You wrote:
"You can use any insulated wire"

I agree.
I hound in tracing the location of large buried undrground pipelines,
the "cooperative method", a transmitter attached to the pipe with a
receiver sensing the signal on the pipe worked best. Due to ground
characteristics, the lower the frequency, the better. I used 12 Hz.

My transmitter used cpmplementary symmetry power transistors in its
output which fed a 400-Hz Variac which proved to work well as a variable
impedance matching transformer to the pipeline. Oscillator-amplifier and
Variac were about the size of a bread box and weighed much less than the
12-volt car battery which powered them.

The receiver was a common relay coil with an iron core which fed a
transistor amplifier which had its output rectified and metered. The
coil was attached to a stick which served as a wand. Divers used the
coil in rivers and oceans where it followed the signal very well, as on
land.

The strongest signal was when the core pointed directly to the pipe, and
checking the distance to either side of the pipe the signal could be
well received gave an estimate of the depth of the ground covering the
pipe, usually 6 to 8 feet in our case. I could go miles down the pipe
before the signal faded (once 8 miles).

I don`t know how applicable any of my pipeline experience is to animal
burrows but it works well with tracing buried conductors or wires in
walls for that matter.

Our pipelines also had rectifiers for cathodic protection and I also
keyed these on and off at low frequency to provide a unique signal to
follow.

Best regards, Richard Harrison, KB5WZI