The I in TEI3M stands for Interfacing. Since the Earth is an analog entity and the Arduino is a digital device, an understanding of the conversion techniques of each others' data is instructive.

Analog	Required Conversion	Digital

Analog to Digital Conversion (ADC) Algorithm: Successive Approximation.

Digital to Analog Conversion (DAC): R-2R Ladder: Wikipedia or OpAmp-Electronics.
(Based on: Thévenin's Theorem)