Geonics Limited EM38-MK2, Operating Manual

9 

Particularly over resistive ground the operator must ensure that, when setting the instrument zero 
he is not near fences, cars or other large metallic objects.  The reason for this is that the theory 
wherein V =2H  applies only for a uniform or a horizontally layered earth.  The presence of 
large metallic objects will usually affect H more than V, and will thus give an error in the zero 
setting.  The operator must also make sure that the ground is laterally uniform for a few meters 
on either side of the site where he is setting the zero.  He must not, for example, attempt to set 
the zero in an abandoned landfill site where there are probably buried metallic objects, without 
first making sure that the surrounding area is reasonably uniform by carrying out a quick 
preliminary EM38-MK2 survey. 
 
It should be noted that if the operator does not want to use the Q/P zero to physically alter the 
zero setting during the survey, he should take readings of V and H (with the instrument at 1.5 
meters every so often during the day.  Then, when he is doing the data plotting he should 
calculate C, using the above quotation and add or subtract the value of C to the survey readings 
to correct them using linear interpretation as necessary.  In the first example given above, 11 
mS/m would be added to the readings, and in the second example 12 mS/m would be subtracted 
from them. 
 
2.4 

Final Inphase Nulling

 
It was stated above that the inphase nulling was required in order to cancel or null the large 
primary signal from the transmitter so that it does not overload the electronic circuitry, thus the 
nulling procedure was carried out before setting the instrument zero, as described above.  
Unfortunately the magnetic susceptibility of soils causes an additional signal to be picked up by 
the receiver coil when the EM38-MK2 is located close to or is lying on the surface of the ground.  
Fortunately this signal decreases rapidly with the instrument height above the ground and is 
negligible at a height of 1.5 meters. 
 
The additional signal is dealt with in one of two ways.  If the survey is to be carried out in only 
the vertical or horizontal dipole mode the instrument is simply placed on the ground in the 
appropriate position and the residual signal arising from the magnetic susceptibility is nulled out 
exactly as described in Section 2.2 (except that the instrument is on the ground). 
 
In the event that the zero level is to be checked during the survey, the nulling procedure should 
be repeated with the instrument at 1.5 meters before the zero check is carried out, and again 
when the instrument is placed back on the ground for further conductivity measurements.  The 
magnetic susceptibility of the soil generally varies relatively slowly with location so that 
checking the null on the ground every half hour or so is usually sufficient. 
 
Even though the EM38-MK2 with two coils separation provides information from two different 
depths, sometimes surveys are carried out in both the vertical and horizontal dipole 
configurations in order to obtain more information about the layering, as described later in this 
note.  Unfortunately, the signal from the susceptibility causes a positive meter deflection in the 
vertical dipole mode and a negative meter deflection in the horizontal dipole mode, so that even 
though the instrument is correctly nulled for a vertical dipole measurement, re-nulling is 
necessary for the horizontal mode measurement, significantly slowing survey speed.  However, 
this procedure must be followed when making accurate measurements in both the vertical and 
horizontal dipole modes on low conductivity ground having significant magnetic susceptibility.