[katieHaylor (OP)]

Devaraj asks:

Can any electromagnetic waves travel through hard rock below the earth's surface in order find deep underground water? Is there any possibility of detecting underground water in this way?

What do you think?

[evan_au]

Radar has been used to peer through the dry sands of the Sahara to map the terrain of the bedrock below.

I assume that this radar would be able to detect water under the sand, as this would show up as a region with reduced radar return, or a suspiciously flat area under the sand.

[chiralSPO]

Sonar (sound waves) is sometimes used to find underground oil, I assume the same principles would allow finding water too...

[glot]

Geology and then drilling.

[evan_au]

What methods can we use to detect underground water?

I understand that GRACE satellites have been able to measure groundwater levels in California during their big drought.
This uses gravitational attraction to peer beneath the surface: this pair of satellites are attracted closer together when they pass over a large mass, and drift farther apart when the mass is in front or behind them.

Because these satellites are traveling 500km above the Earth's surface, and 200km apart, the resolution at ground level is not very good - it can't tell you whether to drill on the left or the right-hand side of your field, for example.

But it can measure the water levels of a whole aquifer, and determine if the water is being used at a rate faster than it is being replenished.
See: https://en.wikipedia.org/wiki/Gravity_Recovery_and_Climate_Experiment#Oceanography.2C_hydrology.2C_and_ice_sheets

[Bass]

IP (induced polarization) and resistivity are very sensitive to water and are used to detect subsurface water. Down-hole EM surveys are used for a variety of reasons, especially in oil exploration, including the detection of groundwater (especially effective for saline water).