Electricity: measuring and testing – Of geophysical surface or subsurface in situ – With radiant energy or nonconductive-type transmitter
Reexamination Certificate
2000-12-15
2003-03-18
Strecker, Gerard R. (Department: 2862)
Electricity: measuring and testing
Of geophysical surface or subsurface in situ
With radiant energy or nonconductive-type transmitter
C324S329000, C324S202000
Reexamination Certificate
active
06534985
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to electromagnetic sensing apparatus and in particular to a method and apparatus for the non-contacting measurement of earth material electrical and magnetic properties with respect to depth below surface and position.
BACKGROUND OF THE INVENTION
Portable devices currently available for in-situ direct estimation of nearby earth materials' apparent conductivity will be referenced in the following as terrain conductivity meters (TCM's) and multi-frequency EM sounders (MEMS). Extensions to the TCM approach, which will be referenced as the ground conductivity meter (GCM) and array conductivity meter (ACM), improve aspects of TCM operation.
TCM's
The first devices which could be described as TCM's were described by Doll (Doll, H. G. 1949, Introduction to Induction Logging and Application to Logging of Wells Drilled with Oil Base Mud, J. Pet. Technol. 1, pp 148-162) in a borehole context and Howell (Howell, M., 1966, A Soil Conductivity Meter, Archaeometry 9, pp 20-23) in a shallow soil conductivity measurement context. Examples of commercially-available TCM's include the Geonics Ltd. EM-31, EM-34 and EM-38 and the Geoftzyka CM-031. These devices use the Low Induction Number Approximation (LINA) to estimate the apparent conductivity of the earth materials over a range of depths by a linear scaling operation from the component of the reflected EM signals which are in quadrature (i.e. at a 90° phase shift) with the primary field emitted by the sensors transmitter coil. The component of the EM measurement arising from signals in phase with the primary field from these devices may also be interpreted, with some effort, in terms of apparent conductivity, as well as the apparent magnetic permeability of the earth materials under test. The terms apparent conductivity and apparent magnetic permeability are defined below.
Existing TCM's incorporate a transmitter coil transmitting a sinusoidal signal at a single stable frequency (e.g. approximately 10 kHz for the EM-31) such that the LINA holds, i.e. that the following inequality is true:
[(&ohgr;&mgr;&sgr;
0
)½&rgr;]<0.5
where &ohgr; is the operating frequency in radians/sec, &mgr; is the magnetic permeability of the earth in henrys/metre, &sgr;
0
is the conductivity of the earth in Siemensim, and &rgr; is the separation in metres between the transmitter and receiver coils (as described further below).
In a TCM, a receiver coil is located at a distance &rgr; from the transmitter coil and substantially coplanar with the transmitter coil. There may or may not be a preamplifier located near this receiver coil to increase its effective output signal level. The peak moment (transmitter coil current times number of turns time area of one turn) of the transmitter coil's magnetic field is such that an acceptable signal to noise ratio (SNR) can be obtained at the receiver.
TCM's also incorporate electronics which can analyse the signal picked up at the receiver coil into components in phase and in quadrature (90 degrees out of phase) with the transmitted field. The quality of calibration of the output of this process and its stability are important factors in the utility of the instrument. These electronics also include circuits and/or software which convert the measured quadrature component into an apparent resistivity using the LINA relationship as stated in McNeill (McNeill, J. D., 1980, Electromagnetic Terrain Conductivity Measurement at Low Induction Numbers, Technical Note TN6, Geonics Limited, Mississauga, Canada) after Wait (Wait, J. R., 1962, A Note on the Electromagnetic Response of a Stratified Earth, Geophysics 27, pp 382-385.), i.e.
&sgr;
a
=4/(&ohgr;&mgr;&rgr;
2
)*(
H
s
/H
p
)
quadrature
The fundamental unit of apparent conductivity under the MKS system of units is Siemensimeter (S(m), although most if not all TCM's present their quadrature data in terms of milliSiemensim. In situations displaying horizontally-layered geology, the apparent conductivity represents a weighted average of the earth materials' conductivity in the vicinity of the sensor. A commonly-accepted rule-of-thumb depth of investigation (DOI) has been defined by McNeill as 1.5 times the transmitter-receiver separation for the horizontal coplanar configuration and 0.75 times this separation for the vertical coplanar configuration, corresponding to a cumulative response value of approximately 30%. Using the same 30% value for the perpendicular configuration's cumulative response yields a DOI for this configuration of approximately 0.5 times the transmitter-receiver separation.
A separate data logging device is typically provided which can acquire, store and display the analog outputs of typical commercial TCM's on demand or at a preset sampling rate.
The Transmitter and Receiver coils are typically installed near the ends of a tubular boom or other support structure, while the electronics and data logging device are mounted in a package near the central point of the support structure. The boom in the EM-31 is designed to be partially disassembled for shipping, with the two outer portions attached via couplings to the central portion of the boom, which is affixed to the electronics package. Short-offset systems like the EM-38 are housed in unitary support structures which incorporate the receiver electronics. The EM-34 does not incorporate a rigid housing joining the transmitter and receiver.
The orientation of the Transmitter and Receiver coils is such that the axis of each coil is approximately vertical when the instrument is held in an upright position (the Horizontal Coplanar orientation).
TCM's incorporating horizontal coplanar coil geometries can be rotated 90 degrees about a line joining the Transmitter and Receiver coils to place the coils into the Vertical Coplanar orientation, wherein the axes of the coils are horizontal. As described above, this approximately halves the effective DOI for the system, and allows the user to investigate vertical variations In the conductivity structure of the earth.
The principal shortcomings of the TCM are its single transmitter-receiver coil pair, which doubles the measurement time per station if two depths of investigation are desired at each site, its substantial weight (12.4 kg for the EM-31), its weak joint structure (for the EM-31), which permits substantial sag and flexibility in the boom when assembled, and its bulky packaging-which generates uncomfortable magnitudes and directions of pressure on the operator's shoulder. TCM's which rely exclusively on the LINA formula quoted above for estimation of earth material conductivity will generate erroneous values when used under very conductive conditions.
MEMS
A related class of non-contacting multi-frequency electromagnetic sounders (MEMS) used for near-surface earth material investigation measure the variation in the instruments electromagnetic coupling with earth materials as a function of frequency. Examples of such instruments include airborne electromagnetic (AEM) sensors developed by various companies over the years, including Barringer Research, Dighem, Geotech, Geoterrex, Geophex, and Aerodat, and ground systems such as the Apex Double-Dipole™ and the Geophex GEM-2™ and Geophysical Survey Systems' GEM-300™ which operate in the range 330 to 20,000 Hz.
Conventional AEM sensors have been thoroughly described in the literature (e.g, Palacky and West, 1987). Over the last fifteen or twenty years, efforts have been made to increase the quantitative capabilities of some AEM sensors through improvements to calibration methodologies and the introduction of electronic calibration methods. These efforts achieved encouraging, though not definitive, results. The multi-frequency, rigid-boom approach used in helicopter electromagnetics (HEM) received the most attention in terms of calibration. In their simplest form, HEM sensors incorporate a linear coil array consisting of a transmitter, a receiver of effective area (turns times
Holladay, III John Scott
Lee James Leonard Corbett
Geosensors Inc.
Hill Nancy E.
Hill & Schumacher
Strecker Gerard R.
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