Glossary of Geophysical Terms Commonly Used with Dualem Instruments

alignedAn array aligned with the traverse direction.
apparent conductivityConductivity measured at a given height above the surface of the earth.  In general, apparent conductivity will be less than the conductivity measured at the surface, as a portion of the volume sampled by the array consists of non-responsive air.
arrayA fixed arrangement of a transmitter coil and a receiver coil.  The adjacent figure shows the arrays available from a DUALEM instrument.  In standard orientation, the instrument incorporates the HCP and PRP arrays.  When rotated ¼ turn about its long axis, the instrument incorporates the VCP and NULL arrays.

broadsideAn array oriented at right angles to the traverse direction.
coilTurns of wire in a fixed form, used to transmit or receive electromagnetic fields.
conductivityThe ratio of electrical current flow to applied voltage, per unit length.  The MKS unit is siemen/m or S/m.
coplanarAn array where both the transmitter coil and the receiver coil lie in the same plane.
depth of investigationSee depth of sounding.
depth of soundingThe depth to which an array, operating at low induction number, accumulates about 70 % of its total response.
dipoleA pair of opposite poles at infinitely small separation.  A coil in an array can be represented by a magnetic dipole aligned with its axis (i.e. perpendicular to the coil) if the coil is separated from the other coil by at least several coil-diameters.
electromagnetic inductionA geophysical method in which the primary time-varying magnetic field of a transmitter induces time-varying electrical current in the earth, which produces a secondary time-varying magnetic field that is sensed by a receiver.
geometryThe situation of the transmitter and receiver in an array relative both to each other and to the earth.
HCPHorizontal coplanar.
horizontal coplanarA coplanar array where both coils are horizontal.  The low induction number for this array is about 0.09, and its depth of sounding is about 1.5 times the transmitter-receiver separation.
induction numberA dimensionless parameter of electromagnetic response.  For a uniform earth, it is (cmf)1/2s, where c is the conductivity and m is the magnetic permeability of the earth, and where f is the angular EM frequency and s is the transmitter-receiver separation of the array.
in-phase or inphaseThe component of the secondary magnetic field that is in phase with the primary magnetic field.  The in-phase component is negligible below the low induction number.
LINLow induction number.
loopOne-or-more turns of wire, usually in a flexible form.
low frequency approximationSee low induction number.
low induction numberThe induction number below which the patterns of electrical current induced in the earth are essentially invariant; thus, the sensitivity of a array at LIN to a given volume of earth is constant, and the response of that volume of earth is directly proportional to its conductivity.
magnetic permeability or susceptibilityA response from the re-orientation of magnetic domains in the earth induced by the primary field of the transmitter, which tends to oppose the in-phase component of induction.
nullAn array that obtains no response from a uniform- or layered-earth.
perpendicularAn array where one coil is horizontal and intersected by the axis of the other coil.  The low induction number for this array is about 0.34, and its depth of sounding is about 0.6 times the transmitter-receiver separation.
pptParts-per-thousand.  A unit for the amplitude of the secondary magnetic field relative to the primary magnetic field sensed by the receiver.  In DUALEM instruments, the in-phase component is expressed in ppt; the quadrature component is scaled linearly from ppt to mS/m, a unit of conductivity.
quadrature or quadphaseThe component of the secondary magnetic field that is shifted 90-degrees relative to the primary magnetic field.  At low induction number, quadrature is linearly proportional to conductivity.
resistive limitSee low induction number.
sfericsAtmospheric EM noise.  Vertical coils with tend to be more sensitive to the noise, especially if the coil is coplanar with the source.
VCPVertical coplanar.
vertical coplanarA coplanar array where both coils are vertical.  The low induction number for this array is about 0.19, and its depth of sounding is about 0.75 times the transmitter-receiver separation.
vertical soundingThe technique of measuring apparent conductivity at a range of heights above a given location, in order to deduce the nature of layering within the depth of sounding.

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