The EM31-MK2 is a conductivity meter that uses an electromagnetic inductive technique that measures the soil’s apparent conductivity as well as its magnetic susceptibility. The EM31’s transducer produces a magnetic field that produces a voltage. This voltage creates a secondary magnetic field that varies depending on the electromagnetic properties of its surroundings (the subsurface.) The EM31 is useful for mapping geological variations, groundwater contamination, or any other feature associated with a change in ground conductivity such as an underground storage tank.
The standard EM31-MK2 possesses an intercoil spacing of 3.66 meters. This means that EM31-MK2 can reach depths of approximately 5.5 meters in the vertical dipole and 2.75 meters in the horizontal dipole. A shorter version, the EM31-SH, possesses an intercoil spacing of 2 meters. This means that EM31-SH can reach depths of approximately 3 meters in the vertical dipole and 1.5 meters in the horizontal dipole.
The EM31 currently comes in two lengths as discussed above. Both the EM31-MK2 and EM31-SH are both carried by the operator via a shoulder strap. The weight and length can make the EM31-MK2 quite cumbersome compared to the EM31-SH but can obviously read much deeper. Often times, when exploration isn’t as deep, the EM31-SH is perfect.
Most operators use the EM31 in auto mode in conjunction with a GPS, letting the instrument free runs while GPS records NMEA strings for location. However, some users prefer laying out an XY grid and manually triggering the EM31 to take readings at station on lines, taking careful note of their data and surroundings. Apparently, there is a EM31 cart and wheel mode that can be deployed but they are scarcely used and hard to find.
In the near future, Geonics is developing an EM31 that possesses eight different intercoil spacings and, thus, creating a higher resolution.
Before renting an EM31, users should think about what anomalies they are trying to find, what depths they are trying to find these anomalies at, and how large an area they need to survey.
We already know that the EM31 can reach either 1.5 times or .75 times its intercoil spacing. However, the depth at which an EM31 can find a ferrous anomaly compared to the depth at which it can map soil types varies. For instance, a standard EM31-MK2 can find a 55 gallon drum at depths up to 2 meters whereas it can map soils to a depth of approximately 5.5 meters. The EM31-SH can map soils variations to approximately 3 meters.
If the depths to which need to be surveyed can mapped by either the EM31-MK2 or EM31-SH, it may be beneficial to use the smaller unit. Likewise, if the area is large, the smaller unit may be more friendly. Further, a large area may require the use of a GPS unit in conjunction with the EM31 rather than trying to keep track of lines and stations on large plots of land. It is also a wise idea to turn large surveys into several smaller surveys in case files become corrupted.
Typical EM31 Packing List
1 x EM31 with Transmitter & Receiver Coils
1 x Hip Pad
1 x Allegro CX Datalogger
1 x Battery Charger for Datalogger
1 x Cable (Console to Datalogger)
1 x Flash Card Reader
1 x USB Flash Drive with Manuals and Software
1 x Shipping Case (57 x 9 x 14, 60lbs)
EM31-MK2 and EM31-SH Specifications
1: Apparent conductivity in millisiemens per metre (mS/m)
2: In-phase ratio of the secondary to primary magnetic field in parts per thousand (ppt)
EM31-MK2 INTERCOIL SPACING: 3.66 meters
EM31-SH INTERCOIL SPACING: 2.0 meters
OPERATING FREQUENCY: 9.8 kHz
POWER SUPPLY: 8 disposable alkaline “C” cells (approx. 20 h continuous)
In-phase: +/- 20 ppt
MEASUREMENT RESOLUTION: +/- 0.1% of full scale
MEASUREMENT ACCURACY: +/- 5% at 20 mS/m
Conductivity: 0.1 mS/m
In-phase: 0.03 ppt
DATA STORAGE:10,000 records (2 components); 16,500 records (1 component); ext. memory available
Boom: 4m ext.; 1.4m stored
Case: 145x38x23 cm
WEIGHTS: 12.4 kg