OVERVIEW
Ground penetrating radar (GPR) is a geophysical survey method and the main component of a PAS 128:2014 utility detection and mapping survey.
GPR is:
- an accurate, fast and high-resolution geophysical technique for subsurface investigation.
- non-invasive, non-destructive and completely safe.
- the only non-intrusive method capable of accurately locating non-metallic subsurface features and utilities (eg. clay, concrete, fibreglass, PVC conduits or fibre-optic cables).
- a geophysical surveying technique based on transmitting pulsed electromagnetic (EM) energy into the subsurface and measuring the strength of the reflected energy.
- successful where a sufficient contrast in material properties (dielectric permittivity) between a buried target and its surroundings exists.
- used by Scantech to detect and map buried pipes, cables, structural reinforcement, voids, disturbed ground, material degradation, subsurface layers and buried objects.
- acquired using transmitting and receiving antennae which can be mounted on a cart, skid plate or vehicle, or can be hand-held.
- a technique which requires qualified and experienced personnel to acquire high quality survey data and geophysical expertise to process and interpret the results.
GPR METHOD
GPR uses high-frequency-pulsed electromagnetic (EM) waves to acquire subsurface information. It is used to investigate the subsurface without drilling, probing or digging. Basic GPR survey equipment consists of a transmitting and receiving antennae, a radar control unit and a data storage and display device.
Energy is radiated into the ground from a transmitting antenna. As the wave spreads out and travels downward, if it encounters a buried object or boundary with different electromagnetic properties, then part of the wave energy is reflected or scattered back towards the surface. The receiving antenna at the surface records the strength (amplitude) of the reflected signal with time. The amplitude of the EM energy reflected from any boundary depends on the change in material properties (dielectric constant, magnetic permeability and electrical conductivity) at the boundary. The reflected signals are recorded over a selected time range for a fixed antenna position to produce a scan or trace of radar data. Scans obtained as the antennae are moved over a surface are placed side by side to produce a radar profile or radargram.
The vertical scale of the recorded radargram is in units of two-way travel time, the time it takes for the EM wave to travel down to a reflector and back to the surface. The travel time is converted to depth by relating it to on-site measurements or assumptions about the velocity of radar waves in the subsurface material under investigation. For most geological materials radar wave speeds vary between 60–175 mm/ns.
GPR waves can reach depths up to 30 metres in low conductivity materials such as dry sand or granite. Clays, shale, and other high conductivity materials may attenuate or absorb GPR signals, decreasing the depth of penetration to 1 metre or less. The depth of penetration is also determined by the GPR antenna.
The antennae are selected on the basis of the depth of interest and the size of the target. Penetration depth varies inversely with frequency and the higher the central frequency of the antenna, the smaller the size of object that can be resolved. Higher antenna frequencies give higher resolution, but less penetration, and vice versa. The lateral and vertical resolution of the results varies from 0.01 to 1.0 meters, depending on the choice of antenna frequency. Low frequency antennae are used in geological mapping to get the maximum penetration depth while higher frequencies are used in non-destructive testing for high resolution imaging. Having a range of frequencies available (usually 50MHz–2.5GHz) makes GPR ideal for locating objects of different sizes at various depths and in different ground conditions.
POST PROCESSING
GPR is a technique where experience and geophysical expertise is required to process and interpret GPR survey data correctly. Distinct PAS 128 utility quality levels are assigned to post-processed utility locations to reflect the resulting improvement in data interpretation and utility detection. Scantechs geophysicists are experienced GPR data processors. We provide basic to advanced GPR post-processing as standard to improve target object detection and survey interpretation.
Utility survey GPR data before and after processing
Typical data processing flow sequence
(bistatic common-offset reflection mode GPR data)
Diagram modified from "Ground Penetrating Radar: Theory and Applications" Jol, 2009.
Steps shown in bold are considered necessary for interpreation.
APPLICATIONS
CONSTRUCTION/ENGINEERING
General site investigation
- GPR is a non-destructive, reconnaissance tool for general sub-surface mapping (soil/rock horizons, depth to water table etc.) and buried object location prior to invasive investigation.
- GPR can assist in the planning and location of follow-up intrusive sampling programs (trial pits, boreholes, coring).
Utility Detection & Mapping (depth, position and direction)
- Detection and mapping of buried structures such as plastic water and gas pipes, fibre optic ducts, concrete sewers, clay drainage and asbestos cement pipes as well as the standard traceable services, made of steel, cast iron and power carrying cables.
- Subsurface piping leak detection and leak impact assessment (presence of washouts or voids)
Concrete Non Destructive Testing
- Identification and location of embedded structures (metallic and non-metallic conduits, rebars and tension cables) within concrete structures (e.g., bridges, dams, reservoirs, foundations, tunnels, runways etc.).
- Identification and location of voids and zones of construction degradation (concrete layer thickness and water content)
- Quality Assurance Control of new structures or existing constructions for rehabilitation purposes
Road & Rail Investigation
- Mapping structure of asphalt and concrete pavement, (layer thickness, layer integrity, water content, voids) for effective maintenance and rehabilitation decisions
- Map railroad ballast thickness
- Identification of embedded utilities
ENVIRONMENTAL
- Landfill delineation
- Brown Field Site Investigation: mapping of existing services, thickness of made up ground, unknown cellars, voids and buried objects prior to excavation.
- Contaminant Plume profiling (from tank leaks, surface spills, piping leaks, landfills)
- Soil stratigraphy mapping (soil condition, compaction and water distribution)
- Underground storage tank (UST) location (steel, concrete or fiberglass)
- Underground Storage Drums location
ARCHAEOLOGICAL/FORENSICS
- General sub-surface investigation of ground conditions and target location around archaeological and forensic sites.
- Location of buried objects and buried remains or disturbed soil associated with a burial.
- Location and delineation of buried objects, walls & foundations, soil disturbances and hidden cavities.
GEOLOGICAL & MINING
- General subsurface mapping (rock quality, fault and fracture detection, soil stratigraphy, depth to bedrock)
- Subsidence and swallow hole investigations, karst mapping.
- Soil & aggregate mapping (soil composition and compaction aggregate depth, quality & quantity)
- Water resources (water table delineations, soil water distribution)
- Quarry and mine mapping (tunnelling, rock mass stability, mineral and ore zone delineation)
FINANCIAL & INSURANCE
- Property assessment (building integrity/construction quality control).
- Risk assessment
STANDARDS AND GUIDELINES FOR GPR AND UNDERGROUND UTILITY DETECTION SURVEYS
Scantech has adopted PAS 128:2014, the recently published BSI specification for underground utility detection, verification and location. PAS 128 sets out the requirements and recommendations for underground utility surveying using GPR and other geophysical methods. GPR is a principal component of a PAS 128 utility detection survey (Type B). As chartered geophysicists we have the required training and expertise to carry out GPR surveying for utilities and we follow established GPR survey procedures, some of which are now specified in PAS 128. Prior to the publication of PAS 128 for utility surveying we used the American Standard ASCE 38-02 Standard Guideline for the Collection and Depiction of Existing Subsurface Utility Data.
As GPR surveyors, in addition to PAS 128 for utilities, we also comply with ASTM International guidelines for the GPR method. ASTM D6432-11 is the standard guide for using the surface ground penetrating radar method for subsurface investigation. It covers the equipment, field procedures, and interpretation methods for the assessment of subsurface materials using impulse GPR. These guidelines were originally published in 1999 but updated in 2011 to take account of new GPR developments including the increased use of GPR for underground utility detection.
GPR technology has undergone significant advances since we first used it in 1994. Basic equipment targeted at the construction and land surveying sector can be used with minimal training and works well in simple situations (a metal pipe in dry sandy soil). However, in complex underground environments with high utility densities or poor soil conditions, GPR surveying can be challenging. A geophysicist has the formal understanding of GPR wave propagation and interaction with the subsurface which is necessary to optimise GPR data acquisition and to process and interpret GPR data in non-ideal conditions.
Unfortunately, even with adherence to the current guidelines and specifications, engaging a practitioner with no formal geophysics qualifications and a limited understanding of GPR theory and the physics of subsurface surveying increases the risk of receiving a sub-standard survey. Scantech are currently the only specialist GPR and utility surveyors in Ireland and the UK with chartered status in geophysics.
References
PAS 128:2014 Specification for underground utility detection, verification and location (British Standards Institution (BSI), 2014)
ASTM D6432-11 Standard Guide for Using the Surface Ground Penetrating Radar Method for Subsurface Investigation. (American Society for Testing and Materials, 2011).
ASCE 38-02 Standard Guideline for the Collection and Depiction of Existing Subsurface Utility Data. (American Society of Civil Engineers, 2002)
GPR LIMITATIONS
- As with any other geophysical technique, GPR performance is site specific and it is unsuitable for use at some locations. Expected subsurface conditions and the target composition, location and size should be taken into account.
- GPR anomalies rely on a detectable contrast in subsurface electrical properties between the target of interest and its surrounding material. In the absence of a detectable contrast, no anomaly will be evident.
- GPR signal cannot penetrate through highly conductive material e.g., beneath metal sheets or very wet ground or in material saturated with salt water or highly conductive fluid.
- Velocity – depth calibration should always be carried out to obtain satisfactory depth estimates.
- GPR data processing and interpretation can be complicated - specialised geophysical analysis and interpretation is often required.
- GPR is unsuited to absolute measurement, e.g., it can find wet areas, but cannot determine actual moisture content.
- GPR is an interpretive method, based on the identification of reflectors, which may not uniquely identify an object. Additional constraining information from ground truthing or other geophysical methods is important to help resolve any ambiguities.
- In common with all surface geophysical methods GPR is inherently limited by decreasing resolution with depth.
“An understanding of the theory, field procedures, and methods for interpretation of GPR data along with an understanding of the site geology are necessary to successfully complete a GPR survey. Personnel not having specialized training and experience should be cautious about using this technique and solicit assistance from qualified practitioners.”
Quote from ASTM D6432–11 Standard Guide for Using the Surface Ground Penetrating Radar Method for Subsurface Investigation