Sonde Locating

Some years ago, having a minimum amount of locator training, I was tasked with my first week of locate tickets. Most were straightforward and completed without issue but one was a real challenge. The construction side of my company needed me to locate an empty duct run that did not show up in our records. The only real technique that I knew was taping a ground wire to a fiberglass duct rod and pushing both of them as far as I could and then locating the wire. It did not go well. First, there was either debris or a break in the pipe which prevented me from pushing the wire more than 70 feet. Second, since the wire was not grounded at the far end, I was forced to use a high frequency (~300 kHz on my locator at the time) and I was selecting a Null locate mode. The locate was extremely difficult. The
neighboring utilities were carrying my signal and the Null mode in my receiver was indicating a path that was not possible. After an hour of this, I called for help.

Tom, an experienced locator who came to help, went about locating with what I had set up and advised me to use  peak mode as a way to minimize the effects of signal bleed onto other utilities. He also pulled back the duct rod and connected a sonde to the threaded end of the rod.

Once assembled, he pushed the sonde about ten feet and located the signal directly above. Tom located along the suspected duct path containing the sonde with the receiver handle rotated 90 degrees to align the receiver’s peak antennas in the direction of the transmitted signal. Tom found the signal and had me push the sonde another ten feet and found it again. The third time (and the rest of the run) he had me locate and flag the run until the unit stopped at about 70 feet. Later we discovered the duct had collapsed which prevented the rod from advancing further into the pipe.

Since that day, I have learned much more about sondes and their applications. A sonde is a small portable  trans-mitter designed to be inserted in buried pipes. They have many uses, but often they are used to find blockages, bends, unknown pipe paths and can be found attached to cameras and other devices for locating.


The signal output has a characteristic shape with a large center peak and two smaller ghost peaks before and after the Sonde position. The receiver response is indicated in red. For accurate locating and depth readings it is important to find the center peak which will also be the strongest.

Sondes can be used to find bends in pipe, blockages, collapsed duct, unknown duct/pipe paths and bore tips of HDD machines. With many applications, there are a few considerations when purchasing:

Check the specifications of the sonde with any rods or attachments you will be using.
• Sondes are tuned to a specific frequency. A high frequency (33 kHz) works well in plastic pipe and a low frequency (512 Hz) may work in metal pipes.
• Verify the frequency of the sonde is a frequency option in the locator to be used.
• Verify the maximum detection depth meets your needs.
• Check the thread size and count to match to a duct rod if this method will be used.

When using a sonde, the best practice is to use a new battery to ensure the job will be completed without the battery dying. Push the sonde in 5’-10’ intervals while locating and marking the peak signal. While locating each position, it is a necessary to find the center peak for marking and depth readings.

Review the path as you mark and be attentive to signal strength changes. A lowering of signal could indicate a depth change or a transition for plastic duct to metal duct and an increase in signal could mean the opposite. Once each job is completed, remove the sonde, clean and disconnect the battery or turn the unit off to stop transmitting.

Gary Morris is a Technical Service Technologist at 3M supporting Locating and Marking products and solutions. He has more than 28 years of experience with outside plant support and operations. The opinions in this article are that of Gary Morris, and in no way reflect the opinions of 3M Company. He can be reached at

Share on facebook
Share on google
Share on twitter
Share on linkedin