Electromagnetic response of a dipping anisotropic half-space

1991 ◽  
Vol 39 (2) ◽  
pp. 268-271 ◽  
Author(s):  
J.R. Wait
Keyword(s):  
Half Space ◽  
Electromagnetic Response

ELECTROMAGNETIC SCATTERING BY CONDUCTORS IN THE EARTH NEAR A LINE SOURCE OF CURRENT

Geophysics ◽  
1971 ◽  
Vol 36 (1) ◽  
pp. 101-131 ◽  
Author(s):  
Gerald W. Hohmann
Keyword(s):  
Magnetic Field ◽  
Half Space ◽  
Line Source ◽  
Numerical Technique ◽  
The Body ◽  
Scale Model ◽  
Electromagnetic Response ◽  
Field Phase ◽  
Horizontal Magnetic Field ◽  
Depth Of Burial

A theoretical solution is developed for the electromagnetic response of a two‐dimensional inhomogeneity in a conductive half‐space, in the field of a line source of current. The solution is in the form of an integral equation, which is reduced to a matrix equation, and solved numerically for the electric field in the body. The electric and magnetic fields at the surface of the half‐space are found by integrating the half‐space Green’s functions over the scattering currents. One advantage of this particular numerical technique is that it is necessary to solve for scattering currents only in the conductor and not throughout the half‐space. The response of a thin, vertical conductor is studied in some detail. Because the only interpretational aids available previously were scale model results for conductors in free space, the results presented here should be useful in interpreting data and in designing new EM systems. As expected, anomalies decay rapidly as depth of burial is increased, due to attenuation in the conductive half‐space. Depth of exploration appears to be greatest for measurements of horizontal magnetic field phase, while vertical field phase is diagnostic of conductivity. Horizontal location and depth of burial are best determined through measurements of vertical or horizontal magnetic field amplitude.

To: “The electromagnetic response of thin sheets buried in a uniformly conducting half‐space,” by R. Clark Robertson in January 1987 GEOPHYSICS, p. 108–117

Geophysics ◽  
1987 ◽  
Vol 52 (4) ◽  
pp. 583-583
Keyword(s):  
Electric Field ◽  
Half Space ◽  
Thin Sheet ◽  
Skin Depth ◽  
Modeling Technique ◽  
Electromagnetic Response ◽  
Thin Sheets ◽  
Incident Electric Field

On p. 112, the caption of Figure 4 should read “The (a) magnitude and (b) phase in radians of the x component of the horizontal electric field obtained for a square thin sheet of integrated conductivity 1 S, 8 skin depths on a side, buried at a depth of 0.1 skin depth when the incident electric field is x polarized. Each segment is 1 skin depth on a side.” On p. 114, the last sentence of the first paragraph in the Discussion should read “It is easy to see why the surface thin sheet is a popular modeling technique for magnetotelluric applications.”

The effect of a conductive half‐space on the transient electromagnetic response of a three‐dimensional body

Geophysics ◽  
1985 ◽  
Vol 50 (7) ◽  
pp. 1144-1162 ◽  
Author(s):  
William A. SanFilipo ◽  
Perry A. Eaton ◽  
Gerald W. Hohmann
Keyword(s):  
Half Space ◽  
Free Space ◽  
Eddy Currents ◽  
Three Dimensional ◽  
Vertical Component ◽  
The Body ◽  
Electromagnetic Response ◽  
Loop Current ◽  
Transient Electromagnetic ◽  
Space Response

The transient electromagnetic (TEM) response of a three‐dimensional (3-D) prism in a conductive half‐space is not always approximated well by three‐dimensional free‐space or two‐dimensional (2-D) conductive host models. The 3-D conductive host model is characterized by a complex interaction between inductive and current channeling effects. We numerically computed 3-D TEM responses using a time‐domain integral‐equation solution. Models consist of a vertical or horizontal prismatic conductor in conductive half‐space, energized by a rapid linear turn‐off of current in a rectangular loop. Current channeling, characterized by currents that flow through the body, is produced by charges which accumulate on the surface of the 3-D body and results in response profiles that can be much different in amplitude and shape than the corresponding response for the same body in free space, even after subtracting the half‐space response. Responses characterized by inductive (vortex) currents circulating within the body are similar to the response of the body in free space after subtracting the half‐space contribution. The difference between responses dominated by either channeled or vortex currents is subtle for vertical bodies but dramatic for horizontal bodies. Changing the conductivity of the host effects the relative importance of current channeling, the velocity and rate of decay of the primary (half‐space) electric field, and the build‐up of eddy currents in the body. As host conductivity increases, current channeling enhances the amplitude of the response of a vertical body and broadens the anomaly along the profile. For a horizontal body the shape of the anomaly is distorted from the free‐space anomaly by current channeling and is highly sensitive to the resistivity of the host. In the latter case, a 2-D response is similar to the 3-D response only if current channeling effects dominate over inductive effects. For models that are not greatly elongated, TEM responses are more sensitive to the conductivity of the body than galvanic (dc) responses, which saturate at a moderate resistivity contrast. Multicomponent data are preferable to vertical component data because in some cases the presence and location of the target are more easily resolved in the horizontal response and because the horizontal half‐space response decays more quickly than does the corresponding vertical response.

To: “The electromagnetic response of thin sheets buried in a uniformly conducting half‐space,” which appeared in the January 1987 issue of GEOPHYSICS, p. 108–117

Geophysics ◽  
1987 ◽  
Vol 52 (10) ◽  
pp. 1455-1455
Keyword(s):  
Half Space ◽  
Electromagnetic Response ◽  
Thin Sheets ◽  
Left Hand

The term “4πσ” should be deleted from the left‐hand side of equations (16), (A-13), and (A-14).

The electromagnetic response of thin sheets buried in a uniformly conducting half‐space

Geophysics ◽  
1987 ◽  
Vol 52 (1) ◽  
pp. 108-117 ◽  
Author(s):  
R. Clark Robertson
Keyword(s):  
Thin Layer ◽  
Half Space ◽  
Thin Sheet ◽  
Three Dimensional ◽  
Horizontal Layer ◽  
Thin Layers ◽  
Electromagnetic Response ◽  
Thin Sheets ◽  
Magnetotelluric Data ◽  
The Earth

The interpretation of magnetotelluric data is hampered by the effect of three‐dimensional (3-D) conductivity variations within the earth. In particular, the effects of deep structures are masked by heterogeneities near the surface. In order to understand the effects of 3-D anomalies on magnetotelluric investigations, the electromagnetic response of 3-D models of the earth must be investigated. One technique used to model a 3-D earth is the thin‐sheet approximation. This technique confines all lateral changes in conductivity to a horizontal layer in a laterally homogeneous earth; however, the thin‐sheet technique can be applied only to anomalies that are electrically thin at the frequency of investigation. The thin‐sheet technique can be extended to include a greater variety of models by stacking heterogeneous thin layers. As a first step, the thin‐sheet technique is extended to model a buried, heterogeneous thin layer. Extension of the method to account for buried thin sheets is theoretically and computationally more involved than for a surface thin sheet, but the buried thin sheet still has computational advantages over other 3-D models.

Approximate calculations of the transient electromagnetic response from buried conductors in a conductive half‐space

Geophysics ◽  
1984 ◽  
Vol 49 (7) ◽  
pp. 918-924 ◽  
Author(s):  
J. D. McNeill ◽  
R. N. Edwards ◽  
G. M. Levy
Keyword(s):  
Half Space ◽  
Free Space ◽  
Target Location ◽  
Electromagnetic Coupling ◽  
Practical Interest ◽  
Electromagnetic Response ◽  
Galvanic Current ◽  
Transient Electromagnetic ◽  
Space Modeling ◽  
Space Response

The transient electromagnetic (TEM) response from a conductive plate buried in a conductive half‐space and energized by a large‐loop transmitter is investigated in a heuristic manner. The vortex and galvanic components are each calculated directly in the time domain using an approximate procedure which ignores the electromagnetic coupling present in the complete solution. In modeling the vortex and galvanic current flows, the plate is replaced with a single‐turn wire loop of appropriate parameters and a distribution of current dipoles, respectively. The results of calculations of the transient magnetic field at the surface of the earth are presented for a few selected cases of practical interest. The relative importance of the vortex and galvanic components varies with the half‐space resistivity. The vortex component dominates if the half‐space is resistive, in which case free‐space algorithms suffice for numerical modeling. Furthermore the measured responses give much useful information about the target, and large depths of exploration should be achieved. As the half‐space resistivity decreases, a significant half‐space response is observed, caused by currents induced in the half‐space itself. This response can be very large. Spatial variations in it caused by relatively small changes in resistivity, i.e., geologic noise, obscure the response from deep targets making them difficult to detect. The effect of the half‐space is also to delay, distort, and reduce the vortex component in comparison with the free‐space response. The behavior of the galvanic component is determined by the haft‐space current flow. The presence of this component explains the large enhancement of overall target response seen at early times over relatively resistive ground and the departure from an exponential decay seen over more conductive ground, again with respect to responses predicted by free‐space modeling. In more conductive ground the galvanic component completely dominates the vortex component, resulting in the loss of useful diagnostic information. Although target location and depth can still be determined, target shape and orientation are poorly defined. Because of galvanic current saturation good conductors are difficult to distinguish from poor ones.

DIFFRACTION OF ELECTROMAGNETIC WAVES BY AN INHOMOGENEOUS SPHERE

Geophysics ◽  
1962 ◽  
Vol 27 (4) ◽  
pp. 480-492 ◽  
Author(s):  
Janardan G. Negi
Keyword(s):  
Half Space ◽  
Electromagnetic Waves ◽  
Homogeneous Medium ◽  
Electromagnetic Response ◽  
General Situation ◽  
Conductivity Distribution ◽  
Conducting Sphere ◽  
Conductivity Variation ◽  
Magnetic Contrast ◽  
Inhomogeneous Sphere

The electromagnetic response of an inhomogeneous conducting sphere (having radial conductivity distribution) embedded in an infinite homogeneous medium is investigated. The effect of linear conductivity variation and magnetic contrast between the two media is discussed in detail and relevant curves are presented. Analytical results are further extended to a more general situation in which this sphere is located in a half‐space and the influence of air‐earth boundary is calculated. These results may be applied to geoelectrical interpretation.

Sign in / Sign up

Export Citation Format

Share Document