Tom Donaly wrote:

"Detecting Natural Electromagnetic Waves," The Amateur Scientist, Shawn
Carlson, Scientific American, May, 1996. 50,000 turns #30 wire on 1/2
inch rebar, 2 feet long. You'll have to make your own amplifier and low
pass filter.

Rebar has a horrible B-H curve, and low permeability. You would do
better with a length of black iron pipe that you annealed after you
sawed a slit in it, lengthwise, to reduce eddy current losses.