Apparent earth conductivity over coal mines as estimated from through-the-earth electromagnetic transmission tests

Electromagnet ic narrow-band s i g n a l s were t r ansmi t t ed through t h e e a r t h a t 27 c o a l mines l oca t ed throughout t h e United S t a t e s . From those Bur eau of Mines tests, apparent e a r t h conduc t iv i t y va lues were der ived based upon a homogeneous half-space model of t h e e a r t h . The der ived conduc t iv i t y va lues were found t o be i nve r se ly p ropor t i ona l t o t h e t r a n s m i t t e d frequency and mine depth. A l i n e a r r eg re s s ion model r e l a t i n g t h e logar i thm of t h e conduc t iv i t y t o t h e mine depth was formulated, and t h e r e s u l t s i n d i c a t e t h a t t h e mine depth can be an adequate pred i c t o r of t h e apparen t e a r t h conduc t iv i t y above c o a l mines. Apparent e a r t h conduc t iv i t y was found t o decrease w i th mine depth. l ~ l e c t r i c a l engineer , P i t t sbu rgh Research Center, Bureau of Mines, P i t t sbu rgh , PA.