1. Field of the Invention
The present invention relates to detection of electromagnetic (EM) signals from targeted concealed conductors and, in particular, to the precise location of such conductors in the presence of signal distortion.
2. Discussion of Related Art
Utility conduits are often buried underground and not readily accessible. It is often necessary to locate these concealed utility conduits in order to repair and replace them. It is also important to know the location of utility conduits so that excavators can avoid them while excavating an area. If these locations are not accurate for an excavation, harmful results may occur such as property damage, serious physical harm to a person, or even death.
In an attempt to overcome these harmful results, there are various ways to locate concealed conduits, for example, using pipe and cable locators. Underground pipe and cable locators (sometimes termed line locators) have existed for many years and are well known. Line locator systems typically include a mobile receiver and a transmitter. The transmitter can be coupled to a target conductor, either by direct electrical connection or through induction, to provide a current signal on the target conductor. The receiver detects and processes signals resulting from the EM field generated at the target conductor as a result of the current signal. The signal detected at the line locator can be a continuous wave sinusoidal signal provided to the target conductor by the transmitter.
The transmitter is often physically separated from the receiver, with a typical separation distance of several meters or in some cases up to many kilometers. The transmitter couples the current signal, whose frequency can be user selected from a selectable set of frequencies, to the target conductor. The frequency of the current signal applied to the target conductor can be referred to as the active locate frequency. The target conductor generates an EM field at the active locate frequency in response to the current signal.
Different location methodologies and underground environments can call for different active frequencies. The typical range of active locate frequencies can be from several Hertz (for location of the target conductor over separation distances between the transmitter and receiver of many kilometers) to 100 kHz or more. Significant radio frequency interference on the EM field detected by the receiver can be present in the environment over this range. Therefore, receivers of line location systems have often included highly tuned filters to preclude interference from outside sources from affecting the measurement of signals at the desired active locate frequency from the target conductor. These filters can be tuned to receive signals resulting from EM fields at each of the selectable active locate frequencies and reject signals resulting from EM fields at frequencies other than the active locate frequencies.
In line location systems, the signal strength parameter determined from detection of the EM field provides a basis for derived quantities of the current signal (i.e., the line current in the targeted conductor), the position of the line locator receiver relative to the center of the conductor, the depth of the conductor from the line locator receiver, and the input to a peak or null indicator (depending on the orientation of the electromagnetic field to which the detector is sensitive). Line location systems measure signal strength at one or more active frequencies, also referred to as measurement channels.
For detection of cables or pipes laying in a continuous path (e.g., buried in a trench or occupying an underground conduit extending over some distance), an assumption is often made that the induced electromagnetic field is concentric around the cable and that signal strength depends only on the local ground conductivity, the depth and horizontal position of the target cable, and the magnitude of AC current flowing in the cable. When this is the case, the EM field at the detector of the line locator, on which the signal strength depends, is said to be free of distortion.
Nearly all locating systems present a “peak” indication that results from a horizontally-oriented coil, with axis orthogonal to the direction of the cable, that has a maximum deflection over the presumed centerline of the cable, assuming the ideal undistorted field. Some locating systems also present a “null” output from a vertically-oriented coil, which has a minimum at the same position, again making the same ideal assumption about the electromagnetic field. Moreover, some locating systems also provide a top-oriented coil to further assist in estimating a conductor lateral position and depth using a non-linear optimization algorithm.
Often in a crowded underground utility environment of metallic pipes and cables, coupling of signals at the active locating frequency from the target conductor to other adjacent underground conductors can occur. These conductors (lines) are not intended to be tracked by the line location system, but coupling of currents from the target conductor to those neighboring conductors through various means (resistive, inductive, or capacitive), termed “bleedover,” can lead a line locator astray such that the operator of the line location system ceases tracking the targeted conductor (e.g., pipe or cable of interest) and instead begins following an adjacent line. In some cases, there may be bias in the targeted conductor's estimated centerline as a result of distortion due to bleedover.
In conventional receivers, it is very difficult to determine whether the receiver is tracking the targeted conductor or whether the receiver is erroneously tracking a neighboring conductor. Therefore, there is a need for refinement of walkover line location systems to allow for more precise locating of an underground conductor in the presence of distortion due to significant bleedover to other conductors. As stated above, the more precise a locating can be, the less likely that property damage, physical injury, or even death will occur due to an erroneous locate.