ASTM D6432-19

5.3.1 Radar Control Unit—The radar control unit synchronizes signals to the transmitting and receiving electronics in the antennas. The synchronizing signals control the transmitter and sampling receiver electronics located in the antenna(s) in order to generate a sampled waveform of the reflected radar waves. These waveforms may be filtered and amplified and are transmitted along with timing signals to the display and recording devices.

5.3.2 Real-time signal processing for improvement of signal-to-noise ratio is available in most GPR systems. When working in areas with cultural noise and in materials causing signal attenuation, time-varying gain is necessary to adjust signal amplitudes for display on monitors or plotting devices. Filters may be used in real time to improve signal quality. The summing of radar signals (stacking) is used to increase effective depth of exploration by improving the signal-to-noise ratio.

5.3.3 Data Display—The GPR data are displayed as a continuous profile of individual radar traces (Fig. 2). The horizontal-axis represents horizontal traverse distance and the vertical-axis is two-way travel time (or depth). Data are commonly presented in wiggle trace display, where the intensity of the received wave at an instant in time is proportional to the amplitude of the trace (see Fig. 2), or as a gray scale or color scale display, where the intensity of the received wave at an instant in time is proportional to either the intensity of gray scale (that is, black is high intensity, and white is low intensity; see Fig. 3) or to some color assignment defined according to a specified color-signal amplitude relationship.

5.4.2.4 Polarization—The type and alignment of polarization of the vector electromagnetic fields may be important in receiving responses from various scatterers. Two linear, parallel polarized, electric field antennas can maximize the response from linear scatters like pipes when the electric field (typically long axis of the dipole antenna) is aligned parallel with the pipe and towed perpendicular across the pipe. Similarly, alignment with the rebar in concrete will maximize the ability to map the rebar, but alignment perpendicular to the rebar will minimize scattering reflections from the rebar to see through or past the rebar to get the thickness of concrete. Similar arrangement may be made for overhead wires and nearby fences. Cross-polarized antennas (perpendicular to each other) minimize the response from horizontal layers.

5.4.3.1 Measurements obtained by the GPR method may contain unwanted signals (noise) caused by geologic and cultural factors.

5.4.3.2 Ambient and Geologic Sources of Noise—Boulders, animal burrows, tree roots, or other inhomogeneities can cause unwanted reflections or scattering of the radar waves. Lateral and vertical variations in EM properties can also be a source of noise.

5.4.3.3 Cultural Sources of Noise—Above-ground cultural sources of noise include reflections from nearby vehicles, buildings, fences, power lines, lampposts, and trees. In cases where this kind of interference is present in the data, a shielded antenna may be used to reduce the noise.

5.4.3.4 Summary—All possible sources of noise present during a survey should be noted so that their effects can be considered when processing and interpreting the data.

1.1.1 This guide covers the equipment, field procedures, and interpretation methods for the assessment of subsurface materials using the Ground Penetrating Radar (GPR) Method. GPR is most often employed as a technique that uses high-frequency electromagnetic (EM) waves (from 10 to 7000 MHz) to acquire subsurface information. GPR detects changes in EM properties (dielectric permittivity, conductivity, and magnetic permeability), that in a geologic setting, are a function of soil and rock material, water content, and bulk density. Data are normally acquired using antennas placed on the ground surface or in boreholes. The transmitting antenna radiates EM waves that propagate in the subsurface and reflect from boundaries at which there are EM property contrasts. The receiving GPR antenna records the reflected waves over a selectable time range. The depths to the reflecting interfaces are calculated from the arrival times in the GPR data if the EM propagation velocity in the subsurface can be estimated or measured.

1.1.2 GPR measurements as described in this guide are used in geologic, engineering, hydrologic, and environmental applications. The GPR method is used to map geologic conditions that include depth to bedrock, depth to the water table (Wright et al 1.2 Limitations: 

1.3 Units—The values stated in SI units are to be regarded as standard. The values given in parentheses are provided for information only and are not considered standard. Reporting of test results in units other than SI shall not be regarded as nonconformance with this standard.

1.4 This guide offers an organized collection of information or a series of options and does not recommend a specific course of action. This document cannot replace education or experience and should be used in conjunction with professional judgment. Not all aspects of this guide may be applicable in all circumstances. This ASTM standard is not intended to represent or replace the standard of care by which the adequacy of a given professional service must be judged, nor should this document be applied without consideration of a project’s many unique aspects. The word “Standard” in the title of this document means only that the document has been approved through the ASTM consensus process.

1.5 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.

1.6 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.