CN-121978765-A - Electromagnetic exploration apparent resistivity imaging method for vertical limited-length line source

CN121978765ACN 121978765 ACN121978765 ACN 121978765ACN-121978765-A

Abstract

The invention discloses a vertical limited-length line source electromagnetic exploration apparent resistivity imaging method which comprises the following steps of (1) deducing radial electric field response analysis type of vertical limited-length line source well-to-well electromagnetism on the ground based on a uniform half-space model, (2) setting a ground model, (3) inputting calculation parameters, (4) initializing and setting, (5) iterative calculation of resistivity, and (6) judging whether abnormal bodies exist or not and identifying abnormal body distribution. According to the invention, by introducing the monotonic relation between the electric field and the resistivity and adopting the numerical solution path more suitable for the electromagnetic response rule, the aim of shortening the apparent resistivity calculation time and realizing rapid imaging can be achieved.

Inventors

MA RUOLONG
CAO HUI
WANG KEWEI

Assignees

成都理工大学

Dates

Publication Date: 20260505
Application Date: 20260109

Claims (7)

1. A method of imaging apparent resistivity for electromagnetic exploration of a vertical finite length line source, comprising: Based on a uniform half-space model, taking a vertical finite length source as an electric dipole model, deducing radial electric field response analysis of well-to-ground electromagnetism of the vertical finite length source on the ground, and determining monotonically decreasing relation between electric field response and underground medium conductivity; Placing a vertical limited-length current source into an underground medium perpendicular to the ground to generate an electromagnetic field in the underground medium; Obtaining ground radial electric field response data, inputting parameters including current magnitude, vertical limited length current source starting point depth and end point depth, emission frequency and radial distance, setting a conductivity search interval, calculating to obtain an electric field response calculated value according to radial electric field response analysis, comparing the electric field response preset value with an actual measured value through a recursion dichotomy, adjusting the interval until convergence, iteratively calculating an underground medium conductivity value, and taking the reciprocal to obtain a apparent resistivity value; drawing a apparent resistivity distribution map of the underground medium according to the obtained apparent resistivity value; judging whether an abnormal body exists or not according to the apparent resistivity distribution diagram and identifying abnormal body distribution.
2. The method of claim 1, wherein the radial electric field response resolution is derived by introducing a magnetic vector and solving a quasi-static Maxwell equation set in a frequency domain, wherein displacement current is ignored, using a Hank transform and an inverse transform thereof, and the radial electric field response resolution is: Wherein E r Wire (C) is the radial electric field strength, sigma 1 is the medium conductivity, epsilon is the medium constant, mu is the magnetic permeability, lambda is the integral constant, omega is the angular frequency, f is the emission frequency, h 1 is the line source start point depth, h 2 is the line source end point depth, and J 1 (lambda r) is a first-order Bessel function.
3. The method for imaging apparent resistivity of electromagnetic survey of a vertical finite length line source of claim 1, wherein the recursive dichotomy is calculated by: Setting a solving interval [ sigma min ,σ max ] and precision beta of underground conductivity, and calculating an initial value sigma mid = (σ min + σ max )/2; Substituting the initial value sigma mid into a radial electric field response analysis formula to calculate an electric field response theoretical value E r0 , comparing the electric field response theoretical value E with an actual measured electric field response value E r , judging whether the difference value meets the precision beta when the actual measured electric field response value E r is larger than the electric field response theoretical value E r0 , outputting the sigma mid at the moment if the difference value meets the precision beta, otherwise, adjusting the solving interval to be [ sigma mid ,σ max ], judging whether the difference value meets the precision beta when the actual measured electric field response value E r is smaller than the electric field response theoretical value E r0 , outputting the sigma mid at the moment if the difference value meets the precision beta, otherwise, adjusting the solving interval to be [ sigma min ,σ mid ], and repeating the steps until the conductivity value at each measuring point is obtained.
4. The method for imaging the apparent resistivity of the electromagnetic exploration of the vertical finite length line source according to claim 3, wherein the contour map of the apparent resistivity in the measuring point range is drawn through Kriging interpolation according to the measuring point positions and the apparent resistivity corresponding to the measuring point positions.
5. The method of imaging the apparent resistivity of a vertical finite length line source electromagnetic survey according to claim 4, wherein when an anomaly occurs in a region of the contour map of apparent resistivity, it is possible to determine whether anomalies exist in the subsurface medium and the spatial distribution characteristics of the anomalies, and to distinguish between low-resistivity and high-resistivity.
6. A method of electromagnetic survey apparent resistivity imaging in a vertical finite length line source as defined in claim 5, wherein the apparent resistivity anomaly location is highly coincident with the true anomaly location when the vertical finite length current source is positioned below the anomaly.
7. A computer readable storage medium having program code stored thereon which, when executed, implements the vertical finite length source electromagnetic survey apparent resistivity imaging method of claims 1-6.

Description

Electromagnetic exploration apparent resistivity imaging method for vertical limited-length line source Technical Field The invention relates to the technical field of geophysical exploration, in particular to a visual resistivity imaging method for electromagnetic exploration of a vertical limited-length line source. Background In geophysical electromagnetic exploration, the electrical change of an underground medium can influence the propagation of electromagnetic waves, so that the observed electric field response is changed, and inversion of observed data by analyzing the propagation characteristics of the electromagnetic field is a key for constructing an underground geological model. However, the conventional inversion method generally relies on priori knowledge to build an initial geological model, and the forward modeling result is enabled to continuously approximate to actual observation data through repeated iterative correction. Although the method is widely applied, numerical forward calculation needs to be carried out again in each iteration, and especially when facing a complex three-dimensional structure, the calculation load is extremely large, so that the inversion process is long in time consumption, and the method becomes a main technical bottleneck for realizing real-time interpretation. Disclosure of Invention The invention aims to provide a vertical limited-length line source electromagnetic exploration apparent resistivity imaging method which can improve the calculation efficiency and accuracy of resistivity and enhance the identification capability of underground abnormal bodies. The invention aims at realizing the following technical scheme: A method of perpendicular finite length source electromagnetic survey apparent resistivity imaging comprising: Based on a uniform half-space model, taking a vertical finite length source as an electric dipole model, deducing radial electric field response analysis of well-to-ground electromagnetism of the vertical finite length source on the ground, and determining monotonically decreasing relation between electric field response and underground medium conductivity; Placing a vertical limited-length current source into an underground medium perpendicular to the ground to generate an electromagnetic field in the underground medium; Acquiring ground radial electric field response data, inputting a current magnitude, a vertical limited length current source starting point depth, a vertical limited length current source ending point depth, a transmitting frequency and a radial distance, setting a conductivity searching interval, calculating to obtain an electric field response calculated value according to a radial electric field response analysis method, comparing the electric field response preset value with an actual measurement value, adjusting the interval until convergence, iteratively calculating an underground medium conductivity value, and taking the reciprocal to obtain a apparent resistivity value; drawing a apparent resistivity distribution map of the underground medium according to the obtained apparent resistivity value; and finding abnormal bodies according to the apparent resistivity distribution diagram and identifying abnormal body distribution. Further, the radial electric field response analytic formula is derived by introducing a magnetic vector position and utilizing hank transformation and inverse transformation thereof to solve a quasi-static maxwell equation set neglecting displacement current in a frequency domain, and the radial electric field response analytic formula is as follows: Wherein E r Wire (C) is the radial electric field strength, sigma 1 is the medium conductivity, epsilon is the medium constant, mu is the magnetic permeability, lambda is the integral constant, omega is the angular frequency, f is the emission frequency, h 1 is the line source start point depth, h 2 is the line source end point depth, and J 1 (lambda r) is a first-order Bessel function. Further, the calculation steps of the recursive dichotomy are as follows: Setting a solving interval [ sigma min,σmax ] and precision beta of underground conductivity, and calculating an initial value sigma mid = (σmin + σmax)/2; Substituting the initial value sigma mid into a radial electric field response analysis formula to calculate an electric field response theoretical value E r0, comparing the electric field response theoretical value E with an actual measured electric field response value E r, judging whether the difference value meets the precision beta when the actual measured electric field response value E r is larger than the electric field response theoretical value E r0, outputting the sigma mid at the moment if the difference value meets the precision beta, otherwise, adjusting the solving interval to be [ sigma mid, σmax ], judging whether the difference value meets the precision beta when the actual measured electric field response value E r