Geonics, Ltd. EM34-3XL earth conductivity meter with data logger:
The EM34 transmits a low-frequency electromagnetic signal into the earth. This signal causes charged particles to flow, which generates a secondary electromagnetic wave. A receiver antenna and electronic analyzer interprets the strength of this secondary wave in terms of the apparent conductivity of the ground. The EM34 transmits at one of three different frequencies and can be deployed in either the vertical or horizontal dipole mode. At the highest frequency, the transmitting and receiving antennae are set 10 meters apart. At the intermediate and lowest frequency, they are set 20 and 40 meters apart respectively. Rather than 'frequency', the operator specifies a 'separation' setting of 10, 20 or 40 meters, and, either the vertical or horizontal dipole.
Apparent conductivity is the weighted average of the conductivity distribution with depth of the ground in the vicinity of the transmitter. Response curves for vertical and horizontal dipole modes show the weighting functions. 3 separations and 2 modes provide 6 possibly different apparent conductivities for a given location. These 6 values can be interpreted to yield a shallow earth model. More complex models can be evaluated through forward modeling. Although results are not as well constrained or detailed as results from electrical resistivity soundings, EM34 surveys are far more rapid than electrical resistivity and do not require penetration of the ground surface. The EM34 can be used to interrogate under asphalt pavement or through frozen soil.
The EM34 is most useful as a reconnaissance tool for mapping lateral conductivity variations from the surface to a depth of about 40 meters. Such lateral variations may be due to changes in water chemistry (plume of contaminated fluids escaping a landfill), the depth to low-conductivity bedrock, or water-filled voids in shallow karst. A graduate student, Ms. Chaffee (M.S., 1994), constructed a pair of low-riding carts that allow us to make measurements (vertical dipole only) almost as fast as a person can walk by towing the array and recording apparent conductivity with the datalogger. Rubber cart wheels are replaced by skis for towing this array across ice or snow.