I use that type of locators for the POCO to mark out cables prior to fault locating especially on screened 11 and 33 kV cables.
They are Radiodetection, cost about 12,000 NZ$, US$ 8,000.
You can induce a frequency in a cable or cables at 655Hz, 8.1kHz 33.1kHz or 65 kHz. Use different frequencies depending upon length of cable or double check a HV cable prior to spiking.
I prefer as what the telephone guy did, Have the cable disconnected at one end, connect one testlead from transmitter to the screen or outer sheeth, the other to an independant earthpin about 1 meter away.
The cable is usually earthed on the other site and a very accurate location can be done.
A small current at selected frequency is injected in the cable, about 50 to 100 mA. In case of a cable fault or leakage to earth a loss in signal is detected which the receiver will pick up. The receiver has adjustable gain which is visible as a bargraph, or percentage with optional audible sound as well.
When a cable is single core or not screened have both ends disconnected, earth one end and put transmitter at other end with independant earth, so a current at selected frequency can flow.
This method i find better than have the cable acting as an areal and you pick up a lot of other cables as well.
It depends what you are looking for, mark out cables for roadworks or trace the one cable you are interested in.
For cables in service which carry a load to a factory or transformer a clip on CT can be used around the cable, and a current loop can be generated in the cable under test to mark out where it goes.