Abstract: A system for the electronic detection of military threats includes an electromagnetic (EM) gradiometer for detecting secondary EM waves scattered from objects illuminated by a primary EM wave existent in a local area. A right magnetic dipole antenna and a left magnetic dipole antenna are included in the EM gradiometer for receiving the secondary EM waves and able to reject the primary EM wave. A synchronizer derives a reference signal to the EM gradiometer from the primary EM wave. A synchronous detection processor makes measurements of the phase and amplitude of the secondary EM waves. A graphical user interface (GUI) is used to display information about the location of objects assessed to be threats in the local area that were computed from the measurements of the phase and amplitude. A vehicle may be used to transport the EM gradiometer over the local area during periods the objects are being radio illuminated.

Abstract: A combined electromagnetic and magnetometer detection system for detecting objects. The system includes a primary electromagnetic transmitter for generating a primary magnetic field; an electromagnetic sensor for sensing secondary magnetic fields generated by a target region subjected to the primary magnetic field; a magnetometer sensor substantially centered at a center of the primary magnetic field; a compensation transmitter for generating a compensating magnetic field that substantially negates the primary magnetic field across the magnetometer sensor; and a signal generator electrically connected to the primary electromagnetic transmitter and the compensation transmitter for driving the transmitters to generate the primary magnetic field and the compensating magnetic field, respectively.

Abstract: At least one antenna array including three mutually orthogonal antennas each sharing a common center point senses an electromagnetic signal emitted by a buried object such as a utility line, pipe or sonde. A circuit at least partially mounted in a housing is connected to the array and determines a location of the buried object by measuring signal strength and field angles in three dimensions without having to align the antenna array relative to the buried object while eliminating nulls and false peaks. A graphical user interface (GUI) has user-friendly icons, symbols, menus, numbers and graphical and auditory representation of signal strength. A plurality of different underground objects can be simultaneously detected and their different locations can be simultaneously indicated to a user via audible sounds and/or visual images on a display.

Abstract: Arrangements, apparatus and associated methods are described for use in a multi-frequency boring tool locating system. The boring tool includes a transmitter for transmitting a locating signal at two or more selectable frequencies. One set of above ground procedures may be applied to the transmitter in order to change the frequency of the boring tool transmitter. Another set of procedures is applicable for changing the frequency during below ground drilling operations, for example, by subjecting the boring tool to a predetermined roll orientation sequence. An enhanced portable locator operates in a manual or automatic mode to receive locating frequency information transmitted from the boring tool transmitter including frequency updates. Boring tool transmitter, as well as above ground locator shutdown/restart procedures are described relating to multi-frequency operation. A tone detector is described which implements one or more digital match filters.

Abstract: A device rotates at least one static magnetic field about an axis, producing a rotating magnetic dipole field, and is movable in relation to the surface of the ground. The field is periodically sensed using a receiver to produce a receiver output responsive to the field. A positional relationship between the receiver and the device is monitored using the output. In one aspect, changing the positional relationship, by moving the device nearer to a boring tool which supports the receiver, causes an increase in accuracy of depth determination. In another aspect, determination of an actual overhead position of the boring tool, and its application, are described. Use of a plurality of measurements over at least one-half revolution of each magnet is disclosed. Establishing a surface radial direction toward a boring tool and resolution of multi-valued parameters is described. Calibration techniques, as well as a three transmitter configuration are also described.

Abstract: A detector for locating a sonde is comprised of a plurality of antennas, a corresponding number of signal isolator modules 1 and a geometrical evaluation module. The magnetic signal generated by the sonde has a fundamental frequency and harmonic frequencies. A frequency spectrum analyzer 29 which forms part of the signal isolator module 1 has frequency bins which are aligned with the fundamental frequency and a plurality of harmonic frequencies of the magnetic signal produced by the sonde. The frequency bins follow variations in the frequency of the magnetic signal produced by the sonde which allows the detector to detect the sonde at a separation of up to 30m.

Abstract: An above ground locator includes an apparatus for determining the strength of the locating signal at a selected point relative to the boring tool. The apparatus includes an antenna arrangement configured for measuring the strength of the locating signal at the selected point along first and second orthogonally opposed receiving axes to produce first and second received signals. A phase shifting arrangement phase shifts the first and second received signals in a predetermined way to generate first and second phase shifted signals, respectively. Thereafter, a summing arrangement adds the first and second phase shifted received signals to generate an output signal which is a vector sum of the first and second received signals. In one feature, third and fourth signals derived from the first and second signals are used to eliminate balance point ambiguity which is present using the first and second signals alone.

Abstract: Continuous-wave radiation is used to detect a target hidden behind a surface. In an embodiment, a transmitter directs a beam of continuous-wave microwave radiation from a transmitting location, and reflected radiation from the target is received at first and second receiving locations closer to the surface than the transmitting location. The transmitting and receiving locations have spatial relationships such that the phase of reflected radiation received at one receiving location is in quadrature with the phase of reflected radiation received at the other receiving location. In an embodiment, direct transmitted radiation is received at the receiving locations in quadrature.

Abstract: A hand-held, wand-type buried object detector includes a motion and position sensor that provides data about the sensor head of the buried object detector. An inertial measuring device including three accelerometers and three gyroscopes is one example of a motion and position sensor. The buried object detector includes a processor/communicator device that is capable of processing the data from the motion and position sensor and communicating feedback, based on the processing results, to the operator about the operation of the buried object detector. In one example, a personal digital assistant may be configured for use as a processor/communicator device of a buried object detector.

Abstract: A method and receiver system for identifying a location of a magnetic field source using two horizontally displaced tri-axial antennas. In a preferred embodiment two tri-axial antennas are positioned at opposite ends of a receiver frame. Each antenna detects in three dimensions a magnetic field from a source or transmitter. The receiver is maintained in a horizontal plane and the receiver is moved in the horizontal plane until a flux angle measured at each of the two points is zero so that the receiver is in the vertical plane perpendicular to the axis of the source. The depth and location of the source in three dimensions relative to the receiver is determined using the detected field values. The receiver is moved in a direction defined by a line containing the two points of the receiver until a magnitude of the magnetic field detected at each of the two points is substantially the same so that the receiver is positioned above the source.

Abstract: A service tee marker fixture comprises a disk marker and a cylindrical attachment sleeve fastened concentrically to the disk marker and constructed of a flexible material which provides a releasable friction fit with a service tee cap. For certain sleeve materials a stiffener plate is used to secure an end portion of sleeve adjacent to the disk marker. Detents are formed inside the attachment sleeve to grip matching ribs on the cap. An integrally formed inwardly-extending annular flange at an open end of the sleeve helps retain the sleeve on the cap. When installed the wire coil is horizontal and the marker shields the tee, and is further centered over a centerline of the main pipe. A novel method is also disclosed for using the service tee marker fixtures to survey and locate service line end points.

Abstract: At least one antenna array including three mutually orthogonal antennas each sharing a common center point senses an electromagnetic signal emitted by a buried object such as a utility line, pipe or sonde. A circuit at least partially mounted in a housing is connected to the array and determines a location of the buried object by measuring signal strength and field angles in three dimensions without having to align the antenna array relative to the buried object while eliminating nulls and false peaks. A graphical user interface (GUI) has user-friendly icons, symbols, menus, numbers and graphical and auditory representation of signal strength. A SEARCH view indicates signal strength by showing a rotating strength indicator, a trace mode MAP view in which line location is shown by a line that moves side-to-side, and a sonde mode MAP view in which sonde location is shown by a moving line, pole and equator.

Abstract: A metal detector transmitting high current pulses and employing the high voltage back EMF signals which occur at their termination. The method employed uses a resonant tuned receive coil which provides a major improvement in the depth of detection of metallic objects and interference rejection over current time and frequency domain metal detectors.

Abstract: A milling machine for milling a paved surface has milling tools connected to an underside of a body of the machine. At least one ferrous metal detector is attached to a front end of the machine. Electronic equipment is disposed within the machine and is in communication with the metal detector, the equipment being adapted to interpret feedback from the detector.

Abstract: A method is disclosed as part of an overall process in which a boring tool is moved through the ground within a given region along a particular path in an orientation which includes pitch. A locating signal is transmitted from the boring tool which signal exhibits a field defined forward point within a reference surface which field defined forward point is vertically above an inground forward point on the particular path through which the boring tool is likely to pass. The method establishes a predicted depth of the boring tool at the inground forward point by first identifying the field defined forward point. The signal strength of the locating signal is then measured at the field defined forward point as being representative of the depth of the boring tool at an inground upstream point which is the current location of the boring tool. With the boring tool at the upstream inground point, the pitch of the boring tool is determined.

Abstract: This invention relates in general to measuring depth of well-tools, such as logging tools or the like, in a borehole. Embodiments of the present invention may provide for disposing transponders along a wireline that may be used to suspend and move the well-tool in the borehole, where the transponders may be disposed along the wireline at predetermined locations. A reader may be located at a reference location and may read when a transponder passes through the reference location and this information may be used to determine the depth of the well-tool in the borehole. Additionally, this invention provides for combining depth measurements from the transponders with measurements from odometer wheels in frictional contact with the wireline and/or time of flight measurements of optical pulses passed along a fiber optic cable coupled with the wireline to accurately and robustly measure the depth of the wireline in the borehole.

Abstract: Magnetometry and pulsed electromagnetic radiation (EM) may be employed during a same survey pass by synchronously interleaving the collection of magnetometer data with pulses from EM sensors. For example, one or more magnetometers may be sampled between pulses of one or more EM coils such that the data from the one or more magnetometers is substantially free of artifacts from the pulsed EM radiation. A detection system may include a vehicle capable of locomotion over a surface of earth, a mount coupled to the vehicle, the mount including at least one magnetometer, at least one EM sensor adapted to produce pulses of EM radiation, and a controller adapted to interleave collection of data from the at least one magnetometer between the pulses of EM radiation.

Abstract: A detection method and apparatus that includes a controller and a plurality of remote sensor units, each containing sensor elements, connected to the controller to achieve custom detection profiles and resolutions that are optimized for a given application by alteration of scanning sequences used by the controller, variation of scanning frequencies, adjustment in response times, and utilization of multi-modal sensing methods.

Abstract: A boring tool is moved through the ground in a region which includes at least one electrically conductive in-ground line and which is subject to static magnetic fields including the magnetic field of the earth.

Abstract: A device rotates at least one static magnetic field about an axis, producing a rotating magnetic dipole field, and is movable in relation to the surface of the ground. The field is periodically sensed using a receiver to produce a receiver output responsive to the field. A positional relationship between the receiver and the device is monitored using the output. In one aspect, changing the positional relationship, by moving the device nearer to a boring tool which supports the receiver, causes an increase in accuracy of depth determination. In another aspect, determination of an actual overhead position of the boring tool, and its application, are described. Use of a plurality of measurements over at least one-half revolution of each magnet is disclosed. Establishing a surface radial direction toward a boring tool and resolution of multi-valued parameters is described. Calibration techniques, as well as a three transmitter configuration are also described.

Abstract: An aerial electronic system for detection of surface and underground threats comprises an electromagnetic (EM) gradiometer flown aloft over the possible ground and underground threats to a convoy. The EM gradiometer is disposed in a Styrofoam torpedo shaped pod that is towed in flight behind an airplane. An illumination transmitter and loop antenna mounted to the airplane radiate a primary EM wave that travels down to the ground surface and penetrates beneath. Frequencies of 80 KHz to 1 MHz are selected according to whether the targets are laying on the surface or deeply buried. Detonation wire pairs, buried cables and pipes, and other conductors will re-radiate a secondary wave that can be sensed by the EM gradiometer. A reference sample of the transmitter signal is carried down a fiberoptic from the airplane to the towed pod. This signal is used in the synchronous detection to measure the secondary EM wave phase.

Abstract: A system for determining the magnetic permeability of a material is provided. Two electrical inductors formed as primary and secondary concentric coils share a common magnetic core space. An AC voltage applied to the primary coil creates a magnetic flux in the core proportional to the magnetic permeability of a sample of the material positioned within the core space. The magnetic flux induces an AC voltage in the secondary coil indicative of the sample magnetic permeability. When the material is a magnetorheological fluid, the magnetic permeability is proportional to the concentration of magnetic particles in the sample and can be back-calculated from the amplitude of the secondary voltage signal. Sensitivity and resolution can be increased by using two identical sets of coils wherein a reference material forms a core for the primary set and the MR fluid sample forms a core for the secondary set.

Abstract: A metal detection apparatus includes a transmitter for generating a magnetic field in the vicinity of the metal object, and a detector for detecting a secondary magnetic field. The detector include a first gradiometer for providing a first output signal representative of a first direction, y, to the metal object, a second gradiometer arranged substantially orthogonal to the first gradiometer for providing a second output signal representative of a second direction, x, to the metal object, the second direction being substantially orthogonal to the first direction and in the same plane as the first direction, and an outer coil and an inner coil arranged to be substantially concentric with one another and each of which is arranged to provide an output signal representative of secondary magnetic field strength in a third direction, z, which is substantially orthogonal to the plane of the first and second directions.

Abstract: Multiple sources are provided for a transmitter cable for use in electromagnetic surveying. The transmitter cable includes a dipole antenna comprising a pair of spaced apart electrodes mounted on their respective cables. Two antennas may be powered from each source. Alternatively, the outputs of each source are connected to a common antenna pair. A single large power supply may be mounted on a vessel to supply power through the tow cable. Alternatively, a number of power supplies may be provided aboard the vessel, with each power supply having dedicated conductors through the tow cable to power the plurality of current sources.

Abstract: A search apparatus and related method facilitate the final stage of a search for a buried person carrying a transmitter. When the searcher is in the close vicinity of the buried transmitter, the search enters a point-localization phase where the apparatus displays a representation of the current target distance from the apparatus to the buried transmitter as well as a representation of a previously-stored value for the target distance. By comparing the current distance to the previously-stored distance, the searcher can readily see whether the apparatus is getting closer to the buried transmitter or further away. Preferably, graphic symbols are displayed to represent the current distance and the previously-stored distance. The display can also provide a representation of whether the current distance is greater than or less than an immediately preceding distance that has been stored.

Abstract: An underground digital locating system includes a receiver capable of acquiring a signal produced by an underground object and displaying a digital representation of the orientation of underground object and of its depth. The system includes a carrying case for the digital locating receiver, the carrying case further including a transmitter for energizing underground objects to emit a magnetic field. A method of locating an underground object using the described system is also disclosed, as well as methods of measuring a flux field and translating a flux field signal of an underground object into a visual representation of the underground object on the display screen of the locating receiver.

Abstract: An apparatus and method to locate and mark the surface position of an underground utility while maneuvering along the path of the utility. The apparatus uses an underground utility detector that responds to the location of an underground utility to continually position a carriage proximate vertical of the utility. Marker systems are aligned with the carriage and apply either a unique paint symbol on pavement or a spike in the ground. The apparatus is configured to use an underground utility detector or positioning equipment that generate positional signals. The apparatus may be configured to mark utility positions at predetermined intervals and mark utility offset positions. The apparatus may be attached to a vehicle, towed by a vehicle, motorized or propelled by a person.

Abstract: Continuous-wave radiation is used to detect a target hidden behind a surface. In an embodiment, a transmitter directs a beam of continuous-wave microwave radiation from a transmitting location, and reflected radiation from the target is received at first and second receiving locations closer to the surface than the transmitting location. The transmitting and receiving locations have spatial relationships such that the phase of reflected radiation received at one receiving location is in quadrature with the phase of reflected radiation received at the other receiving location. In an embodiment, direct transmitted radiation is received at the receiving locations in quadrature.

Abstract: The invention relates to a sensor for locating metallic objects, especially an inductive metal sensor (110) for construction materials, said sensor comprising at least one emission coil (20) and at least one receiving conductor looping system (26) which are inductively coupled to each other. According to the invention, the at least one emission coil (20) is mounted in series with the primary side (30) of a compensation transformer (28). The invention also relates to a measuring instrument, especially a hand-held measuring instrument comprising one such sensor.

Abstract: A new approach for locating an underground line described herein remains accurate in the face of bleedover by including both amplitude and phase from at least two magnetic field strength sensors in the measurement set. A numerical optimization step is introduced to deduce the positions and currents of each of several cables, of which one is the targeted cable and the others are termed bleedover cables. Furthermore, some embodiments of the method accounts for practical problems that exist in the field that relate to reliable estimation of cable positions, like the phase transfer function between transmitter and receiver, the estimation of confidence bounds for each estimate, and the rejection of false positive locates due to the presence of noise and interference.

Abstract: A system for determining the location of and communicating with a downhole tool assembly. A tracker assembly comprises a vertical transmitting antenna, a plurality of receiving antennas, and a processor. The downhole tool assembly has a beacon assembly comprising a plurality of receiving antennas, a transmitting antenna, an orientation sensor, and a processor. The transmitter in the tracker transmits a substantially vertical dipole field. The beacon assembly detects the vertical field and processes the signals to determine the location of the beacon relative to the tracker. The beacon assembly transmits the beacon location and operational information to the tracker assembly. The tracker assembly displays the beacon location and operational information on a visual display. The tracker assembly may also transmit operational commands or requests for information to the beacon assembly.

Abstract: An aerial electronic system for detection of surface and underground threats comprises an electromagnetic (EM) gradiometer flown aloft over the possible ground and underground threats to a convoy. The EM gradiometer is disposed in a Styrofoam torpedo shaped pod that is towed in flight behind an airplane. An illumination transmitter and loop antenna mounted to the airplane radiate a primary EM wave that travels down to the ground surface and penetrates beneath. Frequencies of 80 KHz to 1 MHz are selected according to whether the targets are laying on the surface or deeply buried. Detonation wire pairs, buried cables and pipes, and other conductors will re-radiate a secondary wave that can be sensed by the EM gradiometer. A reference sample of the transmitter signal is carried down a fiberoptic from the airplane to the towed pod. This signal is used in the synchronous detection to measure the secondary EM wave phase.

Abstract: A detector for locating a sonde includes a plurality of antennas, an analogue to digital converter 11 and a digital signal processor to isolate the magnetic signal produced by the sonde. The digital signal processor includes a phase feedback loop 21 to allow the digital processing unit to follow variations in the frequency of oscillation of the magnetic signal produced by the sonde.

Abstract: A sonde includes an elongate flexible ferromagnetic core and a coil surrounds an intermediate segment of the core. A housing surrounds and supports the coil. An electronic drive circuit is mounted in the housing for supplying the coil with an electric signal that will induce the core to emit electromagnetic signals at a predetermined strength and frequency that can be remotely detected. Self-contained battery powered float-type and pill-type sondes are also disclosed.

Abstract: In a method of identifying a passive electromagnetic signal emitted from an underground utility, a portable above-ground antenna is disposed at a location at which the passive electromagnetic signal is expected. Measurement signals are received from the antenna corresponding to electromagnetic signals received by the antenna. The measurement signals are measured at a plurality of different frequency bands within a measurement frequency range expected to include a frequency of the passive electromagnetic signal, and at least one of the plurality of frequency bands is selected based on a predetermined criteria applied to the measurements.

Abstract: A detection method and apparatus that includes a controller and a plurality of remote sensor units, each containing sensor elements, connected to the controller to achieve custom detection profiles and resolutions that are optimized for a given application by alteration of scanning sequences used by the controller, variation of scanning frequencies, adjustment in response times, and utilization of multi-modal sensing methods.

Abstract: A sonde includes an elongate flexible ferromagnetic core and a coil surrounds an intermediate segment of the core. A housing surrounds and supports the coil. An electronic drive circuit is mounted in the housing for supplying the coil with an electric signal that will induce the core to emit electromagnetic signals at a predetermined strength and frequency that can be remotely detected. Self-contained battery powered float-type and pill-type sondes are also disclosed.

Abstract: A pulse induction metal detector includes a pulse transmitter coil for generating and transmitting a first electromagnetic signal and a receiver coil for detecting and amplifying a second electromagnetic signal emitted by a nearby metal target due to eddy currents generated in response to the first electromagnetic signal. The receiver coil is a coil of wire having multiple loops. At least one switch is coupled with the coil of wire such that when the switch is in its open position, the coil of wire is a broken circuit. In this condition, the inductive properties of the receiver coil are minimized during abrupt transmitter coil turn off. The number of switches per coil loop can be varied to achieve varying levels of sensitivity.

Abstract: A device in which the receiver coil of a pulse inductive metal detector is capable of switching between a differential configuration (i.e., gradiometer configuration) to a non-differential or summing configuration under control of the operator or computer.

Abstract: A remote sensing unit for sensing subsurface formation is provided. The remote sensing unit is an active device with the capability of responding to control commands to determine subsurface formation characteristics, and transmitting corresponding data values. Some embodiments of the remote sensing unit include a battery, or a capacitor for storing charge. The embodiments that include the capacitor receive RF power that is converted to a DC signal for storing charge on the capacitor. When the charge is depleted to a specified point, the remote sensing unit prompts the wellbore tool to transmit additional RF power to recharge the capacitor. The remote sensing unit is provided with RF power to wake it up and to place it into an operational mode, and/or to send modulated data values that are then transmitted to the surface where operational decisions for the well may be made.

Abstract: A system for determining the location of and communicating with a downhole tool assembly. A tracker assembly comprises a vertical transmitting antenna, a plurality of receiving antennas, and a processor. The downhole tool assembly has a beacon assembly comprising a plurality of receiving antennas, a transmitting antenna, an orientation sensor, and a processor. The transmitter in the tracker transmits a substantially vertical dipole field. The beacon assembly detects the vertical field and processes the signals to determine the location of the beacon relative to the tracker. The beacon assembly transmits the beacon location and operational information to the tracker assembly. The tracker assembly displays the beacon location and operational information on a visual display. The tracker assembly may also transmit operational commands or requests for information to the beacon assembly.

Abstract: A device and method to improve pulse inductive metal detector (MD) sensitivity uses multiple, current steps to reduce the voltage rating of electronic components. Current reduction in one implementation is done with multiple switched resistors coupled to a voltage source and connected to the transmitter coil. The current reduction in another implementation is done with multiple switched current sources that are connected to the transmitter coil and are switched inactive one by one to reduce the total current to the transmitter coil. The current is stepped down in rapid secession with a time delay between current transitions less than the time constant of the metal target under investigation.

Abstract: A metal detector for detecting objects that are composed of non-ferrous metals. The metal detector provides for a reduced rate of false positive responses to objects composed of ferrous metals. A preferred metal detector according to the invention includes a transmitter for transmitting a time-varying electromagnetic signal and a receiver for receiving a responsive time-varying electromagnetic signal produced by the metal object. The metal detector also includes an analyzer for determining the resistive and reactive components of the responsive signal as respective first and second functions of the spatial position of the detector. A module for determining and comparing rates of change of the first and second functions at one or more spatial positions is included. Preferably, when the rates of change are the first derivatives or slopes of the functions, and when these are of the same sign, a result indicating that the metal object is non-ferrous is provided by the module.

Abstract: A detection system is provided which is configured to have a transmitter capable of interacting with an object by generating a field, and a multiplicity of receivers operative to measure changes in the environment caused by the object's response to the generated field and mounted to a closed-looped belt, which is displaceable in a proximity to the object.

Abstract: A locator capable of tracking buried, non-metallic utility lines (fiber optic, gas, water, waste, conduits) using ground penetrating radar (GPR) and an inertial position sensor is described. In some embodiments, a tracking filter is applied to a hyperbolic trajectory model based on the radar range data to determine a predicted track of the target utility line. After comparison of the predicted track to the measured inertial position, the centerline variance of the tracked line can be deduced. In some embodiments, an electromagnetic pipe and cable locator may also be included. Some embodiments of the invention can include accurate depth calibration and line depth tracking. Further, a display may provide results to a user in a simplified fashion.

Abstract: A method for identifying a buried metal object using a three-dimensional steerable magnetic field (3DSMF) system. The method comprises generating a magnetic field vector (MFV) at a first MFV position above the buried metal object; measuring a time decay response at the first MFV position; generating a MFV at a next MFV position above the buried metal object; measuring a time decay response at the next MFV position; repeating the above steps until a complete 360-degree measurement of time decay responses of the buried metal object is completed; processing all measured time decay responses with a target search algorithm to determine a magnetic polarizability tensor of the buried metal object; and identifying the buried metal object by matching the magnetic polarizability tensor of the buried metal object to a known magnetic polarizability tensor of an object.

Abstract: A method of and system for locating a single passive underground marker type may distinguish false indications caused by other marker types. In accordance with aspects of the present invention, minimization or elimination of false marker indications may be accomplished by processing the response from a marker to determine which frequency has the largest response.

Abstract: A metal detector having a plurality of phase delay discrimination regions with corresponding selectable exception spaces therein.

Abstract: In a method of identifying a passive electromagnetic signal emitted from an underground utility, a portable above-ground antenna is disposed at a location at which the passive electromagnetic signal is expected. Measurement signals are received from the antenna corresponding to electromagnetic signals received by the antenna. The measurement signals are measured at a plurality of different frequency bands within a measurement frequency range expected to include a frequency of the passive electromagnetic signal, and at least one of the plurality of frequency bands is selected based on a predetermined criteria applied to the measurements.

Abstract: Tracking a boring tool is performed within an underground region using a locating signal. The boring tool is moved through the ground during a series of distance movements such that potential movement of the boring tool during any one of the distance movements is less than a maximum movement value. A current positional relationship is determined for a current one of the distance movements based on: a last-determined positional relationship established for an immediately preceding one of the distance movements, certain orientation parameters, the maximum movement value and the determined signal strength of the locating signal in the current positional relationship. Target coordinates are accepted and a target position, based on the target coordinates, is included as part of the current positional relationship. The position of the target is unconstrained with respect to system geometry. Steering command features are provided along with steering warnings.