Apparent resistivity curves in controlled‐source electromagnetic sounding directly reflecting true resistivities in a layered earth

Geophysics ◽  
1995 ◽  
Vol 60 (1) ◽  
pp. 53-60 ◽  
Author(s):  
Umesh C. Das
Keyword(s):  
Direct Current ◽  
Apparent Resistivity ◽  
Dipole Source ◽  
Current Dipole ◽  
Text Data ◽  
Electromagnetic Sounding ◽  
Controlled Source ◽  
Electric And Magnetic Fields ◽  
Em Field ◽  
Subsurface Resistivity

Conversion of the measured voltages in direct current resistivity sounding methods into apparent resistivity [Formula: see text] is a useful step since [Formula: see text] data provide information about the subsurface resistivity variations with depth. This resistivity information then helps select a model for inverting the sounding data. In the controlled‐source electromagnetic method (CSEM), conversion of the measured electric and magnetic fields into apparent resistivity values has not been popular. This attitude may be attributed to the difficulties in the inversion of the resistivity of a half‐space from the electromagnetic (EM) field components as well as to the nonunique nature of the inversion giving two resistivity values for a single measurement. Two measured components—the vertical magnetic field [Formula: see text] and the tangential electric field [Formula: see text] as a result of a vertical magnetic dipole source—are combined to derive an exact apparent resistivity in a way similar to that used in direct current resistivity methods. Conversion of the measured [Formula: see text] and [Formula: see text] field components into apparent resistivity is found to be simple and can be carried out on a programmable pocket calculator. Theoretical apparent resistivity curves for frequency‐domain electromagnetic sounding show features similar to magnetotelluric (MT) and direct current dipole‐dipole apparent resistivity curves.

Reply by the author to Pierre Valla

Geophysics ◽  
1996 ◽  
Vol 61 (3) ◽  
pp. 919-919
Author(s):  
Umesh C. Das
Keyword(s):  
Asymptotic Behavior ◽  
Direct Current ◽  
Apparent Resistivity ◽  
Intermediate Step ◽  
Controlled Source ◽  
Layered Earth ◽  
Asymptotic Expressions ◽  
Higher Order Terms ◽  
Earth Structures ◽  
Subsurface Resistivity

I thank Pierre Valla for his interest in my paper (Das, 1995a). Transformation of controlled source electromagnetic (CSEM) measurements into apparent resistivities is carried out as an intermediate step in order to enhance interpretation. Duroux (1967; and hence Valla, 1984) derives, using asymptotic expressions (higher order terms are dropped out), apparent resistivities from CSEM measurements. Valla mentions, ‘those apparent resistivities do not have the nice asymptotic behavior’, and they can not be used as an intermediate step to estimate the layer resistivities and thicknesses in the subsurface. My aim in the paper has been not to work a ‘miracle’ but to derive a function to reflect the subsurface resistivity distributions of the layered earth structures directly. The calculations on a few models indicate that such a function can be derived which yields an unambiguous apparent resistivity. The apparent resistivity curves are similarly useful in interpretation as the direct current and magnetotelluric apparent resistivity curves. Inclusion of Duroux’s work would have given the readers a chance to appreciate my definition.

On: “Apparent resistivity curves in controlled‐source electromagnetic sounding directly reflecting true resistivities in a layered earth”, by U. C. Das (January‐February 1995 GEOPHYSICS, 60, 53–60).

Geophysics ◽  
1996 ◽  
Vol 61 (3) ◽  
pp. 918-918 ◽  
Author(s):  
Pierre Valla
Keyword(s):  
Magnetic Dipole ◽  
Apparent Resistivity ◽  
Electromagnetic Sounding ◽  
Controlled Source ◽  
Layered Earth ◽  
Vertical Magnetic Dipole ◽  
Em Field ◽  
Depth Sounding

Using a clever mix of two components of the EM field caused by a vertical magnetic dipole, U. C. Das derives what he claims to be an exact apparent resistivity for use in EM depth sounding.

On: “Apparent resistivity curves in controlled‐source electromagnetic sounding directly reflecting true resistivities in a layered earth”, by U. C. Das (January‐February 1995 GEOPHYSICS, 60, 53–60).

Geophysics ◽  
1996 ◽  
Vol 61 (3) ◽  
pp. 917-917
Author(s):  
Brian R. Spies ◽  
James R. Wait
Keyword(s):  
Apparent Resistivity ◽  
Previous Literature ◽  
Electromagnetic Sounding ◽  
Controlled Source ◽  
Layered Earth

Das has made a number of fundamental errors in his paper on apparent resistivity in controlled‐source EM sounding, and has ignored the previous literature.

Multiseparation, multisystem electromagnetic depth sounding‐An extension for unification

Geophysics ◽  
1997 ◽  
Vol 62 (1) ◽  
pp. 56-62 ◽  
Author(s):  
Umesh C. Das
Keyword(s):  
Apparent Resistivity ◽  
Mutual Coupling ◽  
Random Noise ◽  
Field Measurements ◽  
Dipole Source ◽  
Controlled Source ◽  
Definition Of ◽  
Depth Sounding ◽  
Central Loop ◽  
The Given

A recent definition of controlled‐source electromagnetic apparent resistivity has been adopted, and it is shown that this definition is unique. It produces a single apparent resistivity value by transforming any of the given combinations of the mutual coupling ratios measured by five different source‐receiver configurations, namely, horizontal coplanar loops (HCP), vertical coplanar loops (VCP), vertical coaxial loops (VCA), electric dipole source and horizontal receiver loop (EDL), and central loop (in‐loop) configurations. Synthetic field data for the commercially available MaxMin system, which can be operated with HCP, VCP, and VCA configurations, are fabricated and they are transformed to apparent resistivities. An analysis of apparent resistivity curves so obtained reveals the requirements of the ranges of frequencies and transmitter‐receiver separations needed for given exploration depth. A concise analysis of the effect of the random noise errors in the MaxMin data on stability of apparent resistivity is carried out. From this analysis, it is expected that apparent resistivities from field measurements will be stable, even when the measurements are corrupted with random noises.

Frequency‐ and time‐domain electromagnetic responses of layered earth—A multiseparation, multisystem approach

Geophysics ◽  
1995 ◽  
Vol 60 (1) ◽  
pp. 285-290 ◽  
Author(s):  
Umesh C. Das
Keyword(s):  
Magnetic Field ◽  
Direct Current ◽  
Apparent Resistivity ◽  
Field Measurements ◽  
Initial Guess ◽  
Dipole Source ◽  
Resistivity Sounding ◽  
Electromagnetic Methods ◽  
Time Domain Electromagnetic ◽  
Depth Sounding

EM depth soundings by controlled‐source electromagnetic methods (CSEM) are made to determine the vertical resistivity distribution of the earth. The two variations of soundings, namely frequency sounding and geometric sounding, are used for exploring the subsurface. Although in field applications electromagnetic (EM) sounding has relative advantages over direct current resistivity sounding, quantitative use of EM depth sounding has not been used as much as direct current resistivity sounding (Mundry and Bohlm, 1987). Mundry and Bohlm (1987) have pointed out that one of the problems in the application of frequency EM sounding is the lack of sophisticated interpretational tools. The interpretation of the mutual coupling ratio (MCR) from the EM field component measurements, [Formula: see text], invariably relies on numerical inversion routines. However, unlike the direct current (DC) or magnetotelluric (MT) apparent resistivity curves, MCR curves do not reflect the subsurface resistivity distributions, and an initial guess model from MCR required for its inversion is difficult. Conversion of single component EM measurements into apparent resistivity is ambiguous because for a single measurement, two apparent resistivity values are obtained. Combining the general expressions for MCR obtained from the quasi‐static tangential electric and vertical magnetic field components of a vertical magnetic dipole source on the surface of a half‐space, Das (1995) defined an apparent resistivity for the use of the CSEM. The CSEM apparent resistivity curves show features similar to DC and MT apparent resistivity curves and they greatly enhance the interpretation. I refer to this paper (Das, 1995, published in this issue) as Paper I. In the present paper, difficult measurements of the electric field have been avoided by combining [Formula: see text] and [Formula: see text] obtained from the magnetic field measurements of two different configurations, i.e., the horizontal coplanar loops (HCP) and vertical coplanar loops (VCP) systems, respectively. This combination of measurements provides operational simplicity in the field and gives the same CSEM apparent resistivity described in Paper I. However, complementary behavior of the two combinations would be realized.

2-D electromagnetic modeling by finite‐element method with a dipole source and topography

Geophysics ◽  
2000 ◽  
Vol 65 (2) ◽  
pp. 465-475 ◽  
Author(s):  
Yuji Mitsuhata
Keyword(s):  
Finite Element ◽  
Delta Function ◽  
Dipole Source ◽  
Electromagnetic Modeling ◽  
Secondary Field ◽  
Controlled Source ◽  
Transient Electromagnetic ◽  
Isoparametric Finite Element ◽  
Anomalous Bodies ◽  
Element Technique

I present a method for calculating frequency‐domain electromagnetic responses caused by a dipole source over a 2-D structure. In modeling controlled‐source electromagnetic data, it is usual to separate the electromagnetic field into a primary (background) and a secondary (scattered) field to avoid a source singularity, and only the secondary field caused by anomalous bodies is computed numerically. However, this conventional scheme is not effective for complex structures lacking a simple background structure. The present modeling method uses a pseudo‐delta function to distribute the dipole source current, and does not need the separation of the primary and the secondary field. In addition, the method employs an isoparametric finite‐element technique to represent realistic topography. Numerical experiments are used to validate the code. Finally, a simulation of a source overprint effect and the response of topography for the long‐offset transient electromagnetic and the controlled‐source magnetotelluric measurements is presented.

scholarly journals Megasource time-domain electromagnetic sounding at Poggi del Sasso - Cinigiano (GR)

1994 ◽  
Vol 37 (5 Sup.) ◽  
Author(s):  
G. V. Keller ◽  
P. Cantini ◽  
R. Carrara ◽  
O. Faggioni ◽  
E. Pinna
Keyword(s):  
Depth Profile ◽  
Time Domain ◽  
Environmental Conditions ◽  
Dipole Source ◽  
Electromagnetic Noise ◽  
Electromagnetic Sounding ◽  
Piecewise Constant ◽  
Time Domain Electromagnetic ◽  
The Time Domain ◽  
Single Set

An experiment was carried out in the vicinity of the “I Terzi” area in Southeastern Tuscany (fig. 1) to evaluate the applicability of the Time Domain Electromagnetic (TDEM) sounding method under the geological and environmental conditions prevailing in that area. An electromagnetic source was established using a motor-generator set and heavy cable. Measurements were attempted at four sites. Numerous samples of electromagnetic noise were recorded at each of these sites. At one site, signals transmitted for a grounded dipole source at 1.6 km distance were also recorded with the noise. The single set of observations has been processed and inverted to yield a six-layer piecewise constant resistivity depth-profile to a depth of about 2 km. The primary achievement of the experiment was demonstration of the praeticability of TDEM methods under the conditions prevailing in the site.

Remote Detection of Hydrocarbon Filled Layers Using Marine Controlled Source Electromagnetic Sounding

2002 ◽  
Author(s):  
L.M. MacGregor ◽  
T. Eidesmo ◽  
S. Ellingsrud ◽  
S. Constable ◽  
M.C. Sinha ◽  
...  
Keyword(s):  
Remote Detection ◽  
Electromagnetic Sounding ◽  
Controlled Source
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