How deep could a subterranean communications network penetrate on Mars using Apollo-era technology?

I have been wondering what type of communications network could be constructed to maintain contact with an autonomous subsurface habitat or research station buried deep within the Martian crust, constrained by the technical capabilities available during the Apollo era (late 1960s to early 1970s).

As you descend below the Martian regolith, radio propagation becomes increasingly difficult due to signal attenuation through dense, rocky materials. We know that low-frequency (LF) or very-low-frequency (VLF) radio waves can penetrate soil to some degree, similar to how early ground-penetrating experiments were conceived. However, at significant depths—approaching the transition where the crustal material becomes more consolidated or potentially moisture-rich—electromagnetic (EM) waves face severe skin-depth limitations.

Using only 1960s-era electronics, materials science, and power generation (e.g., RTGs, early battery tech, and vacuum tube/discrete transistor circuitry), what would be the ideal "layered" communication architecture?

• Surface to Near-Surface: Presumably, we would rely on high-gain parabolic antennas similar to the Apollo Lunar Communication Relay Unit (LCRU) to beam data back to Earth or orbiting satellites.

• Near-Surface to Subterranean: At what depth would standard radio frequencies (VHF/UHF) begin to fail? Could a network of buried, relay-style "repeaters" using VLF or ELF (Extremely Low Frequency) be feasible, given the massive antennas required by 1970s standards?

• Deep Subsurface: If EM waves become unviable, would it be practical to implement a mechanical or seismic communication system? Similar to how the Apollo Lunar Surface Experiments Package (ALSEP) utilized seismic sensors, could we leverage piezoelectric transducers to send coded seismic pulses (vibrations) through the Martian crust to relay information?

I am interested in how one would architect a "deep-reach" network using the constraints of that specific era:

1. At what specific depths or conditions do EM-based communication methods become untenable, and what would be the frequency-spectrum trade-offs?

2. Is there a theoretical limit where seismic or mechanical signaling becomes the only viable medium for an Apollo-era setup?

3. What would the physical infrastructure look like? (e.g., relay distance, power requirements for driving seismic actuators, or the feasibility of laying hard-wired cable networks over vast distances in that environment).

Essentially, I am trying to imagine how Apollo-era engineers might have solved the "deep-crust communication" problem, and where the hard physical limits of that technology would have forced a transition from electromagnetic signaling to mechanical or seismic methods.