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.

STATISTICAL EVALUATION OF ELECTRICAL SOUNDING METHODS. PART II: APPLIED ELECTROMAGNETIC DEPTH SOUNDING

Geophysics ◽  
1976 ◽  
Vol 41 (6) ◽  
pp. 1222-1235 ◽  
Author(s):  
S. H. Ward ◽  
B. D. Smith ◽  
W. E. Glenn ◽  
L. Rijo ◽  
J. R. Inman
Keyword(s):  
Magnetic Dipole ◽  
Electromagnetic Sounding ◽  
The Earth ◽  
Vertical Magnetic Dipole ◽  
Vertical Electric Sounding ◽  
Data Points ◽  
Electrical Sounding ◽  
Schlumberger Sounding ◽  
Dar Zarrouk Parameters ◽  
Depth Sounding

Schlumberger resistivity and electromagnetic sounding data have been obtained over prehistoric Lake Bonneville sediments near Delta, Utah. Inverse interpretation of these data illustrates that vertical electric sounding with the Schlumberger array is superior, for this environment, to electromagnetic sounding with either a vertical magnetic dipole or a horizontal magnetic dipole. Combined parametric and geometric electromagnetic sounding using 84 data points is inferior to Schlumberger sounding using 21 data points. However, superiority in these contexts pertains to parameter resolution. The earth models obtained with all three soundings are virtually identical within the limits of resolution. The importance of percent parameter standard deviations, parameter correlations, and least‐squares residuals are all illustrated for a field example in this manuscript. Reparameterization in terms of the Dar Zarrouk parameters S and T reduces the parameter correlations as expected. A generalization of reparameterization beyond the Dar Zarrouk parameters is suggested.

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.

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.

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.

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