December 1, 2007Locating systems for horizontal directional drilling (HDD) are responsible for the directional aspects of this construction method. These systems consist of a transmitter, a receiver and most often a remote display.
The transmitter inside the drillhead generates a magnetic field, which the walkover receiver interprets as depth and direction. A second type of signal is broadcast from the transmitter, often referred to as data that includes pitch, roll, temperature and battery status. Assuming correct calibration, proper use and no outside interference, these receivers do a good job of providing the locator person with the information required to accurately steer the drillhead.
Most HDD projects are in or around urban areas and therefore interference is practically always present. The following is a brief discussion on interference, how it can be detected, how one can evaluate its effect on the receiver and finally some ways to deal with it.
Interference is often labeled as either active or passive. Active interference can be defined as anything that emits a signal or generates its own magnetic field. Some examples of active interference include power lines, traffic loops, fiber trace lines, invisible dog fences and an unknown source. You should not assume that just because there is no evidence or markings on the ground that there is nothing there. Some of the possible effects of active interference on a receiver include erratic signal strength and depth readings, loss of pitch and roll data and inaccurate calibration, which may lead to depth errors.
Passive interference as the name implies does not generate a signal. It could be defined as anything that blocks, absorbs or distorts a magnetic field. Examples include metal structures such as chain-link fences, rebar, saltwater and other unknown sources. Anything that is conductive has the potential to act as passive interference. Possible effects include depths appearing greater (or in some cases shallower) than they actually are, incorrect drill head location and direction, all information being blocked and incorrect calibration that may lead to depth errors.
Since you are more likely to encounter interference during drilling operations, it is beneficial to be able to identify where the interference is coming from and how you can estimate its effect on the locating equipment. This is a two-step process, with the first step being to walk the bore path without a transmitter turned on looking for signal readings on the receiver. The higher the signal being registered, the greater the interference. Walk the entire path making notes of the signal being read. Since each receiver brand displays signal in a different format, you cannot set hard and fast rules. Needless to say, however, the amount of signal being read by the receiver from the transmitter needs to exceed the interference by a significant amount to ensure an adequate signal for locating.
Once the intended drill path has been walked, it should become clear which parts of it might be influenced by interference. It should be emphasized that this first walkthrough tests primarily for effects on depth readings. The second part of the test involves the transmitter. The purpose is now to see how reliable the pitch and roll signal will be.
Let us assume that the planned depth for this particular bore is 10 ft. At this depth, the receiver will see a pitch/roll signal from the transmitter in the ground of a given strength. The issue now becomes: Is this signal powerful enough to overcome the interference? One way to find out is to simulate the bore in the following manner. Insert batteries in the transmitter and carry it so that it is one and a half times the anticipated drill depth removed from the receiver, which is being walked down the bore path. If the pitch and roll information is not affected during this test, it is reasonable to assume clear sailing during the bore. Although this test is quite effective, it cannot always pick up all the potential problems.
When dealing with interference, there are four main options available. The first is identifying the interference source and checking if it can be turned off. Security systems, invisible dog fences and, in some cases, power can often be turned off temporarily. Secondly, create separation from the interference source. This often means locating off the actual drill path using more advanced locating methods such as off-track guidance or remote steering. This way you may effectively be out of range of the interference but still within range of your transmitter. Third, using a stronger (higher signal) transmitter, which in some cases may mean going to a cable transmitter, is often an effective way of overcoming the interference. Fourth, using a transmitter operating on a different frequency may get better results in this particular area.
The most crucial thing, however, is to fully understand how your locating equipment is supposed to function so that it becomes immediately obvious when something out of the ordinary is happening.
Siggi Finnsson is product manager at Digital Control Inc. and is a member of the Electronic Drillmaster Advisory Board. All Drillmaster Reports are reviewed by the Electronic Drillmaster Advisory Board: Finnsson; John Bieberdorf, The Charles Machine Works Inc.; John Archambeault, McLaughlin Mfg.; and Ed Savage, Vermeer Mfg. Co.