Richard Clark wrote:
"These sounds were the RF emissions of lightning in the AF band."

Richard also noted whistling sounds in army field telephones. I`ve seen
these audio radiations called "whistlers".

My efforts were to locate buried pipelines by a more convenient method
than bouncing a metal pick off of them. Pipeline rights of way can be
wide and pipes are often not where one may think but may be at odd
distances from the center of the right of way.

Connecting a signal to the pipe is much more effective than bouncing
signals off the pipe or detuning a metal locator with the pipe. Another
pipe tracing method in use is a vehicle (pig) launched into the pipe.
The pig contains a pneumatic hammer which produces so much noise that
it is easily heard through all the ground covering the pipe. Effective,
but it is not cheap to launch and recover pigs.

I`ve found broken pipes at the bottom of the Brazos River in Texas and
around offshore platforms in the Gulf of Mexico.

My efforts began with the availability of cheap SCR`s. I placed them to
key the output of cathodic protection rectifiers at a low audio
frequency rate. It worked well.

For more portability, I constructed a complimentary symmetry bipolar
power transistor generator which incorporated a 400-cycle Variac. The
generator was adjusted to produce a 12-Hz switching rate which proved to
work well. Output of the Variac was adjusted to give maximum power tnto
the pipeline wherever it was used from the switched 12 V automobile
battery. Connections to the pipeline are readily available at the
cathodic protection test points at intervals of several miles all along
the pipe.

Best regards, Richard Harrison, KB5WZI