Geonics Limited EM38-MK2, Operating Manual

6 

2.  EM38-MK2 OPERATING INSTRUCTIONS - Expanded Method 

 
The following procedures are used to prepare the EM38-MK2 for survey operation.  The 
suggested time interval between the various procedures are typical values; under adverse 
conditions they may have to be carried out more often and under good conditions less often, as 
dictated by survey experience. Since the procedure is similar for both coil separations, only the 
procedure for the 1 m coil separation will be described 
 
2.1  

Battery Test

 
The battery test should be carried out at the beginning of each day or when the battery voltage is 
suspected of being low.  Check the battery voltage as follow: set the MODE switch to BAT.   
In the case of a good battery meters will read over 720 units.  If meters read below 720 units 
replace the battery.  To remove the battery, undo the two thumb screws holding the cover of the 
battery compartment.  A Mallory MN1604 9-volt Alkaline battery or equivalent gives about 5 
hours of continuous operation.  For longer service (about 12 hours) we recommend a 
ENERGIZER L522 9-VOLT Lithium battery. 
 
Geonics also provides as an option, an external battery pack that give about 25 hours of 
continuous service. For more details see section 10. 
 
2.2  

Initial Inphase Nulling

 
The initial inphase nulling should be carried out at the beginning of the day at the first survey 
station.  As described in Technical Note TN-6 the EM38 measures ground conductivity by 
inducing very small electrical "eddy" currents in the ground and measuring the magnetic field 
that these currents generate.  A small transmitter coil located at the front of the EM38-MK2 is 
used to generate the time-varying primary magnetic field which induces the eddy currents in the 
ground, and a small receiver coil located at the end (middle for the 0.5 m separation) measures 
both this strong primary magnetic field and the much smaller secondary magnetic field arising 
from the eddy currents in the ground. 
 
It is the task of the receiver electronics to measure the very small signal from the eddy currents in 
the presence of the much larger signal arising from the primary magnetic field.  To facilitate this 
measurement an internally generated signal is used to cancel or "null" the large primary signal so 
that it does not overload the electronic circuitry.  Because of the high sensitivity of the EM38-
MK2 it is advisable to remove all metal objects from wrists, fingers, neck and pockets when 
making this adjustment (sensitivity to metal objects, which is discussed further in Section 2. is 
greatest near the coils at either end of the instrument). 
 
To null the EM38-MK2, lift the instrument to a height of about 1.5 meters (5 feet) above the 
ground and place in the horizontal dipole mode of operation (Fig. 1).  Set the Mode switch to the 
1 m position and null the I/P meter to indicate zero by 1 m control.  The EM38-MK2 is 
satisfactorily nulled when the instrument at a height of 1.5 meters the I/P meter reads 
approximately zero (± 10 mS/m).