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Geophysics foundations:Quick overview:

 EM terrain conductivity

 

General concept

Electromagnetic methods involve using oscillating electromagnetic energy whichpenetrates the ground and causes (induces) secondary EM fields in regions of

elevated electrical conductivity. Terrain conductivity surveys usually involve ahandheld instrument operating at a single frequency. Some systems estimateterrain conductivity at several frequencies. One transmitter coil generates the EMenergy and a second receiver coil detects EM fields caused by the transmitter aswell as fields induced in subsurface conductive regions. The combination of EM fields which is seenby the receiver is a signal that is not generally in phase with the transmitted sinusoidal signal.Therefore the received signal can be thought of as consisting of two parts one which is in phase withthe transmitter (called the in phase component) and the other which is !" degrees out of phase (calledthe quadrature or quad phase). These two terms are used in e#planations below.

$arge data sets can be collected efficiently but results can not be used directly to learn about

variations with depth. %ata are usually plotted as maps or line profiles of apparent conductivity andinterpreted to find the ground positions directly above conductive features. &ote that terrainconductivity mapping is a special case of frequency domain electromagnetic methods.

EM-31; a special case of terrainconductivity

The 'eonics EM* instrument isused to map average variations ofelectrical conductivity at depthsbetween +ero and three to si#metres. ,apid acquisition of

spatially dense data sets is usuallythe most important requirement.-hen searching for discretetargets the most important designconsideration is to avoid spatialaliasing (defined in the glossary).or small % targets (such as buried drums) a tightly spaced grid would be required. or /% targets(such as buried utility pipes) data spacing along profile lines would li0ely be tighter than spacing betweenlines assuming lines can be placed perpendicular to the target orientation.Mapping electrical conductivity of the ground

The estimation of ground conductivity is carried out using an appro#imation formula that relates the quadphase measurement to the conductivity of the ground. The formula is valid for the socalled 1low inductionnumber1 situation2 that is when the coil spacing is much smaller than s0in depth (defined in the glossary)of EM signals at the frequency being used. The EM* provides a good appro#imation for true groundconductivity when that conductivity is less than roughly *""" mS3m. 4s the true conductivity of the groundbecomes larger than *""" mS3m the value provided by the EM* becomes more of an underestimate.The figure to the right illustrates this. %etails can be found in technical note T&5 on the 'eonics website.

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These comments about terrain conductivity hold when the ground within range of the instrument isuniform. 6n nonuniform ground the instrument yields a value called the 1apparent  conductivity1

which is a complicated weighted average of conductivities of all materials within range. The insetimage to the right illustrates with a fading yellow +one how the effect of ground causes thetotal result to decay at greater distances from the instrument.

Response to long or s!all targets

$ong targets (called /% targets) have onedimension much larger than theinstrument7s coil spacing and the otherdimension much smaller. E#amplesinclude pipes trenches etc. 1Small1targets such as small metal ob8ects have

all dimensions smaller than the coilspacing.

or /% targets the patterns on response curves or maps depends upon whether the .55m long instrumentboom is oriented parallel or perpendicular to the target. This is because the geometry of source fieldsinduced currents in the target and the sensing receiver is different for the two situations.

9oth the quadrature phase (apparent conductivity) and inphase responses depend upon orientation.:lic0 the buttons below to see the orientation and response for these two configurations.

9oom perpendicular to targets.

 9oom parallel to targets.

9oth parallel and perpendicular datasets.

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 9oom perpendicular to targets.

 9oom parallel to targets.

9oth parallel and perpendicular datasets.

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 9oom perpendicular to targets. 9oom parallel to targets.9oth parallel and perpendicular data

sets.

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The response to small ob8ects will loo0 similar to that of /% ob8ects e#cept that the orientation ofthe instrument boom does not ma0e any difference. The response to small buried metal targets willbe more evident on the inphase measurements than the quad phase (apparent conductivity) measurements.

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