Depth, direction, pitch and roll are the most basic information displayed by all directional drilling receivers. The transmitter inside the drillhead generates a magnetic field, which the receiver interprets as depth and direction. A second type of a signal is used to broadcast data which includes pitch, roll, transmitter temperature and battery status.
Assuming correct calibration, proper use and no outside interference, these receivers do a fine job of providing the locator with quite accurate information required to steer the head.
Since much of drilling takes place in or around urban areas, interference is practically always present. The following is a brief discussion on interference, how it can be detected, how you can evaluate its effect on the receiver and, finally, some ways to deal with it.
Interference is often referred to as active or passive. Active interference can be defined as “anything that emits a signal or generates its own magnetic field.” That being said, all things electrical emit a magnetic field to a differing extent.
Some examples of active interference include power lines, traffic loops, fiber trace lines, invisible dog fences and an unknown source. Do 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 emit a signal of its own. It is defined as “anything that blocks, absorbs or distorts a magnetic field.” Examples include metal structures, such as chain link fences, rebar, salt water 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 we are more likely than not to encounter interference during drilling operations, we need to know how to identify where this interference is coming from and how you can estimate its effect on the locating equipment.
This is a two-step process, the first being to walk the borepath without a transmitter turned on, looking for signal readings on the receiver. The higher the signal present, the greater the interference level. 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 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 walk through tests primarily for effects on depth readings (the magnetic signal). The second part of the test involves the transmitter. The purpose is now to see how reliable the pitch and roll signal reception will be.
Let’s 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 question is, 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 such that it is 1.5 times the anticipated drill depth removed from the receiver, which is being walked down the borepath. In our example, this would be 1.5 x 10 = 15 ft. 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 potential problems.
When dealing with interference, there are four main options available. The first is after identifying the interference source; see if it can be turned off. Security systems, invisible dog fences and, in some cases, power can often be shut off temporarily. Secondly, is 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 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 with a different operating 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.