Arrayed Sensing for 3D Utility Detection and Subsurface Characterization: Surveying and Mapping the Underground Using Arrayed Sensors

This paper discusses two technologies that use arrays of antennae to derive precise locations of utility lines and other subsurface features...employed “ahead of the drill” (design phase) to prevent conflict and damage during excavation. While the CART (Computer Assisted Radar Tomography, RT for short) system employs an array of ground-penetrating-radar (GPR) antennae, the AIR (Array of Inductive Receivers) system uses specially-designed tri-axial electromagnetic receivers. Both patented systems use laser-positioning and image/data post-processing software to render 3D imagery and, ultimately, CAD/GIS plan-and-profile drawings showing what’s down there and where it all is. While the CART system detects all utility types, it is limited in depth. Conversely, AIR is limited to conductive utilities but has unlimited depth of penetration. This is especially relevant to the HDD industry, which is often asked to place new facilities at HDD depths where others (with bad records and “unlocatable” lines) have gone before. This paper reviews historical and technical background, explains how these systems work, where and when they’re applied, their respective strengths and limitations, and the synergies between them. In addition, it describes how both systems fit in the subsurface utility engineering (SUE), design, and construction processes to create risk, cost, and damage avoidance well beyond that of conventional SUE. Lastly, attendees will be exposed to several real-world applications of these technologies. Background The technological concepts that underlie CART and AIR go back more than 40 years, when future WTI founders Alan Witten and Tony Devaney were roommates at the University of Rochester. It was 1964, and Dr. Emil Wolf had just published “The Principles of Optics,” a seminal work (then and now) on the topic, and considered Devaney his best student. 20 years later, while Devaney was working at Schlumberger, he patented “Geophysical Diffraction Tomography (GDT),” the basic theory behind CART. Simply put, GDT allows precise 3D location of subsurface objects and features by using an array of receiving antennae to interpret signals passing through the earth (and whatever’s contained therein). Given the complexity of early GDT theory, Schlumberger was not able to reduce Devaney’s patent to commercial practice for 6-7 years. Meanwhile, Dr. Witten seized on Devaney’s concept and ran with it. By the early-90’s Witten had used GDT (in acoustic form, both on land and on water) to 3D-image everything from pipes, to caves, to tunnels, to sunken treasure ships (Fig.1), to Seismosaurus (Fig.2)...the largest dinosaur on record...whose skeleton (in reconstructed form) ultimately became Witten Technologies’ company logo. By this time, Schlumberger had begun using acoustic GDT for petrochemical exploration. Its advent, and the additional reserves it found, resulted in a temporary leveling-off of oil prices worldwide. In 1992, Dr. Witten, while working for Oak Ridge National Labs on a project in Jacksonville, FL, met Robert Green, a second generation utility contractor. This was a pivotal moment. TARGET Figure 3: GPR diagram Green was tired of hitting poorly (or un-) marked utilities, and saw GDT as a possible solution. Upon meeting Dr. Witten, Green asked what proved to be a very important question...”What if we could take this ‘geophysical telescope’ (deep/low resolution) and make it a ‘geophysical microscope’ (shallow/highresolution)?” When Dr. Witten replied “Ground-penetrating-radar,” Computer Assisted Radar Tomography (CART...RT for short) was born. In 1994, Witten Technologies, Inc. (WTI) was formed by Green and Dr. Witten, with the specific purpose of commercializing RT. At the time, Schlumberger was also trying to commercialize RT but, when they realized that they were trying to do something that was already being done, they transferred ownership of their technology development contract with the Electric Power Research Institute to WTI in exchange for WTI shares. Additionally, several of Schlumberger’s key scientists joined the WTI development team. Using specifications provided by WTI, Mala Geosciences provided a first prototype array in 1997 and, by the turn of the century, RT was fully developed. The first real test of the system came in 2001, when it was used to help reconstruct infrastructure near the WTC damaged in the 9/11 tragedy. Since then, RT has covered over 16 million square feet, and WTI has continued to improve on the process, introducing a complementary electromagnetic (vs. radar-based) array to fill some of RT’s “gaps.” Computer Assisted Radar Tomography Technological Background Since RT is based on ground-penetrating-radar (GPR), it is important to make a clear distinction between the two. To make this distinction, an understanding of GPR is required. GPR is a “pulse” technology, where a pulse (wave) sent by a transmitting antenna is received (after traveling through the ground) by a receiving antenna. The variations in the speed of signal return when it is reflected back from a given target(s) allow determination of the vertical position of the target(s) with respect to the antenna. (Fig. 3)Therein lies the major difference between GPR and RT. Given it’s “singular” nature (1 transmitter/1 receiver), GPR produces two-dimensional imagery (shown above), which reveals only the position of the target. It does not reveal what the target is, or anything else about it. The array of antenna used in RT, via plurality and physical distance (between outermost antennas), plays a major part in solving this spatial interpretation problem. (Fig. 4) Figure 4: RT diagram Figure 5: GPR-RT comparison Of course, since GPR is RT’s “foundation,” there are also similarities between the two. Since GPR’s effectiveness is a function of soil resistivity (more resistive/less conductive = better penetration), so too is RT’s. There are certain (conductive) soil types and conditions that limit effectiveness. Since GPR is a pulse/wave technology, it has limited depth of penetration...the wave degrades over time to a point where it is not reflected back by targets. Lastly, there is a direct relationship between frequency, resolution, and depth...the higher the frequency of the radar, the higher the resolution, the shallower the depth of penetration. The higher the frequency, the shorter the waveform, the shorter the distance it can travel, the smaller the object it can discern. Lower frequencies “see” deeper but with lower resolution. Higher frequencies (900+ MHz) are used for very-near-surface applications, like roadbed and bridge deck analysis, while lower frequencies (100-600 MHz) are used to detect deeper concerns, such as utilities, sinkholes, archaeological/burial sites, etc. The chart below (Fig. 5) highlights the differences and similarities between GPR and RT. System Overview For ease of explanation, RT can be broken down into three separate but related components: Hardware, Process and Software. SAME: Materials detected: conductive AND non-conductive Penetration depth: shallow (0-10m) Soil dependent: conductive soil = poor results Resolution vs Depth: higher frequency =better resolution (function of wavelength) =shallower depth DIFFERENT: GPR RT Footprint: small large Data collection rate: low high Cost per sq ft: high low Imagery: 2D 3D Interpretation: high low