Choosing the frequency that will give the locator the clearest signal depends on many variables. Different frequencies may behave differently depending on the line conductivity or differing ground conditions.
The most common, almost "standard", frequencies utlised on the Pipe & Cable Locators throughout the world are 8kHz and 33kHz. These frequencies predominately cover most locating requirements.
The High Frequency, 131kHz is ideal for the Telecommunication industry to induce the signal on small copper pairs.
Very Low to Low (<1 kHz)
Frequencies in the range of 1 kHz and under need a good circuit. If the line being located is traceable, these lower range frequencies will travel very long distances and will not jump onto other pipes or cables unless they are a part of the same good circuit.
The low range frequency does not perform well where there are breaks, gaskets, poor connections and dry soil resistance to current flow. These deterrents will stop low frequency flow immediately as it does not flow easily into the ground and around these breaks.
Medium (4 – 40 kHz)
Frequencies in the medium range are a bit stronger and less susceptible to ambient noise (e.g. other frequency sources like power lines) compared to lower frequencies. In the medium frequency mode the effects of signal bleed into the surrounding earth and onto adjacent utilities is limited. If the soil and circuit properties are conductive, and the distance is reasonable, the medium frequency is usually a good frequency range to start with in most circumstances.
High Frequencies (40 - 300 kHz)
In the high frequencies range the properties of the utility system such as stubs, dead-ends, and poorly grounded laterals can more likely be energized and detected compared to using lower frequencies. This frequency range can also help the operator trace the signal through high-resistant rubber coated pipe joints and insulated cable. The signal is more likely to penetrate the gap of broken trace wire and other discontinuities and still impose signal on the downstream side of the conductor.
The drawback to this current range is that it bleeds heavily into the ground surrounding the conductor. This significantly reduces its range and compromises the accuracy of the signal. Higher frequency signals may “jump” and couple more readily onto nearby conductors. As a general rule, using higher frequencies in congested areas increases signal distortion, causing greater confusion for the locate operator. This unreliable signal can result in incorrect marking of the target utility.
Very-high Frequencies (>300 kHz)
Care should always be taken to minimise distortion. This is especially true in the higher frequency ranges. Frequencies in the very high range are most often used as a last resort on highly resistant circuits that cannot be located with a lower frequency. These frequencies penetrate the soil easily, energizing all nearby conductors. Very-high frequencies also bleed heavily into the ground, which significantly limits the distance they can travel.
Another application for very-high frequencies is to use them as a final site investigation check of an area before excavation is to occur. These higher frequencies tend to couple onto all metallic lines in the area, making very-high frequencies a good choice when trying to avoid unknown metallic lines in an area.
Information on Actual Locator Frequencies available