Dielectric permittivity and resistivity mapping using high‐frequency, helicopter‐borne EM data

Geophysics ◽  
2002 ◽  
Vol 67 (3) ◽  
pp. 727-738 ◽  
Author(s):  
Haoping Huang ◽  
Douglas C. Fraser
Keyword(s):  
Dielectric Permittivity ◽  
Half Space ◽  
High Frequency ◽  
Free Space ◽  
Apparent Resistivity ◽  
Phase Component ◽  
High Frequencies ◽  
Space Model ◽  
Homogeneous Half Space ◽  
Displacement Currents

The interpretation of helicopter‐borne electromagnetic (EM) data is commonly based on the transformation of the data to the apparent resistivity under the assumption that the dielectric permittivity is that of free space and so displacement currents may be ignored. While this is an acceptable approach for many applications, it may not yield a reliable value for the apparent resistivity in resistive areas at the high frequencies now available commercially for some helicopter EM systems. We analyze the feasibility of mapping spatial variations in the dielectric permittivity and resistivity using a high‐frequency helicopter‐borne EM system. The effect of the dielectric permittivity on the EM data is to decrease the in‐phase component and increase the quadrature component. This results in an unwarranted increase in the apparent resistivity (when permittivity is neglected) for the pseudolayer half‐space model, or a decrease in the apparent resistivity for the homogeneous half‐space model. To avoid this problem, we use the in‐phase and quadrature responses at the highest frequency to estimate the apparent dielectric permittivity because this maximizes the response of displacement currents. Having an estimate of the apparent dielectric permittivity then allows the apparent resistivity to be computed for all frequencies. A field example shows that the permittivity can be well resolved in a resistive environment when using high‐frequency helicopter EM data.

Mapping of the resistivity, susceptibility, and permittivity of the earth using a helicopter‐borne electromagnetic system

Geophysics ◽  
2001 ◽  
Vol 66 (1) ◽  
pp. 148-157 ◽  
Author(s):  
Haoping Huang ◽  
Douglas C. Fraser
Keyword(s):  
Dielectric Properties ◽  
Dielectric Permittivity ◽  
Half Space ◽  
Magnetic Permeability ◽  
Free Space ◽  
Electromagnetic System ◽  
Space Model ◽  
The Earth

Interpretation of helicopter‐borne electromagnetic (EM) data is commonly based on the mapping of resistivity (or conductivity) under the assumption that the magnetic permeability is that of free space and dielectric permittivity can be ignored. However, the data obtained from a multifrequency EM system may contain information about the magnetic permeability and dielectric permittivity as well as the conductivity. Our previous work has shown how helicopter EM data may be transformed to yield the resistivity and magnetic permeability or, alternatively, the resistivity and dielectric permittivity. A method has now been developed to recover the resistivity, magnetic permeability, and dielectric permittivity together from the transformation of helicopter EM data based on a half‐space model. A field example is presented from an area which exhibits both permeable and dielectric properties. This example shows that the mapping of resistivity, magnetic permeability, and dielectric permittivity together yields more credible results than if the permeability or permittivity is ignored.

Shallow subsurface mapping by electromagnetic sounding in the 300 kHz to 30 MHz range: Model studies and prototype system assessment

Geophysics ◽  
1994 ◽  
Vol 59 (8) ◽  
pp. 1201-1210 ◽  
Author(s):  
Duff C. Stewart ◽  
Walter L. Anderson ◽  
Thomas P. Grover ◽  
Victor F. Labson
Keyword(s):  
Dielectric Permittivity ◽  
High Frequency ◽  
Low Frequency ◽  
Computer Programs ◽  
Thin Layers ◽  
Depth Range ◽  
Frequency Range ◽  
Model Studies ◽  
New Instrument ◽  
Displacement Currents

A new instrument designed for frequency‐domain sounding in the depth range 0–10 m uses short coil spacings of 5 m or less and a frequency range of 300 kHz to 30 MHz. In this frequency range, both conduction currents (controlled by electrical conductivity) and displacement currents (controlled by dielectric permittivity) are important. Several surface electromagnetic survey systems commonly used (generally with frequencies less than 60 kHz) are unsuitable for detailed investigation of the upper 5 m of the earth or, as with ground‐penetrating radar, are most effective in relatively resistive environments. Most computer programs written for interpretation of data acquired with the low‐frequency systems neglect displacement currents, and are thus unsuited for accurate high‐frequency modeling and interpretation. New forward and inverse computer programs are described that include displacement currents in layered‐earth models. The computer programs and this new instrument are used to evaluate the effectiveness of shallow high‐frequency soundings based on measurement of the tilt angle and the ellipticity of magnetic fields. Forward model studies indicate that the influence of dielectric permittivity provides the ability to resolve thin layers, especially if the instrument frequency range can be extended to 50 MHz. Field tests of the instrument and the inversion program demonstrate the potential for detailed shallow mapping wherein both the resistivity and the dielectric permittivity of layers are determined. Although data collection and inversion are much slower than for low‐frequency methods, additional information is obtained inasmuch as there usually is a permittivity contrast as well as a resistivity contrast at boundaries between different materials. Determination of dielectric permittivity is particularly important for hazardous waste site characterization because the presence of some contaminants may have little effect on observed resistivity but a large effect on observed permittivity.

Depth sounding by means of a coincident coil frequency‐domain electromagnetic system

Geophysics ◽  
1997 ◽  
Vol 62 (1) ◽  
pp. 49-55 ◽  
Author(s):  
Kenneth Duckworth ◽  
Edward S. Krebes
Keyword(s):  
Frequency Domain ◽  
Half Space ◽  
Free Space ◽  
Thin Sheet ◽  
Electromagnetic System ◽  
Homogeneous Half Space ◽  
Scale Modeling ◽  
Physical Scale ◽  
Depth Sounding ◽  
Theoretical Developments

The concept of electromagnetic depth sounding by means of a coincident‐coil frequency‐domain electromagnetic system is developed in theory and demonstrated by means of physical scale modeling. The concept is based on the use of distance from the target as the sounding variable. The theoretical developments are confined to soundings conducted in free‐space with respect to either a homogeneous half‐space or a thin sheet conductor in conditions that approach the resistive limit. The use of distance from the target as the sounding variable becomes practical when the sounding system is a single compact unit of the type that a coincident coil concept inherently provides. In this method of sounding, the distance from the target is determined by taking the ratios of the fields measured at a variety of distances from the target conductor. This permits not only the distance to the target to be determined but also the direction to that target as may be of interest in soundings conducted in mines.

A model for the high‐frequency electrical response of wet rocks

Geophysics ◽  
1983 ◽  
Vol 48 (6) ◽  
pp. 775-786 ◽  
Author(s):  
Peter C. Lysne
Keyword(s):  
High Frequency ◽  
Present Model ◽  
Electrical Response ◽  
Bulk Conductivity ◽  
High Frequencies ◽  
Aspect Ratios ◽  
Dielectric Data ◽  
Bulk Electrical Conductivity ◽  
Wet Rocks ◽  
Displacement Currents

In experiments on the electrical properties of rocks at high frequencies, the measured current has contributions from both conduction and displacement currents, and these are related to the bulk conductivity and dielectric permittivity, respectively. In the present model, the bulk electrical conductivity of a specimen is taken to be a constant given by Archie’s rule, whereas its frequency‐dependent permittivity is taken to be a generalization of Sillars’ model of a composite dielectric. The generalized Sillars’ model treats the pores as being an assemblage of spheroidally shaped inclusions with different orientations and aspect ratios. The conductivity of these spheroids, that is, the conductivity of the pore fluid, influences the frequency dependence of the permittivity in a manner that is in reasonable accord with available data. Furthermore, when applied to the dielectric data obtained in experiments on saturated rocks, the model yields distributions of pore shapes. These distributions are used to estimate the electrical response of oil‐ and water‐wet rocks that are unsaturated.

On: “A grounded vertical long‐wire source system for plane wave magnetotelluric analog modeling” by R. N. Edwards (GEOPHYSICS, October 1980, p. 1523–1529).

Geophysics ◽  
1981 ◽  
Vol 46 (6) ◽  
pp. 934-935 ◽  
Author(s):  
James R. Wait
Keyword(s):  
Plane Wave ◽  
Half Space ◽  
Radial Distance ◽  
Skin Depth ◽  
Space Model ◽  
Homogeneous Half Space ◽  
The Earth ◽  
Analog Modeling ◽  
Vertical Wire ◽  
Electrical Skin

In an interesting analysis, Edwards shows that a vertical long wire source will produce electromagnetic (EM) fields that satisfy simple impedance relationships for a homogeneous half‐space model of the earth. The important restriction is that the radial distance to the observer be large compared with an electrical skin depth. Certainly the vertical wire structures provide a very convenient modeling scheme for the “average prospector” to interpret magnetotelluric (MT) data collected over confined inhomogeneities within the conductive host region.

Conductivity-depth imaging of helicopter-borne TEM data based on a pseudolayer half-space model

Geophysics ◽  
2008 ◽  
Vol 73 (3) ◽  
pp. F115-F120 ◽  
Author(s):  
Haoping Huang ◽  
Jonathan Rudd
Keyword(s):  
Half Space ◽  
Mineral Exploration ◽  
Synthetic Data ◽  
Depth Imaging ◽  
Space Model ◽  
Depth Images ◽  
Homogeneous Half Space ◽  
Time Domain Electromagnetic ◽  
Speed Up ◽  
Effective Depth

Helicopter-borne time-domain electromagnetic (HTEM) systems with a concentric horizontal coil configuration have been used increasingly in mineral exploration. Conductivity-depth imaging (CDI) is a useful tool for mapping the distribution of geologic conductivity and for identifying conductive targets. A CDI algorithm for HTEM systems with a concentric coil configuration is developed based on the pseudolayer half-space model. Primary advantages of this model are immunity to altimeter errors and better resolution of conductive layers than other half-space models. Effective depth is derived empirically from the diffusion depth and apparent thickness of the pseudolayer. A table lookup procedure is established based on the analytic solution of a half-space model to speed up processing. This efficiency makes generation of real-time conductivity-depth images possible. Tests on synthetic data demonstrate that the pseudolayer conductivity-depth-imaging algorithm maps a wider range of conduc-tivities and does a better job of resolving highly conductive layers, compared with that of the homogeneous half-space model. Effective depths are close to true depths in many circumstances. Field examples show stable and geologically meaningful conductivity-depth images.

Research on Application of all Time Apparent Resistivity Translation Algorithm for Large Fixed Loop TEM

2014 ◽  
Vol 644-650 ◽  
pp. 3620-3624
Author(s):  
Fei Long Li ◽  
Xi’an Zhu
Keyword(s):  
Electromagnetic Field ◽  
Half Space ◽  
Apparent Resistivity ◽  
Observation Time ◽  
Test Point ◽  
Homogeneous Half Space ◽  
Transient Electromagnetic ◽  
Analytic Expression ◽  
Research On Application ◽  
Translational Expansion

In order to calculate the all-time apparent resistivity for large fixed loop more quickly, the transient electromagnetic field analytic expression for lager fixed loop in homogeneous half space is analyzed in this paper. The facts that the curve has a characteristic of translational expansion and contraction with the conductivity, the test point coordinates and observation time are discovered. A new method using translation algorithm to calculate all-time apparent resistivity is proposed for the large fixed loop. A new calculation equation of all-time apparent resistivity for large fixed loop is derived which shows that the method is feasible and effective.

The use of quad–quad resistivity in helicopter electromagnetic mapping

Geophysics ◽  
2002 ◽  
Vol 67 (2) ◽  
pp. 459-467 ◽  
Author(s):  
Haoping Huang ◽  
Douglas C. Fraser
Keyword(s):  
Apparent Resistivity ◽  
Single Frequency ◽  
Magnetic Polarization ◽  
Phase Component ◽  
Misleading Information ◽  
High Frequencies ◽  
Quadrature Component ◽  
The Earth ◽  
The Impact ◽  
Dependent Choice

The apparent resistivity from a helicopter-borne frequency-domain electromagnetic (EM) system is typically obtained from the in-phase and quadrature responses arising from the flow of conduction currents in the earth. The most commonly used resistivity algorithms, derived from half-space models and using single-frequency data, do not account for magnetic polarization and consequently do not yield a reliable value for apparent resistivity in highly magnetic areas. This is because magnetic polarization modifies the EM response, causing the computed resistivity to be erroneously high. The impact of magnetic permeability on the EM response is much greater for the in-phase component than for the quadrature component. If magnetic polarization is to be ignored, the calculation of the apparent resistivity using the quadrature component at two frequencies (the quad–quad algorithm) is less subject to error from magnetic polarization than if the in-phase and quadrature responses at a single frequency are used (the in-phase–quad algorithm). The quad–quad algorithm, however, can display undesirable behavior for large induction numbers, i.e., when conductivities and frequencies are large. Determining which algorithm is optimum is a data-dependent choice, which, of course, is area dependent. We have studied the behavior of the quad–quad (apparent) resistivity and its comparison to in-phase–quad resistivity to determine the conditions under which the use of quad–quad resistivity is appropriate. For a two-layer earth, the behavior of the quad–quad resistivity depends mainly upon the ratio of the lower frequency fL to the upper-layer resistivity ρ1. If this ratio is low, the quad–quad resistivity will behave well. In areas yielding a high value of the ratio fL/ρ1, the quad–quad resistivity may lie outside of the range of the true resistivities of the earth and therefore provide misleading information. Our studies therefore suggest that the quad–quad resistivity algorithm should be avoided in areas where the ratio is large, i.e., when using high frequencies in conductive areas. The term large is relative. For a two-layer case, for example, the use of quad–quad resistivity is only recommended for magnetic areas where fL/ρ1 < 500 Hz/ohm-m, when conductive cover exists, and where fL/ρ1 < 50 Hz/ohm-m when a conductive basement underlies resistive cover. In spite of these limitations, quad–quad resistivity is often preferable to in-phase–quad resistivity in highly magnetic areas.

Sign in / Sign up

Export Citation Format

Share Document