WO2021077234A1 - Underground localization using ground penetrating radar - Google Patents

Abstract

A system and method of locating and ranging to an underground structure uses ground penetrating radar to determine time-of-flight (ToF) to and from the structure.

Description

UNDERGROUND LOCALIZATION USING GROUND PENETRATING RADAR

The present application claims priority to United States Provisional Patent Application No. 62/926,304, filed on October 25, 2019, entitled UNDERGROUND LOCALIZATION USING MULTI- ANTENNA ARRAYS, the entire contents of which are incorporated herein by reference, where permitted.

Field of the Invention [0001] The present invention relates to methods and apparatuses for locating an underground structure.

Background

[0002] The determination of the location of underground structures is a difficult task as known methods are not generally accurate or even possible in many cases. For example, a large majority of underground pipelines are only known to be located approximately within a pipeline right-of-way (ROW), which may be 50 feet wide. Digging or trenching in the vicinity of pipelines is dangerous. Upgrading or adding new pipelines to an existing ROW is also expensive and dangerous, because the location of the existing pipeline may not be known with certainty or accuracy. Geotechnical slippage and motion make even previously surveyed pipeline locations uncertain years later.

[0003] There is a large and growing need to instrument pipelines to detect leakages. Leaks from a pipeline transporting high pressure fluids can be detected by using laser interferometry within a fiber optic cable that is laid in close proximity of (<lm) and parallel to the pipeline. Sound pressure waves emitted by a leak physically distort the fiber optic cable affecting the laser propagation, which enables not only the detection of a leak, but also the determination of its precise location. However, it is cost prohibitive and dangerous to install sensor cables adjacent to existing pipelines because of the location uncertainty of the buried assets. As a result, fewer than 0.1% of pipelines have appropriate local leak detection technology installed.

[0004] While installing the fiber optic cable at the time of deploying a new pipeline is a straightforward operation, plowing a trench to install such a cable next to existing and active pipelines is a high-risk proposition that would only be permitted if the plow-to-pipeline distance could be accurately measured on an ongoing basis as the trenching proceeds. Actual routing of older pipeline installations may differ significantly from available routing documentation and even if GPS location information is available for such cases, GPS accuracy and resolution cannot guarantee the fiber optic cable laying accuracy that is required.

[0005] While methods using ground penetrating radar (GPR), may be available to determine the location of the pipelines, the accuracy and resolution of the system depends greatly on the soil material make-up, frequencies used and power for penetration of the EM waves.

[0006] This background information is provided simply to facilitate understanding of the invention described herein, and is not an admission that any particular art is prior art or is relevant.

Summary of the Invention

[0007] In general terms, the invention comprises methods and systems for localization of an underground structure based on a multi-antenna electromagnetic (EM) array. Signal processing is used to determine an accurate location of an underground structure, such as a pipeline.

[0008] In some embodiments, methods of measuring distance to a buried structure use multiple sensors that emit EM radiation to penetrate soil in a refraction-free manner by ensuring that the angles of incidence are orthogonal to the sensor/soil interface, thus eliminating the need to ascertain the EM properties of the soil.

[0009] In some embodiments, a system for measuring distance to a buried structure uses ground penetrating radar (GPR), preferably a system that sweeps frequency over a wide range, combined with a frequency-modulated continuous-wave (FMCW) scheme and adjustable power output, which can be used to locate buried targets in soil within range from the soil surface.

[0010] In some embodiments, a system comprises a sensor module which comprises a transmitting (Tx) antenna and a receiving (Rx) antenna, which antennas may be any suitable GPR antenna. In preferred embodiments, the antennas are directional antennas, such as horn antennas, which provide a suitable front-to-back ratio and side-to-side isolation, without extra shielding.

[0011] In some embodiments, the sensors comprise antennas filled with or immersed in dielectric materials with high permittivity, to improve the EM power coupling to the soil.

Preferably, the material has a relative permittivity £RI greater than about 10, more preferably greater than about 20, and most preferably greater than about 30. [0012] Thus, in one aspect, the invention may comprise a system for determining the location of an underground structure, comprising:

(a) at least two sensor assemblies spaced laterally apart, configured to be placed in position above the underground structure, each sensor assembly mounted to a frame configured to allow each sensor assembly to pivot about a distal end, and each sensor assembly comprising a GPR transmitter Tx and receiver Rx with associated antennas; and

(b) wherein each sensor assembly is configured with circuitry:

1. to determine received signal level (RSL) while the sensor assembly rests on and pivots around its distal end when contact with a point on the ground, to ascertain and record the angle of the assembly at which maximum RSL is attained; and

2. to calculate the structure location and/or depth of the underground structure, such as with algorithms described and illustrated herein.

[0013] In some embodiments, the underground structure is a pipeline, and the sensor assemblies are configured to pivot in a plane which is perpendicular to the longitudinal axis of the pipeline.

[0014] In some embodiments, the target depth, location, and/or distance from the sensors to the target can be obtained without requiring knowledge of soil permittivity, and once those parameters are obtained, they can be used in reverse to calculate the permittivity of each path by using the beams time-of-flight (ToF). Average permittivity values can then be calculated for the soil surrounding a survey site. The ToF for an EM beam is the time the Tx beam takes to travel from the the Tx antenna to the target plus the time for the reflection to travel back from the target to the Rx sensor. In some embodiments, the ToF may be determined in the time domain, preferably by windowing the time frame for the received signal. In some embodiments, ToF can be established using FMCW, in which path delay is translated into a readily measurable frequency shift. [0015] In some embodiments, the system may be configured for mapping soil properties, as the permittivity of the soil may be determined and correlated to soil properties.

[0016] In some embodiments, the system comprises reduced size antennas, such as those which would fit within a small (< 3" diameter) pipe, which allows integration with Tx and Rx electronics to implement compact sensors with lower RF interconnection losses and better system dynamic range. As an example, an antenna that is filled with/immersed in a dielectric material with relative permittivity £RI = 37 results in antenna dimensions reduced by a factor of V37 = 6 relative to the size the same antenna would have in air. Such dramatic reduction in antenna dimensions make embodiments of this invention a better approximation of point sources of EM radiation, and facilitate the rotation of an integrated sensor to align the main radiation lobe of its Tx/Rx antennas with a buried target (to optimize the level of the signal reflected by the target), and to simultaneously make it possible to use the weight of the sensor assembly to force the soil surface to be orthogonal to the incident EM beam and to maintain intimate contact with the antenna aperture (for refraction-free soil penetration), without upturning a significant volume of soil. [0017] In one embodiment, the system comprises a vehicle with a plow to prepare a trench adjacent to an underground pipe, for continuous laying of an optical fiber cable at a depth that matches that of the pipe, the vehicle further comprising:

(a) a trenching tool to implement continuous tilting of the soil to facilitate EM soil penetration with no refraction while aiming the sensor antennas at the pipe, and

(b) a boom extending laterally which may be positioned above the underground pipe, wherein at least two, and preferably three sensor modules are attached to the boom.

The system may be configured to determine a lateral distance D between the trenching tool and the pipe and/or a vertical distance H between the pipe and a surface.

[0018] In some embodiments, the sensor antennas employ circular polarization with the Tx and Rx antennas operating with opposite rotation to take advantage of the polarization reversal experienced by EM waves reflected by conductive surfaces. This affords better isolation between Tx and Rx antennas, particularly in situations where the nature of the soil does not mask the polarization rotation reversal created by the conductive target.

[0019] In some embodiments, each Tx and Rx antenna is angled or positioned such that the antenna aperture is directed to substantially face the target.

[0020] Data to calculate D and/or H may be surveyed at regular intervals, and is used to optimize the alignment of the Tx and Rx antennas to maximize the signal to noise ratio at subsequent survey points, as it may be presumed that if the distance between surveys is a small percentage of the radius of curvature of the pipeline trajectory, the changes in M, D, and H between surveys will be small. [0021] In one embodiment, the method comprises the use of at least two sensor modules, laterally spaced apart, positioned on the surface above and at an angle from the underground object, to determine a lateral distance between the sensor modules and the underground object, and/or a vertical distance between the object and the surface.

[0022] In another aspect, the invention may comprise a system of locating an underground pipe or utility target comprising the steps of:

(a) scanning an area with a system as described herein, and obtaining the depth and location of the target and/or soil EM properties in a rasterized point cloud;

(b) identifying the underground pipe or utility from the point cloud.

Brief Description of the Drawings [0023] In the drawings shown in the specification, like elements may be assigned like reference numerals. The drawings are not necessarily to scale, with the emphasis instead placed upon the principles of the present invention. Additionally, each of the embodiments depicted are but one of a number of possible arrangements utilizing the fundamental concepts of the present invention. [0024] Figure 1 A shows a front view of a pipeline fiber optic cable trenching vehicle having a dual-antenna array. Figure IB shows a schematic representation of a sensor assembly.

[0025] Figures 2 shows the front and top views of sensor assemblies relative to a target pipeline with large antennas implemented with low permittivity materials requiring significant trenching and sloping of the soil to ensure EM soil penetration without refraction. [0026] Figure 3 shows the front and top views of sensor assemblies relative to a target pipeline with small antennas implemented with high permittivity materials requiring minimal soil shaping to enable EM soil penetration without refraction.

[0027] Figure 4 shows a schematic flow chart for one embodiment of a method to calculate D, H, and soil properties. [0028] Figures 5 A and 5B show schematic depictions of the application of Snell's Law.

[0029] Figure 5C shows cross-sectional view of terrain with fiber and tilted sensor trenches having angled antennas aiming EM beams at the center of a pipeline for maximum reflection with substantially no refraction.

[0030] Figures 6A and 6B shows a schematic representation of methods to locate multiple pipelines using a raster scanning method and/or a tracking method.

[0031] Figure 7 shows a schematic representation of the location coordinates of Figure 6A.

[0032] Figure 8 shows a chart with the percentages of the power that arrives at an orthogonal interface between two materials of dissimilar permittivity that get transmitted and reflected as a function of the ratio of the permittivity values present at the interface. [0033] Figures 9A and 9B shows the side and front views respectively of an alternative implementation of pivoting sensor assemblies operated from a transporting vehicle.

[0034] Figure 10 shows integration details of a pipe sensor assembly, and its pivoting motion in a plane perpendicular to the longitudinal axis of a pipeline.

[0035] Figure 11 shows a schematic representation of a PIN diode switch. [0036] Figure 12 shows the relative timing of fast transmit and receive RF pulses

Detailed Description of Embodiments

[0037] The following describes exemplary embodiments of the invention, and is not intended to limit the scope of the claimed invention.

[0038] As used herein, GPR refers to "ground penetrating radar", which uses radar signals in the form of electromagnetic (EM) or RF radiation in the microwave band of the radio spectrum, typically in the range of about 10 MHz to about 3 GHz (UHF/VHF frequencies). Conventional GPR techniques detect, locate and/or image underground structures by analyzing reflected radar signals.

[0039] In this description, reference may be made to an underground pipe or pipeline, however, it will be understood by those skilled in the art, that the systems and methods may be configured to detect and/or locate any underground object or structure which is reflective of GPR.

[0040] A GPR Tx antenna emits electromagnetic or RF energy into the ground. When the energy encounters a buried object or a boundary between materials having different permittivity, it may be reflected, refracted or scattered back to the surface. The Rx antenna can then record the variations in the return signal. RF energy is reflected at boundaries where subsurface EM properties change.

[0041] The electrical conductivity of the ground, the transmitted center frequency, and the radiated Tx power all may limit the effective depth range of GPR investigation. Increases in electrical conductivity attenuate the introduced electromagnetic wave, and thus the penetration depth decreases. Because of frequency-dependent attenuation mechanisms, higher frequencies do not penetrate as far as lower frequencies. However, higher frequencies may provide improved resolution. Thus, operating frequency is always a trade-off between resolution and penetration. Optimal depth of subsurface penetration is achieved in ice where the depth of penetration can achieve thousands of meters at low GPR frequencies. Dry sandy soils or massive dry materials such as granite, limestone, and concrete tend to behave as dielectric materials rather than conductive, and the depth of penetration in them could be up to about 15 meters. However, in wet or clay -laden soils and materials with high electrical conductivity, penetration may be as little as a few centimetres.

[0042] Conventionally, resolution is important in GPR systems as they attempt to locate and differentiate between buried objects with a small radar cross-section that may be in close proximity of each other. As an example, in laying fiber next to a pipeline, measuring the distance to a target pipeline from points along a narrow corridor parallel to the pipeline at a relatively short distance between the pipeline and the corridor D, typically where D = about lm. The location process may start from a known location with a known distance D from the pipeline, but maintaining the distance is required.

[0043] In some embodiments, the system comprises a 1 to 10 kW transmitter Tx, with peak output power manually or automatically adjusted between about lkW to about 10 kW continuous wave pulsing or chirped, where signal frequency is in the GPR range of about 30 MHz to about 1 GHz. In some embodiments, the system may be configured to adjust Tx Power either manually or automatically to progressively increase the probability of obtaining echoes with a received signal level (RSL) discernible from background noise. [0044] The receiver Rx is preferably a low-noise configuration with high-power input protection limiters optionally equipped with logarithmic detection.

[0045] In one embodiment, the antennas are directional antennas, such as horn antennas, to maximize system gain and soil penetration, and to reduce radiated interference to meet regulatory emission standards. In some embodiments, high dielectric fillers for the antenna horns may be provided in an effort to match the dielectric constant of soils to optimize antenna-to-soil EM match and maximize EM penetration.

[0046] One significant disadvantage of GPR relative to over-the-air radar is the power loss experienced by EM waves when initially emitted in air (with £RJ =i) to then transition to soil with a much higher relative permittivity £R2 , typically 25 <£R2 < 55. The chart in Figure 8 shows the coupling effectiveness of EM perpendicular penetration at an interface between dielectrics with different permittivity. From Figure 8 it can be concluded that the Transmitted

Power at an interface is equal to 100 % of the Incident Power only when £R2 /£RI 1, which corresponds to the case where £R2 ⁼ £RI i.e. when there is no change in relative permittivity. For the Transmitted Power to be better than 90% of the Incident Power the permittivity ratio at an interface must be constrained to 0.25 <£R2 /£R1 < 4

[0047] In some embodiments, the antennas are filled or immersed in a lossless dielectric material, in order to approximate the equality of permittivities at the soil interface in a GPR application. The material is thus chosen to make the ratio £R2 / £RI as close as possible to unity. As an example, for soils with 25 <£R2 < 55, the closest value of £R2 /SRI to unity occurs for Qu = V25 * 55 = 37, which yields (£R2 I SRI) MIN = 0.68 and ( £R2 /Oil) MAX = 1.49.

This makes [sqrt ( £R2 /£R)]MIN = 0.82 and [sqrt ( £R2 /£RI]MAX = 1.22, which results in

Transmitted power > 95% of the Incident Power. For relative comparison, the Transmitted Power would be only 55% of the Incident Power if the Antenna was filled with and/or immersed in air, when £R2 = 25 and less than 45% of the Incident Power would be transmitted into the soil if 8R2 = 55.

[0048] Unlike over-the-air radar, the electromagnetic waves used in GPR undergo refraction, if the angle of incidence oci (with reference to FIG. 5A and 5B) is different from 0°. In general, the refracted waves that penetrate the soil adopt a different angle of transmission 0C2, per Snell’s law that is dependent on the two relative permittivity values, 8R2 and 8R2 , present at the interface.

[0049] The soil relative permittivity £R2 is generally not known and varies considerably

(typically between 20 and 40) over long stretches of terrain. The uncertainty of the terrain properties adds an unknown to the process of radar-location of targets with oblique angles of incidence. [0050] Embodiments of the present invention ensure that GPR EM wave penetration of the soil is substantially perpendicular to the ground surface to minimize refraction and making the observable angle above surface equal to the angle below surface, even when the underground target is not directly below the Tx transmitter.

[0051] Assuming a perfectly horizontal ground surface and assuming that “visibility” to a buried target was possible, the only surface point from which the “line-of-sight” to the target is perpendicular to the ground surface, which could be used by an EM wave to penetrate without refraction, is the one vertically above the target. GPR scanning the target from this point ensures knowledge of the pipeline location but the EM wave time-of-flight (ToF) information that can be obtained from the GPR equipment does not by itself yield pipeline depth due to the uncertainty of the value of the soil permittivity, responsible for the velocity of EM propagation in that medium.

[0052] In other embodiments of the present invention, the orthogonality of the angle of EM wave incidence and the sensor/soil interface is maintained while the angle of incidence is adjusted to search for the sensor orientation that results in the maximum Rx signal level (RSL) reflected from the target. The refraction-free EM penetration of the soil ensures that the angles of the waves in soil with respect to vertical are replicated by the corresponding angles of alignment of the sensors on the surface with respect to vertical. When a sensor achieves maximum RSL, its angle of alignment points at the target.

[0053] With reference to Figure 2, target depth H can be obtained by triangulation if two or more non-refracted EM beams are aimed at a buried target from different points on the surface. Enforcing the requirement of EM soil penetration with no refraction when using antennas implemented in a medium with low relative permittivity requires significant soil removal or trenching, as shown in Figure 2. Additionally, large antennas make their integration with associated Tx and Rx electronics, which results in high RF interconnection losses and in the sacrifice of system dynamic range.

[0054] Implementing antennas in a medium with a high relative permittivity SRI reduces the antenna dimensions by a factor

and the antenna aperture by a factor £RI_, relative to an equivalent implementation in air. By selecting an appropriately high value of SRI, the antenna apertures can be reduced such that a sensor assembly can be pivoted around its lower end resting on the ground, and the weight of the assembly be used to shape the ground surface to ensure EM penetration without refraction and with minimal soil upturning, as shown in Figure 3.

[0055] For ease of operation, sensor assemblies comprise an elongated support structure and high permittivity Tx and Rx antennas which are attached at the distal end of the elongated support structure. The antennas point their main radiation lobes in the same direction away from the support structure. The longitudinal axis of the support structure is aligned with the main radiation lobe of the antennas such that the angle of penetration of the Tx EM beam in the ground is replicated by the angle of the sensor support structure above ground. An inclinometer in the support structure provides the value of the angle of the support structure relative to vertical. When a Tx EM beam is reflected by a target surface that is perpendicular to the beam, the reflection is picked up by the associated Rx antenna. [0056] In some embodiments, two or more sensors are located in different known positions on the soil surface. After pivoting the sensors to attain maximum RSL, their angles of alignment with respect to vertical enable the use of triangulation to calculate the target position and depth relative to the points on the surface.

[0057] Pivoting an active sensor assembly in a plane perpendicular to the longitudinal axis of a pipeline while monitoring the Rx signal level (RSL) can be used to find the angle at which the reflection from the pipeline is maximum. This occurs when the sensor longitudinal axis is pointing at the centre of the pipeline. With reference to Figure 3, aligning two sensors with a pipeline in this manner, yields angles Bl and B2. These two angles with a given distance D between pivot points, and a known pipeline diameter d enable the calculation of:

DsinCB 1) d V = - - — - - sin(180° — Bl — B2) 2

_T Dsin(B2 ) d

^{U ~} sin(180° — Bl — B2) ^~ 2

X = Ucos ( Bl )

H = JU² + X²

[0058] The pipeline location X and depth H are fully determined without the need to know the soil properties.

[0059] The soil properties can be derived from the ToF values associated with distances U and V, as provided by the GPR equipment. If the ToF associated with the distance U is ΐ_u and the ToF associated with distance V is t_v, then the associated velocities of propagation n_u and v_v are given by: vu ⁼ U/tu v_v = V/ty and the permittivity of paths V and U are:

en ⁼ l/(tv)² and the average permittivity for the location is:

¾VG = (l/(tv)² + l/Ou)²)/2

[0060] The operation of an exemplary embodiment may now be described, with reference the schematic configuration of Figure 3 and to the flowchart shown in Figure 4. In this example, a dual sensor system as depicted in Figure 9 is being used to scan a pipeline to: a) measure the horizontal distance X between a point on the surface to a point directly above the longitudinal axis of a buried pipeline at one point of its trajectory, b) measure the pipeline depth H at the same location, c) calculate the average EM properties of the local soil, and d) record measured and calculated data, as well as the GPS coordinates of the location to process the data further for display and guidance of subsequent pipeline scanning steps. [0061] In some embodiments, advantage is taken of the small size of antennas filled with or immersed in a high permittivity material, to mount an antenna inside one end of a pipe assembly that includes Tx electronics, which acts as a "radiation pointer" that emits RF energy, much like a laser pointer emits optical energy. The same type antenna with Rx electronics forms a Rx pipe assembly. A Tx pipe assembly and a Rx pipe assembly are tied together, with their antennas pointed in the same direction, to form a sensor assembly. Two or more sensor assemblies may be used together in a system of the present invention, as shown in Figure 3 or 9.

[0062] In some embodiments, the antenna end of each pipe assembly may be sealed with a material that physically isolates the antennas, but which is transparent to the EM radiation.

The material preferably has a high permittivity e, to reduce or eliminate antenna/soil dielectric mismatch, and is resistant to abrasion and shock, as it is in contact with the soil.

[0063] In alternative embodiments, a physical shutter protects the antennas when shut and enables the pipe assembly to be pressed firmly on the ground to shape its surface flat and perpendicular to the sensor pipe assembly longitudinal axis which is colinear with the Tx and Rx main radiation lobe. When the shutter is opened, the antennas project out to press against the flattened ground. The antennas may be projected and retracted by mounting them onto hydraulic pistons or linear actuators (not shown). This automatically ensures perpendicular EM penetration of the soil with zero diffraction angle, and minimizes antenna /soil interface mismatch losses. The antennas are retracted and the shutter is closed on completion of a measurement. [0064] The pipe assemblies may be used to implement sensors mounted on a transportation vehicle with hydraulic assisted sensor control to implement fast sequential location of pipeline points at regularly spaced intervals along the trajectory of the asset. With reference to Figure 9, the sensors are operated as follows:

• Tie Rod Hydraulic Cylinder TRC1 lowers SENSOR ASSY 1 and presses its lower end into the ground at point A .

• Optionally, the SHUTTER at the bottom of SENSOR ASSY 1 is opened

• ASSY Tie Rod Hydraulic Cylinder TRC2 pivots SENSOR ASSY 1 around point A while the system monitors Receive Signal Level (RSL1)

• The angle B1 at which RSL1 is maximum occurs when SENSOR ASSY 1 is pointing at the pipeline and Angle B1 is then recorded

• Tie Rod Hydraulic Cylinder TRC3 lowers SENSOR ASSY 2 and presses its lower end into the ground at point B .

• Optionally, the SHUTTER at the bottom of SENSOR ASSY 2 is opened

• ASSY Tie Rod Hydraulic Cylinder TRC4 pivots SENSOR ASSY 2 around point B while the system monitors Receive Signal Level (RSL2). The angle B2 at which RSL2 is maximum occurs when SENSOR ASSY 2 is pointing at the pipeline. Angle B2 is recorded • The distance D between points A and B is recorded; Pipeline diameter d is known, theefore X is calculated as X = D * tan(Bl)/(tan(Bl) + tan(B2)) and H is calculated as H = D * tan(Bl) * tan(B2)/((tan(Bl) + tan(B2))

• ToF for signals in Sensors 1 and 2 are recorded as ToFl and ToF2 respectively

• Soil EM permittivity properties are calculated using X, H, d, ToFl and ToF2 e)

[0065] Both Time-of-Flight (ToF) techniques (in the time domain) and FMCW techniques (in the frequency domain) can be used to resolve distance to target in over-the-air radar applications. However, the low-pass nature of soil as a medium in which to propagate EM radiation rapidly erodes the high frequency content of the sharp pulses required to ascertain ToF with the accuracy and resolution required by the intended applications of the present invention, even over short distances in the ground.

[0066] Thus, preferred embodiments use FMCW GPR techniques that convert short range ToF into a down-converted frequency shift in a frequency range below 1MHz that can be easily handled with DSP techniques to improve Signal-to-Ratio and minimize the interference of ground surface scatter.

[0067] In some embodiments, the sensor system may be configured to use conventional FMCW radar with chirped frequency. Pulsed FMCW (Frequency Modulated Continuous Wave) signals consist of low duty cycle short pulses during which the amplitude remains constant and the frequency is linearly swept between values f_MIN and †_MAX· The Rx antenna detects and amplify the signals that emerge, after a round-trip delay, from reflections underground. The Rx combines the reflections with the original pulses in a coherent down conversion mixing process that translates the arrival delays into frequency domain information. Since wave propagation delays are proportional to distance travelled to and from the point of reflection, the output can be processed into a representation of that distance.

[0068] With FMCW GPR, objects buried close to the surface produce stronger reflections with shorter delays than shallower targets, which translate at the output of the Rx into a continuous distribution of down-converted frequencies superimposed on the signals that correspond to desired targets. Signal processing may be used to separate such sharper features from the background scatter.

[0069] In the frequency domain, spurious reflections constitute noise that can be filtered or suppressed using known techniques, such as any suitable adaptive filter algorithms, including a Kalman filter, a Fast Fourier Transform filter, or a Constant False- Alarm Rate (CFAR) radar signal processing technique. CFAR is an adaptive algorithm well known in radar signal processing to isolate target signals by minimizing noise, clutter or interference.

[0070] In some embodiments, additional signal detection improvements can be attained by filtering the received signal power P on a F⁺⁴ (with F representing frequency) so that close signals are attenuated and far away signals are amplified. This accounts for the fact that the power of GPR signal returns are proportional to the inverse fourth power of distance D (i.e. P o D ⁴).

[0071] In some embodiments, sensor assemblies may be mounted on a motorized vehicle, and transported between measurements at survey points. The sensor assemblies may be lifted, lowered, rotated and shutters may be opened and closed using hydraulic systems well known in the art. The vehicle may be fitted with the required GPS navigation equipment, computers and software to process the measured data, log GPS coordinates, D, H, X, U, and/or V of survey points, and display to the driver the required information to steer the vehicle to the next survey point.

[0072] Methods of the present invention may be used to survey and geolocate underground structures and to classify soil types in a defined area. The sensor modules may be deployed in a pattern to locate underground structures in the area, and correlated to geographic location provided by conventional GPS technology. For example, when plowing a trench to install a fiber optic cable next to existing and active pipelines, it is important to locate such pipelines first. Exemplary methods may be described, with reference to Figures 6A, 6B and 7, where three existing subsurface pipelines PI, P2, and P3 are present but with uncertain location in a common Right-of-Way (ROW).

[0073] In order to geolocate an underground structure in an area, the previously described process of point scanning may be repeated in a pattern over the desired area, which may be geomapped using conventional GPS locating technology. Figure 6A shows an example of three pipelines being traversed to cross their longitudinal axes. The GPS coordinates of the points A, B ... J at which maximum RSL is measured get recorded for appropriate asset mapping.

[0074] Figure 6B shows an example of continuous plowing of a fiber optic cable next to pipeline P2 using heavy plowing machinery that also cuts sensor trenches to slope the soil as shown in Figure 5C to ensure EM soil penetration without refraction. In Figure 5C, a minimum of two sensors perform triangulation using angles B1 and B2 and the known surface distances L and M to calculate D and H. The sensors move on skids securely connected to the plow maintaining contact with the tilted soil. The measurement and calculation process is repeated at regular intervals, and the values obtained for D inform the adjustment of plow direction, so as to maintain constant distance from the pipeline, and the values of H inform the adjustment of plow depth, so as to make the fiberoptic cable depth equal to that of the pipeline.

[0075] In some embodiments used for longitudinal asset tracking, the calculated values of D and H may be entered as variables into an autonomous or semi-autonomous system which simultaneously and on a continuous basis: a) Steers the trenching vehicle, or prompts an operator to steer the vehicle, to constrain the variance of D; b) Adjusts plow depth to constrain the difference between actual and desired depth; and c) Adjusts the tilting of the auxiliary trench cutting implements to keep EM beams aimed at the target with no refraction.

[0076] FIG.6A depicts raster scanning as the process of using surface distance sensors to perpendicularly cross the ROW to generate an accurate cloud of cross points with correlated GIS data that reveals the exact location and alignment of the existing subsurface facilities. In order to locate P2, the mobile system is moved across the ROW in directions roughly perpendicular to the presumed direction of the pipe. The raster scanning process generates an accurate cross point cloud (cross points A through J), which enables the identification of cross points (B, E, and H) as belonging to a section of pipeline P2 for which location and alignment become accurately known, having been calculated by using the GIS information provided by cross points B, E, and H.

[0077] Once the pipe's location and alignment are known, the mobile system may then be used to follow a surface path with way points that maintain a constant distance to pipeline P2 within a prescribed tolerance, as shown in Figure 6B.

[0078] In other embodiments of the invention, algorithms use the parameters obtained from the last few surveys of a pipeline to estimate the direction and distance to the next survey point. Surveys that track a buried asset with the purpose of laying a fiberoptic cable must always start from a point where the asset is visible or for which location and depth are guaranteed to be accurate.

[0079] In some embodiments, ground penetration is improved by using very short intense pulses of EM energy. However, significant noise is present at the receiver accompanying the reflected signal. In some embodiments, the noise may be mitigated by switching the receiver on and off, such that the receiver is on only in a gated time frame. Thus, the receiver ignores front end and back end, and listens only in a window of opportunity.

[0080] The gated receiver may be implemented with pin diodes which switch on and off in picoseconds. For example, the receiver may comprise a sampling switch configured as shown in Figure 11. The received RF pulse (the reflected transmitted pulse shown in Figure 12) has a leading edge which is dominated by close in-ground surface reflection. The sampling window is chosen to isolate the time-of-flight window of the trailing edge of the signal. The TOF is the time difference between the trailing edge of the transmitted pulse (shaped by RF switch in transmitter) and the trailing edge of the received pulse (determined by the most distant reflection, which is presumed to be the scanned pipe.

[0081] Aspects of the present invention may be described with reference to flowchart illustrations and/or block diagrams of methods, apparatus (systems) and computer program products according to embodiments of the invention. It will be understood that each block of the flowchart illustrations and/or block diagrams, and combinations of blocks in the flowchart illustrations and/or block diagrams, can be implemented by computer program instructions. These computer program instructions may be provided to a processor of a general purpose computer, special purpose computer, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, create means for implementing the functions/acts specified in the flowchart and/or block diagram block or blocks.

[0082] The flowchart and block diagrams in the Figures illustrate the architecture, functionality, and operation of possible implementations of systems, methods and computer program products according to various embodiments of the present invention. In this regard, each block in the flowchart or block diagrams may represent a module, segment, or portion of code, which comprises one or more executable instructions for implementing the specified logical function(s). It should also be noted that, in some alternative implementations, the functions noted in the block may occur out of the order noted in the figures. For example, two blocks shown in succession may, in fact, be executed substantially concurrently, or the blocks may sometimes be executed in the reverse order, depending upon the functionality involved.

It will also be noted that each block of the block diagrams and/or flowchart illustration, and combinations of blocks in the block diagrams and/or flowchart illustration, can be implemented by special purpose hardware-based systems that perform the specified functions or acts, or combinations of special purpose hardware and computer instructions.

[0083] The corresponding structures, materials, acts, and equivalents of all means or steps plus function elements in the claims appended to this specification are intended to include any structure, material, or act for performing the function in combination with other claimed elements as specifically claimed.

[0084] References in the specification to "one embodiment", "an embodiment", etc., indicate that the embodiment described may include a particular aspect, feature, structure, or characteristic, but not every embodiment necessarily includes that aspect, feature, structure, or characteristic. Moreover, such phrases may, but do not necessarily, refer to the same embodiment referred to in other portions of the specification. Further, when a particular aspect, feature, structure, or characteristic is described in connection with an embodiment, it is within the knowledge of one skilled in the art to affect or connect such module, aspect, feature, structure, or characteristic with other embodiments, whether or not explicitly described. In other words, any module, element or feature may be combined with any other element or feature in different embodiments, unless there is an obvious or inherent incompatibility, or it is specifically excluded.

[0085] It is further noted that the claims may be drafted to exclude any optional element. As such, this statement is intended to serve as antecedent basis for the use of exclusive terminology, such as "solely," "only," and the like, in connection with the recitation of claim elements or use of a "negative" limitation. The terms “preferably,” “preferred,” “prefer,” “optionally,” “may,” and similar terms are used to indicate that an item, condition or step being referred to is an optional (not required) feature of the invention.

[0086] The singular forms "a," "an," and "the" include the plural reference unless the context clearly dictates otherwise. The term "and/or" means any one of the items, any combination of the items, or all of the items with which this term is associated. The phrase "one or more" is readily understood by one of skill in the art, particularly when read in context of its usage.

[0087] The term "about" can refer to a variation of ± 5%, ± 10%, ± 20%, or ± 25% of the value specified. For example, "about 50" percent can in some embodiments carry a variation from 45 to 55 percent. For integer ranges, the term "about" can include one or two integers greater than and/or less than a recited integer at each end of the range. Unless indicated otherwise herein, the term "about" is intended to include values and ranges proximate to the recited range that are equivalent in terms of the functionality of the composition, or the embodiment.

[0088] As will be understood by one skilled in the art, for any and all purposes, particularly in terms of providing a written description, all ranges recited herein also encompass any and all possible sub-ranges and combinations of sub-ranges thereof, as well as the individual values making up the range, particularly integer values. A recited range includes each specific value, integer, decimal, or identity within the range. Any listed range can be easily recognized as sufficiently describing and enabling the same range being broken down into at least equal halves, thirds, quarters, fifths, or tenths. As a non-limiting example, each range discussed herein can be readily broken down into a lower third, middle third and upper third, etc.

[0089] As will also be understood by one skilled in the art, all language such as "up to", "at least", "greater than", "less than", "more than", "or more", and the like, include the number recited and such terms refer to ranges that can be subsequently broken down into sub-ranges as discussed above. In the same manner, all ratios recited herein also include all sub-ratios falling within the broader ratio.

ASPECTS OF THE INVENTION

[0090] ] In view of the description above, certain more particularly described aspects of the invention are presented below. These particularly recited aspects should not however be interpreted to have any limiting effect on any different claims containing different or more general teachings described herein, or that the "particular" aspects are somehow limited in some way other than the inherent meanings of the language literally used therein. In particular, various aspects not specifically recited below may combine features, elements, or steps described herein in any combination in alternative embodiments. Aspect 1 : A system for measuring distance to an underground target comprising a ground penetrating radar (GPR) system configured to range the target from the soil surface, by determining time-of-flight (ToF) of an RF signal to and from the target, preferably by windowing the received signal in the time domain, or converting to the frequency domain and filtering using an adapting filter. Aspect 2. The system of Aspect 1 comprising a sensor module comprises a GPR transmitter Tx antenna and a receiver Rx antenna; and wherein the sensor module is configured with circuitry to emit and receive an electromagnetic (EM) signal, and to calculate time-of-flight from the transmitter antenna to the pipe and the return of a reflected signal, and to calculate one or both of (i) a lateral distance (D) between the trenching tool and the pipe or utility and (ii) a vertical distance (H) between the pipe and a surface, wherein ToF is preferably determined by windowing the received signal in the time domain, or converting to the frequency domain and filtering using an adapting filter.

Aspect 3. The system of Aspect 1 or 2, mounted on a vehicle for preparing a trench adjacent an underground pipe or utility, comprising a boom extending laterally, the boom bearing at least two sensor modules which are spaced apart laterally.

Aspect 4. The system of Aspect 2 or 3, wherein each sensor module comprises a directional antenna.

Aspect 5. The system of Aspect 4 wherein each directional antenna is angled such that a plane of an antenna aperture substantially faces the target or pipe.

Aspect 6. The system of Aspect 5 further comprising at least one trenching implement to create a trench having a wall which is substantially perpendicular to a straight line between the trench and the target or pipe, wherein a sensor module may be positioned in the trench such that the plane of the antenna aperture is parallel to and in contact with the trench wall.

Aspect 7. The system of any one of Aspect 1-6, wherein the received signal comprises a scatter of amplitudes and delays in the time domain, and further comprising the step of mapping these reflected signals into a scatter of amplitudes and phases in the frequency domain, and filtering the signal with an adaptive filter.

Aspect 8. The system of Aspect 7, wherein the adaptive filter comprises a Kalman filter, a Fast Fourier Transform filter, or a Constant False-Alarm Rate (CFAR) radar signal processing. Aspect 9. The system of Aspect 8 wherein the adaptive filter implements filtering by the fourth power of the frequency in the signal processing of GPR signal returns.

Aspect 10. The system of any one of Aspect 2-9 wherein the transmitter antenna and the receiver antenna employ circular polarization with the Tx and Rx antennas operating with opposite rotation. Aspect 11. A method of measuring distance to an underground target, comprising the step of ranging the target from the soil surface, by determining time-of-flight (ToF) of a ground penetrating radar (GPR) RF signal to and from the target .

Aspect 12. The method of Aspect 11, comprising the use of at least two sensors that each emit RF signal EM radiation to penetrate soil in a refraction-free manner by positioning each sensor such that the angles of incidence are orthogonal to a sensor/soil interface.

Aspect 13. The method of Aspect 12, comprising the step of determining a lateral distance between a trenching tool and an underground pipe or utility, and/or a vertical distance between the pipe and a surface, comprising the steps of: a) determining ToF values for an electromagnetic signal from each of a plurality of sensor modules positioned above and laterally from the pipe and a reflected signal back to the sensor module; b) converting TOF values to distances using an estimated propagation velocity.

Aspect 14. The method of Aspect 13 wherein the estimated propagation velocity is iteratively refined by comparing a calculated value of a time-of-flight value for one sensor module to an actual time-of-flight value.

Aspect 15. The method of any one of Aspect 11-14, wherein the reflected signal is received in a sampling window comprising a falling edge of the reflected signal.

Aspect 16. The method of Aspect 15 wherein the length of the sampling window is substantially the same or less than the length of a transmitted signal pulse.

Aspect 17. The method of any one of Aspect 11-16, wherein the transmitted signal and the received signal have opposite circular polarization.

Aspect 18. The method of any one of Aspect 11-17, wherein the reflected signals comprise a scatter of amplitudes and delays in the time domain, and further comprising the step of mapping these reflected signals into a scatter of amplitudes and phases in the frequency domain, and filtering noise.

Aspect 19. The method of Aspect 18, wherein the step of filtering noise uses a Kalman filter, a Fast Fourier transform filter, or a Constant False- Alarm Rate (CFAR) radar signal processing. Aspect 20. The method of Aspect 19, wherein the step of filtering noise comprises filtering by the fourth power of the frequency in the signal processing of GPR signal returns.

Aspect 21. A method of locating an underground pipe or utility, comprising the steps of:

(a) scanning an area with a system of any one of claims 1-10, and obtaining TOF values and/or soil composition in a rasterized point cloud; (b) identifying the underground pipe or utility from the point cloud.

Aspect 22. A method of mapping soil types in a defined area, comprising the steps of:

(a) scanning the area with a system of any one of claims 1-10, and determining soil permittivities at each of a plurality of locations in the defined area;

(b) correlating the soil permittivities to corresponding soil types. Aspect 23. A system for determining a horizontal distance X and a vertical depth H of an underground object, comprising two elongate sensor assemblies having a longitudinal axis, laterally spaced apart by distance D, wherein each sensor assembly may be pivoted to vary its longitudinal axis relative to a vertical axis, and wherein each sensor assembly comprises a GPR Tx and Rx and is configured to determine an angle of maximum received signal level, and to determine X and D from the angles of maximum RSL for each sensor assembly.

Aspect 24. The system of Aspect 23 wherein each sensor assembly is configured to determine ground permittivity of each path from each sensor assembly to the underground object.

Aspect 25. The system of Aspect 23 or 24 wherein the GPR system comprises any one or more of the features or elements described herein, in any combination. Aspect 26. A method of determining a horizontal distance X and a vertical depth H of an underground object from a sensor system comprising two elongate sensor assemblies each comprising a GPR Tx and Rx, spaced apart by a lateral distance D, and each having a longitudinal axis, comprising the steps of

(a) pivoting each sensor assembly to vary its longitudinal axis relative to a vertical axis,

(b) transmitting a GPR signal into the ground;

(c) determining an angle of maximum received signal level for each sensor assembly, and

(d) determining X and D from the angles of maximum RSL for each sensor assembly.

Aspect 27. The method of Aspect 26 further comprising the step of determining ground permittivity of a path from each sensor assembly to the underground object.

Aspect 28. The method of Aspect 26 or 27, further comprising any step or step plus function described herein, in any combination.

REFERENCES

[0091] The following references are indicative of the level of skill in the art, and are incorporated herein by reference as if fully reproduced herein.

United States Patent 4,825,223, Moore - Planar antenna

United States Patent Application 20180266854, Moore - Bragg sensor United States Patent US2223161 A, Black, Feedback amp .

United States Patent Application US8949024B2 Stanely . automobile GPR United States Patent 8164837B2, Bowers, negative refraction antenna

US Patent 9360558B2, Young, UG utilities

Frequency -Dependent Attenuation Analysis of Ground-Penetrating Radar Data, Society of Exploration Geophysicists in Geophysics. DOI: 10.1190 1.2710183

Propagation of Radiofrequency Electromagnetic Fields Geological Conductors, JOURNAL OF RESEARCH of the National Bureau of Standards-D. Radio Propagation, Vol. 67D, No.2, March- April 1963.

A resistively loaded, printed circuit, electrically short dipole element for wideband array applications, R.E. Clapp, IEEE Antennas and Propagation Society Int. Symp. Digest, vol. 1, 1993, pp. 478-481.

Claims

1. A system for measuring distance to an underground target comprising a ground penetrating radar (GPR) system configured to range the target from the soil surface, by determining time-of-flight (ToF) of an RF signal to and from the target.

2. The system of claim 1 comprising a sensor module comprises a GPR transmitter Tx antenna and a receiver Rx antenna; and wherein the sensor module is configured with circuitry to emit and receive an electromagnetic (EM) signal, and to calculate time-of-flight from the transmitter antenna to the pipe and the return of a reflected signal, and to calculate one or both of (i) a lateral distance (D) between the trenching tool and the pipe or utility and (ii) a vertical distance (H) between the pipe and a surface.

3. The system of claim 1 or 2, mounted on a vehicle for preparing a trench adjacent an underground pipe or utility, comprising a boom extending laterally, the boom bearing at least two sensor modules which are spaced apart laterally.

4. The system of claim 2 or 3, wherein each sensor module comprises a directional antenna.

5. The system of claim 4 wherein each directional antenna is angled such that a plane of an antenna aperture substantially faces the target or pipe.

6. The system of claim 5 further comprising at least one trenching implement to create a trench having a wall which is substantially perpendicular to a straight line between the trench and the target or pipe, wherein a sensor module may be positioned in the trench such that the plane of the antenna aperture is parallel to and in contact with the trench wall.

7. The system of any one of claims 1-6, wherein the received signal comprises a scatter of amplitudes and delays in the time domain, and further comprising the step of mapping these reflected signals into a scatter of amplitudes and phases in the frequency domain, and filtering the signal with an adaptive filter.

8. The system of claim 7, wherein the adaptive filter comprises a Kalman filter, a Fast Fourier Transform filter, or a Constant False-Alarm Rate (CFAR) radar signal processing.

9. The system of claim 8 wherein the adaptive filter implements filtering by the fourth power of the frequency in the signal processing of GPR signal returns.

10. The system of any one of claims 2-9 wherein the transmitter antenna and the receiver antenna employ circular polarization with the Tx and Rx antennas operating with opposite rotation.

11. A method of measuring distance to an underground target, comprising the step of ranging the target from the soil surface, by determining time-of-flight (ToF) of a ground penetrating radar (GPR) RF signal to and from the target .

12. The method of claim 11, comprising the use of at least two sensors that each emit RF signal EM radiation to penetrate soil in a refraction-free manner by positioning each sensor such that the angles of incidence are orthogonal to a sensor/soil interface.

13. The method of claim 12, comprising the step of determining a lateral distance between a trenching tool and an underground pipe or utility, and/or a vertical distance between the pipe and a surface, comprising the steps of: a) determining ToF values for an electromagnetic signal from each of a plurality of sensor modules positioned above and laterally from the pipe and a reflected signal back to the sensor module; b) converting TOF values to distances using an estimated propagation velocity.

14. The method of claim 13 wherein the estimated propagation velocity is iteratively refined by comparing a calculated value of a time-of-flight value for one sensor module to an actual time-of-flight value.

15. The method of any one of claims 11-14, wherein the reflected signal is received in a sampling window comprising a falling edge of the reflected signal.

16. The method of claim 15 wherein the length of the sampling window is substantially the same or less than the length of a transmitted signal pulse.

17. The method of any one of claims 11-16, wherein the transmitted signal and the received signal have opposite circular polarization.

18. The method of any one of claims 11-17, wherein the reflected signals comprise a scatter of amplitudes and delays in the time domain, and further comprising the step of mapping these reflected signals into a scatter of amplitudes and phases in the frequency domain, and filtering noise.

19. The method of claim 18, wherein the step of filtering noise uses a Kalman filter, a Fast Fourier transform filter, or a a Constant False- Alarm Rate (CFAR) radar signal processing.

20. The method of claim 19, wherein the step of filtering noise comprises filtering by the fourth power of the frequency in the signal processing of GPR signal returns.

21. A method of locating an underground pipe or utility, comprising the steps of:

(a) scanning an area with a system of any one of claims 1-10, and obtaining TOF values and/or soil composition in a rasterized point cloud;

(b) identifying the underground pipe or utility from the point cloud.

22. A method of mapping soil types in a defined area, comprising the steps of: (a) scanning the area with a system of any one of claims 1-10, and determining soil permittivities at each of a plurality of locations in the defined area;

(b) correlating the soil permitivities to corresponding soil types.

23. A system for determining a horizontal distance X and a vertical depth H of an underground object, comprising two elongate sensor assemblies having a longitudinal axis, laterally spaced apart by distance D, wherein each sensor assembly may be pivoted to vary its longitudinal axis relative to a vertical axis, and wherein each sensor assembly comprises a GPR Tx and Rx and is configured to determine an angle of maximum received signal level, and to determine X and H from the angles of maximum RSL for each sensor assembly.

24. The system of claim 23 wherein each sensor assembly is configured to determine ground permittivity of each path from each sensor assembly to the underground object.

25. A method of determining a horizontal distance X and a vertical depth H of an underground object from a sensor system comprising two elongate sensor assemblies each comprising a GPR Tx and Rx, spaced apart by a lateral distance D, and each having a longitudinal axis, comprising the steps of (a) pivoting each sensor assembly to vary its longitudinal axis relative to a vertical axis,

(b) transmitting a GPR signal into the ground;

(c) determining an angle of maximum received signal level for each sensor assembly, and

(d) determining X and H from the angles of maximum RSL for each sensor assembly.

26. The method of claim 25 further comprising the step of determining ground permittivity of a path from each sensor assembly to the underground object.