out that for those of ordinary skill in the art, without departing from the principle of the present invention, some improvements and modifications can also be made, and these improvements and modifications should also be It is regarded as the protection scope of the present invention.

Claims (5)

1. A transient electromagnetic and nuclear magnetic resonance integrated detection method based on a new emission waveform is characterized by comprising the following steps:

s1: emitting a new waveform which simultaneously meets the excitation conditions of transient electromagnetism and nuclear magnetic resonance, and then simultaneously observing the transient electromagnetism and the nuclear magnetic resonance;

s2: acquiring underground electrical distribution by using a transient electromagnetic detection technology, and performing nuclear magnetic resonance inversion on the basis to obtain the water content distribution of underground water;

s3: according to a set research scale, a research space is discretized into grid units, and physical properties in each grid are uniformly distributed to obtain a calculation formula of the nuclear magnetic resonance initial amplitude:

wherein N is the number of subdivision units, K_jInitial amplitude, n, generated for water contained in the jth cell_jIs the water content of the jth cell;

under multiple pulse interval excitation, the above formula (1) can be expressed in a matrix form:

Kn＝E (2)

wherein,K_ijthe initial amplitude generated by the water quantity in the jth unit under the excitation of the ith pulse distance;

solving the linear equation set (2) to obtain the water content value of each unit;

in the step S1:

s11: the new waveform is as follows: rising edge of trapezoidal wave is 0-t₁Cosine wave t of segment and Larmor frequency₁～t₂And falling edge t of trapezoidal wave₂～t₃A combination of segments; direct current component I₀5 to 50A, cosine wave amplitude I₁0.5-5A, duration delta t₁I.e. t₂-t₁100-1000 ms, and a turn-off time Deltat₂I.e. t₃-t₂10-500 us;

s12: and simultaneously exciting transient electromagnetism and nuclear magnetic resonance response through the new waveform, and simultaneously acquiring data of the transient electromagnetism and the nuclear magnetic resonance through one-time observation.

2. The method of claim 1, wherein the step of inverting nuclear magnetic resonance in step S2 is:

s21, in the external magnetic field perpendicular to the direction of the geomagnetic field, the interaction between the macroscopic magnetic moment and the external magnetic field generates a moment, so that the macroscopic magnetic moment is not parallel to the geomagnetic field any more, but generates a pulling chamfer angle with the geomagnetic field;

s22, after the external magnetic field is removed, the macroscopic magnetic moment is enabled to act with the earth magnetic field so that the macroscopic magnetic moment is gradually restored to the state before the magnetic field is applied, and in the process, a free attenuation signal equal to Larmor frequency is released;

and S23, deducing the distribution and the water content of the underground water-containing body according to the initial amplitude of the free attenuation signal.

3. The method of claim 2, wherein the initial amplitude of the free fading signal is formulated as:

in the formula, E₀For freely attenuating the initial amplitude, ω, of the signal₀Is the local Larmor frequency, M₀Is proton magnetic moment, I is emission current, gamma is proton magnetic rotation ratio, n is water content at r point, q is emission pulse distance, B_⊥Is the component of the applied magnetic field perpendicular to the geomagnetic field.

4. The method of claim 3, wherein the initial amplitude of the free fading signal is calculated by: the distribution of the external magnetic field is obtained by utilizing a finite element method; obtaining an overall matrix by adopting vertical hexahedron subdivision and linear interpolation, and then solving an equation set with the left end containing electrical distribution information and the right end containing source item information to obtain magnetic field value distribution; and forming a right-end term by a method of back-calculating a source term through a half-space field value, and then obtaining the nuclear magnetic resonance response of the model according to the magnetic field value calculated by a finite element method.

5. The method of claim 1, wherein in the step S2, in the transient electromagnetic method, the signal receiving device is a coil or a probe formed by winding a plurality of turns of wire, and the rate of change of the secondary magnetic field with time is received, and according to the equivalent conducting plane theory:

wherein S is the longitudinal conductance, K is a constant related to transmission and reception, I is the transmission current, a is the transmission loop radius, F is a function related to both time and conductance, and the expression of F is:

wherein h is the sheet depth, and t is the receiving time;

the longitudinal conductance S can be calculated as follows:

equation (6) is a nonlinear equation about the longitudinal conductance S, and solving the equation results in the longitudinal conductance S;

according to the definition of the longitudinal conductance, there is a resistivity value ρ_τ(h) dh/dS, corresponding to depths:

wherein, mu₀＝4π×10^-7The magnetic permeability constant is vacuum;