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.

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.

A MAGNETOTELLURIC STUDY OF RESISTIVITY ANISOTROPY

Geophysics ◽  
1969 ◽  
Vol 34 (3) ◽  
pp. 438-449 ◽  
Author(s):  
D. Rankin ◽  
I. K. Reddy
Keyword(s):  
Magnetic Field ◽  
Apparent Resistivity ◽  
Field Measurements ◽  
Impedance Analysis ◽  
Complete Agreement ◽  
Precambrian Basement ◽  
University Of Alberta ◽  
The North ◽  
Principal Axes ◽  
Linearly Polarized

The apparent resistivity curves obtained from the magnetotelluric field measurements at the University of Alberta Geophysical Observatory, near Leduc, Alberta, reveal a strong anisotropy. The significance of the direction of the incident magnetic field in interpreting the structure from the apparent resistivity curves is examined in detail for the case of a linearly polarized incident field. The tensor impedance analysis of the data indicates that the directions of the principal axes of anisotropy are aligned parallel to the north‐south and east‐west measuring axis. The high coherency between the orthogonal horizontal components of the incident magnetic field is in complete agreement with this interpretation. The two apparent resistivity curves are interpreted and an anisotropic layered model is derived. The anisotropy is found to lie in the Precambrian basement down to a depth of 14 km. The isotropic upper layer has a resistivity of 8 ohm‐m which is characteristic of the sedimentary section. The anisotropic resistivity structure proposed for the Precambrian basement may be attributed either to local lithological changes in the basement or to major but more distant structure.

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.

Comparison of apparent resistivity functions for transient electromagnetic methods

Geophysics ◽  
1983 ◽  
Vol 48 (6) ◽  
pp. 787-789 ◽  
Author(s):  
A. P. Raiche
Keyword(s):  
Magnetic Field ◽  
Apparent Resistivity ◽  
Time Derivative ◽  
Mineral Resources ◽  
Transient Electromagnetic ◽  
Research Organizations ◽  
Electromagnetic Methods ◽  
Direct Measurements ◽  
The World ◽  
The Magnetic Field

The use of transient electromagnetic (TEM) methods is increasing throughout the world because of their success in finding conductive anomalies in regions previously thought inappropriate for EM techniques. Most TEM systems now in use (SIROTEM UTEM, Crone Pulse EM, etc.) employ voltage measurements, i.e., measurements of the time derivative of the magnetic field. However, the success of SQUID based receiving systems in many fields has led research organizations (such as the Bureau of Mineral Resources, Australia) to investigate the use of SQUIDs in TEM systems to obtain direct measurements of the magnetic field.

Magnetic field measurements in space

1963 ◽  
Vol 1 (3) ◽  
pp. 399-414 ◽  
Author(s):  
Laurence J. Cahill
Keyword(s):  
Magnetic Field ◽  
Field Measurements ◽  
Magnetic Field Measurements
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