I'm interested in developing a bespoke through-the-earth (TTE) radio transceiver for a measurement instrument in a limestone cave system.
The issue I have is that conventional RF signals cannot penetrate the surface due to the conductivity of the permeable material so I've turned to frequencies in the 30kHz to around 130kHz range (potentially higher, not sure how effective this would be as per current research).
The plan is to use ground spikes to inject current into the earth and for the conducting medium to serve as a channel for the signals to reach the transceiver underground, and vice versa.
My question is: As I aim to make this project as cost effective as possible, what do I need to consider when designing this hardware? Are there any existing RF modules (preferably for Arduino) that can produce signals at these frequencies directly through a home-made antenna across the ground, or do I have to build one myself? If so, how would I go about designing such a module?
I'm not interested in transmitting audio or video from the cave, only .CSV files via MySQL. I've already identified the need for a GSM module to transmit the data from the surface to an online server for remote data collection purposes - the main issue is the data transmission between the transceivers. Attached is a diagram summarising a similar system given to me by a developer of a TTE radio system.
Any help would be greatly appreciated, even if just in the form of a nudge towards some appropriate sources.