A. Peter Annan

Acquisition and processing of wide‐aperture ground‐penetrating radar data

A 40-channel wide‐aperture ground penetrating radar (GPR) data set was recorded in a complicated fluvial/aeolian environment in eastern Canada. The data were collected in the multichannel format usually associated with seismic reflection surveys and were input directly into a standard seismic processing sequence (filtering, static corrections, common‐midpoint gathering, velocity analysis, normal‐ and dip‐moveout corrections, stacking and depth migration). The results show significant improvements, over single‐channel recordings, in noise reduction and depth of penetration (by stacking), and in spatial positioning and reduction of diffraction artifacts (by migration). These characteristics increase the potential for reliable interpretation of structural and stratigraphic details. Thus, without having to develop any new software, GPR data processing technology is brought to the same level of capability, flexibility, and accessibility that is current in seismic exploration.

Ground-penetrating radar monitoring of a controlled DNAPL release: 200 MHz radar

A controlled release of tetrachloroethylene was performed in a saturated, natural sandy aquifer to evaluate the effectiveness of various geophysical techniques for detecting and monitoring dense nonaqueous phase liquids (DNAPLs) in the subsurface. Tetrachloroethylene, typical of most DNAPLs, has a low relative dielectric permittivity (2.3), which contrasts with the high relative permittivity (80) of the pore water it displaces, making it a potential target for detection by ground‐penetrating radar (GPR). GPR data were acquired using 200 MHz antennas. Radar sections collected at different times over the same spatial location clearly show the changes induced by the movement of DNAPL in the subsurface. Temporal changes can be examined through the evolution of a radar data trace collected at a single spatial location. Normal moveout analysis of common‐midpoint (CMP) data demonstrates induced changes in electromagnetic (EM) wave velocities of up to 30 percent caused by the presence of DNAPL. The distribution o...

Examples of reverse-time migration of single-channel, ground-penetrating radar profiles

A single‐channel, ground‐penetrating radar (GPR) profile portrays a distorted, unfocused image of subsurface structure due to apparent position shifts associated with dipping reflectors and to diffractions from corners and edges. A focused image may be produced from such data by using any of the migration algorithms previously developed for seismic data; we use reverse‐time migration based on the scalar wave equation. Field work was performed over a simple stratigraphic soil sequence and a complicated fluvial environment. In the migrated images, reflector continuity is enhanced and the level of detail available for high‐resolution interpretation is significantly increased.

Using an induction coil sensor to indirectly measure the B-field response in the bandwidth of the transient electromagnetic method

The traditional sensor used in transient electromagnetic (EM) systems is an induction coil. This sensor measures a voltage response proportional to the time rate of change of the magnetic field in the EM bandwidth. By simply integrating the digitized output voltage from the induction coil, it is possible to obtain an indirect measurement of the magnetic field in the same bandwidth. The simple integration methodology is validated by showing that there is good agreement between synthetic voltage data integrated to a magnetic field and synthetic magnetic‐field data calculated directly. Further experimental work compares induction‐coil magnetic‐field data collected along a profile with data measured using a SQUID magnetometer. These two electromagnetic profiles look similar, and a comparison of the decay curves at a critical point on the profile shows that the two types of measurements agree within the bounds of experimental error. Comparison of measured voltage and magnetic‐field data show that the two sets ...

Resistive‐limit, time‐domain AEM apparent conductivity

On-time measurements, which can be obtained with existing airborne transient electromagnetic (AEM) systems, are used to map the apparent conductivity of the ground. Such measurements can extend the conductivity aperture of these systems substantially at the low end of the conductivity range. A theoretical analysis is given that provides a simple, approximate, resistive-limit solution for any transient EM waveform. The approximate result is compared to the full-numerical result for the specific case of the GEOTEM system over a conductive half-space. Examples of GEOTEM data reduced to apparent conductivity using the approximate analytical solution are presented from surveys for kimberlites and massive sulfide deposits.

Application of a modified GEOTEM® system to reconnaissance exploration for kimberlites in the Point Lake area, NWT, Canada

Airborne geophysical surveying with electromagnetic (EM) and magnetic methods is an effective reconnaissance exploration tool for kimberlite pipes because the target can have an associated EM and magnetic anomaly. The EM response of kimberlite pipes is most often attributed to weathering alteration in a near‐surface layer, whereas the magnetic response is attributed to magnetite and ilmenite within the deeper unweathered kimberlite pipe. The discrete shape of kimberlite diatremes results in an easily identifiable anomaly pattern. Diamondiferous kimberlites have recently been found in the Northwest Territories (NWT) of Canada, an area glaciated in the Pleistocene and therefore devoid of a strongly weathered zone. By configuring the GEOTEM® airborne EM system to operate at high frequencies (270 Hz) and to take measurements while the transmitter is switched on, weakly conductive bodies may be detected because there is an adequate contrast with the surrounding highly resistive country rock. System modificatio...

GPR, TDR, and geochemistry measurements above an active gas vent to study near-surface gas-migration pathways

The migration of deep gas to the atmosphere along faults and associated structures is important in many fields, from studying the natural contribution of atmospheric greenhouse gases leaking from geothermal areas to ensuring the safety of man-made natural gas and carbon dioxide (C O2 ) geologic-storage sites. Near-surface geophysical and geochemical techniques were applied to a naturally occurring gas vent located along a deep terrestrial fault to better understand the structure and geophysical response of this gas-migration pathway. A number of ground-penetrating radar (GPR) profiles were first conducted across the vent. Spot samples were then measured along one of these profiles for in situ apparent permittivity (using time-domain reflectometry — TDR), complex permittivity on dried samples (using a capacitivecell), soil-gas composition, and clay and bulk mineralogy. Results show how the migrating gas induces secondary effects that modify the signature of the vent as seen in the GPR profiles. In particul...

RADIO INTERFEROMETRY DEPTH SOUNDING: PART II—EXPERIMENTAL RESULTS

In such highly resistive geologic environments as ice sheets, salt layers, and the moon’s surface, radio waves penetrate with little attenuation. The field strengths about a transmitting antenna placed on the surface of such an environment exhibit interference maxima and minima which are indicative of the in‐situ electrical properties and the presence of subsurface layering. Experimental results from an analog scale model and from field tests on two glaciers are interpreted on the basis of the theoretical results of Part I. If the upper layer is thick, the pattern is very simple and the dielectric constant of the layer can be easily determined. An upper bound on the loss tangent can be estimated. For thin layers, the depth can be determined if the loss tangent is less than about 0.10, and a crude estimate of scattering can be made.

Concrete Bridge Deck Deterioration Assessment Using Ground Penetrating Radar (GPR)

Ground penetrating radar (GPR) has become an effective means for assessing deterioration in concrete bridge decks. While success has been demonstrated, the method is still not adopted widely. Constant technical development is making such high speed GPR mapping more affordable with systems more widely available and easier to deploy. The American Society for Testing and Materials (ASTM) has a standard procedure for performing bridge deck deterioration using GPR. The current standard, initially written for air-launched GPR devices and then modified to include ground-coupled GPRs, has many simplifying assumptions that could lead to fallacious evaluations. Both field experience and numerical simulations indicate that ground-coupled GPR systems are preferable to air-launched GPRs in this application, delivering larger signal-to-noise and higher spatial resolution data, which enhance extraction of both electromagnetic wave velocity and attenuation. We describe advances in analysis and interpretation that go beyond the current ASTM approach which ignores the impact of depth and other variables. We demonstrate these advances using a high speed, ground-coupled GPR system with examples of deck deterioration mapping. We describe the workflow for using GPR to evaluate the deterioration of concrete bridge decks, highlight the basic interpretation assumptions, demonstrate successful applications and discuss limitations with the methodology.

Spatially-filtered FDTD subgridding for ground penetrating radar numerical modeling

Subgridding schemes are often implemented in the standard finite-difference time-domain (FDTD) method especially when fine geometric features and/or media with a high relative permittivity need to be modeled. In this paper, we propose an FDTD subgridding scheme which employs spatial filtering, to numerically model ground penetrating radar (GPR), using time steps well beyond the FDTD stability limit. To demonstrate the accuracy and efficiency of the proposed approach, comparisons of its numerical modeling results with the standard FDTD method and a previously proposed subgridding scheme are provided.