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).

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.

Download Full-text

Apparent resistivity curves in controlled-source electromagnetic sounding directly reflecting true resistivities in a layered Earth

International Journal of Rock Mechanics and Mining Sciences & Geomechanics Abstracts ◽

10.1016/0148-9062(95)92464-s ◽

1995 ◽

Vol 32 (7) ◽

pp. 332

Keyword(s):

Apparent Resistivity ◽

Electromagnetic Sounding ◽

Controlled Source ◽

Layered Earth

Download Full-text

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 ◽

10.1190/1.1487025 ◽

1996 ◽

Vol 61 (3) ◽

pp. 918-918 ◽

Cited By ~ 1

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.

Download Full-text

Reply by the author to Pierre Valla

Geophysics ◽

10.1190/1.1487026 ◽

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.

Download Full-text

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

Geophysics ◽

10.1190/1.1443762 ◽

1995 ◽

Vol 60 (1) ◽

pp. 53-60 ◽

Cited By ~ 8

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.

Download Full-text