Harry M. Jol

GPR in sediments: advice on data collection, basic processing and interpretation, a good practice guide

Abstract Within sedimentological studies, ground penetrating radar (GPR) is being used with increasing frequency because it yields images of the shallow subsurface that cannot be achieved by any other non-destructive method. The purpose of this paper is to provide an introduction to the collection, processing and interpretation of GPR data so that future sedimentary studies can be improved. With GPR equipment now being lightweight, robust and portable, proper data collection and survey design methods need to be followed in order to acquire high resolution, subsurface digital data. Various factors are discussed including: reflection profiling, velocity soundings, test surveys, topography, logistics, data quality and extreme environments. Basic data processing and visualization are then reviewed, followed by a discussion on GPR interpretation strategies including a background to radar stratigraphy. For the sedimentary geologist or geomorphologist, GPR offers unique data of the shallow subsurface including stratigraphy, geometry, architecture and structure.

Ground penetrating radar: antenna frequencies and maximum probable depths of penetration in Quaternary sediments

Abstract Ground penetrating radar (GPR) experiments were carried out in a gravel pit near Brigham City, Utah, USA,to determine maximum probable depths of penetration for 25, 50, 100 and 200 MHz antennas. We have found that this sedimentary field environment (quartzose-rich, thick, inclined gravel strata) is the most appropriate site known and available for the experimental objectives. With a 1000 V transmitter, 25 MHz antennas are capable of detecting stratigraphy to 52 m and possibly 57 m deep. Excessive signal losses for the 50, 100 and 200 MHz antennas occur at depths below 47, 37 and 28 m, respectively, preventing effective detection of stratigraphic interfaces. From 250 different field experiment sites, we suggest that these profiles represent the maximum probable GPR depths that can be confidently interpreted from any Quaternary unconsolidated sediments. A comparison of results shows a linear trend between different antenna frequencies and the maximum probable depth of penetration, suggesting that the 12.5 MHz antennas can detect strata to 66 m deep. Results obtained using the 25 MHz antennas indicate that at least 52 m of inclined strata, assumed to be foreset facies of gravel, occur beneath the gravel pit floor.

Evidence for eight great earthquake-subsidence events detected with ground-penetrating radar, Willapa barrier, Washington

A new approach to detect Holocene subduction-zone earthquakes combines the results from ground-penetrating radar (GPR), Vibracores, and accelerator mass spectrometry (AMS) dates from a barrier spit located west of Willapa Bay, southwest Washington. GPR data show a 10-m-thick facies of beach sand within which we identify, and Vibracores confirm, beach-parallel, wave-eroded, buried scarps mantled with multiple beds of magnetite. The eight GPR-detected buried scarps are interpreted to be eroded by minor transgressions caused by instantaneous barrier subsidence during earthquakes associated with the Juan de Fuca plate subducting under the North American plate. Of these scarps, four have been AMS dated at 300, 1110, 2540, and 4250 (radiocarbon) yr B.P. No datable material has yet been found for the other four radar-detected scarps, but we interpolate and extrapolate dates of 1800, 3400, 5000, and 5800 yr B.P.

Incipient tunnel channels

Abstract Glaciated terrains in east-central Alberta and south-central Michigan contain channels that have hummocks and transverse ridges separating depressions along their floors. This association imparts a linked pothole appearance. Similar channels are often interpreted as tunnel channels or subaerial channels, partly filled with sediment from a subsequent glacial advance, a stagnating ice roof, or slumped sediment from the channel margins. However, the truncation of sedimentary packages in the channel walls and intrachannel hummocks indicates that they are erosional landforms, cut into glacial sediments (till), bedrock, or gravel. Eskers overlie and are found within a few channels, indicating that these channels formed before the final stagnation that produced the eskers. These two characteristics, combined with the observation that many channels have convex-up long profiles, indicate that the channels were eroded by pressurized, subglacial water. Because the formative mechanisms for this type of channel are not clear, and modern environments that could produce this type of landform are inaccessible, we draw on several morphologic analogues to propose mechanisms for channel erosion. We conclude that the erosion of these linked pothole channels (incipient tunnel channels) was the product of the complex interaction between complex turbulent flow structures and various scales of roughness elements.

Radar structure of a Gilbert-type delta, Peyto Lake, Banff National Park, Canada

Abstract Ground-penetrating radar (GPR) was used to image the internal structural of the gravelly Peyto Creek delta in Banff National Park, Alberta, Canada. This was a test of the ability of GPR to evaluate the internal structure of a small river delta in less than a day of field time. Radar facies include topset beds of braided river gravel, foresets with an apparent dip of 25° and bottomset facies. The radar facies and stratigraphy support the long-held belief that the internal structure of the delta is similar to those in the literature termed as Gilbert-type. Also GPR imaged the presence of bedrock extending at least 100 m downvalley beneath the delta sediment. At the proximal (upstream) end of the delta, gravel-bedrock contact plunges at 18° downvalley for a distance of 100 m before radar signals were unable to further detect it. On average the delta slopes 19.2 m/km. Bedding planes within the bedrock dip upvalley at 12°, opposite to the 25° downvalley dip of foresets. At 100 m downdelta, the topset facies is 17 m thick, which represents the vertical aggradation by Peyto Creek in response to 900 m of delta progradation. Topsets pinch out at the delta front. At this point gravel foreset beds ae 28 to 32 m thick below which there is a substantial loss of signal return, possibly due to fine-grained lower foresets or bottomset beds.

Ground-penetrating radar investigation of a Lake Bonneville deita, Provo level, Brigham City, Utah

Field experiments carried out in the Brigham Sand and Gravel Company pit, near Brigham City, Utah, show an impressive similarity between observed gravelly deltaic foreset facies in highwall exposures and inclined reflections detected by ground-penetrating radar (GPR). Beneath the pit floor, the radar indicates additional inclined strata more than 30 m deep. In a test to compare different antenna frequencies (50, 100, and 200 MHz), using a 400 V transmitter, 50 MHz antennas are capable of detecting gravel facies in excess of 30 m, whereas signal loss for the 100 and 200 MHz antennas occurs below 23 and 17 m, respectively, though providing higher resolution.

Ground penetrating radar surveys of peatlands for oilfield pipelines in Canada

Abstract Placement of buried pipelines in thick peat deposits is difficult because of the low bearing strength and high water content of the material for support of heavy construction equipment. Previously, ground penetrating radar (GPR) has been used to assess thickness and volume of peat as a fuel resource and horticultural material in Scandinavia and Canada. To our knowledge, GPR has not been applied to the site assessment and placement of pipelines crossing peatlands. Field experiments were conducted in the Mitsue oilfield operated by Chevron Canada Resources Ltd., located immediately southeast of Lesser Slave Lake in north-central Alberta Province, Canada. Surficial deposits consist of Holocene, linear, sandy beach ridges separated by peatlands underlain by sand. Several GPR surveys assessed the thickness of the peat along two oil pipeline right-of-ways. Results show the peat-sand contact as irregular and undulating, ranging from 0 to 3.7 m deep. Each survey, 460 and 550 m long, was completed in two hours. Such results from 1 m station spacings (sampling interval) can considerably reduce the uncertainties in planning and placement of oil, gas, and water pipelines crossing peatlands. Results indicate that thickness variations of peat can be detected more effectively in terms of quality of results, lower cost, and less time with GPR than with a peat probe or by coring.

Stratigraphy of Back-Barrier Coastal Dunes, Northern North Carolina and Southern Virginia

Abstract Ground penetrating radar studies of four representative active back-barrier dunes, combined with radiocarbon and photon-stimulated-luminescence dating techniques and soils analysis, reveal phases of alternating dune activity and stabilization along the North Carolina–Virginia coast. Two smaller dunes represent only the current phase of dune activity. Two larger dunes preserve evidence of three phases of dune development (ca. 740, 1260 and 1810 AD) and intervening phases of soil development. Climate, particularly moisture conditions, played a part in the timing of dune activity and stabilization events. All three dune phases are associated with drier conditions whereas soils formation is associated with humid conditions. Modern (phase 3) dunes are more widespread along the coast and their formation is attributed to a combination of dry conditions, increased storminess associated with the Little Ice Age, and rising sea level. Tidal inlet closing and storm overwash processes likely provided sediment point sources for individual dune masses. The longer history and much greater volume of dune sand in the area of the two larger dunes suggests a greater sediment supply in this locality.

An introduction to ground penetrating radar (GPR) in sediments

In sedimentary geology, ground penetrating radar (GPR) is used primarily for stratigraphic studies where near-continuous, high-resolution profiles aid in determining: (1) stratigraphic architecture, (2) sand-body geometry, and (3) correlation and quantification of sedimentary structures. In the past, to investigate lateral continuity and variability of sediments, we had to infer the correlation between boreholes, outcrops or shallow trenches. Nowadays, with suitable ground conditions (sediment with high resistivity, e.g. sands and gravels), we can collect GPR profiles that show the subsurface stratigraphy. In addition, 3-D GPR can provide much greater appreciation of sand-body geometry and architecture. GPR is, however, not a universal panacea; in some cases, ground truth is still required because lithological determination is by no means unequivocal, therefore borehole or outcrop data may be required to corroborate the results of a GPR survey. Indeed, the latest GPR survey data, including 3-D depth migration, required both boreholes and outcrop data to generate a 3-D velocity model (e.g. Corbeanu et al. 2001). In addition, fine-grained sediments (low resistivity) and areas with saline groundwaters cause rapid attenuation of the radar signal, leading to poor signal penetration. This book begins with an introductory paper (Jol & Bristow 2003) aimed at those with little or no experience of GPR and including the basics of data collection, processing and interpretation. The book is then divided into sections on sedimentary environments, including aeolian and coastal, fluvial and alluvial fan, glacial, and lakes; ancient sediments (reservoir analogues); tectonics; and engineering and environmental applications. The final section looks at various aspects of GPR methodology.

Ground penetrating radar: 2-D and 3-D subsurface imaging of a coastal barrier spit, Long Beach, WA, USA

Abstract The ability to effectively interpret and reconstruct geomorphic environments has been significantly aided by the subsurface imaging capabilities of ground penetrating radar (GPR). The GPR method, which is based on the propagation and reflection of pulsed high frequency electromagnetic energy, provides high resolution (cm to m scale) and shallow subsurface (0–60 m), near continuous profiles of many coarser-grained deposits (sediments of low electrical conductivity). This paper presents 2-D and 3-D GPR results from an experiment on a regressive modern barrier spit at Willapa Bay, WA, USA. The medium-grained sand spit is 38 km long, up to 2–3.5 km wide, and is influenced by a 3.7-m tidal range (spring) as well as high energy longshore transport and high wave energy depositional processes. The spit has a freshwater aquifer recharged by rainfall. The GPR acquisition system used for the test was a portable, digital pulseEKKO™ system with antennae frequency ranging from 25 to 200 MHz and transmitter voltages ranging from 400 to 1000 V. Step sizes and antennae separation varied depending on the test requirements. In addition, 100-MHz antennae were used for conducting antennae orientation tests and collecting a detailed grid of data (50×50 m sampled every meter). The 2-D digital profiles were processed and plotted using pulseEKKO™ software. The 3-D datasets, after initial processing, were entered into a LANDMARK™ workstation that allowed for unique 3-D perspectives of the subsurface. To provide depth, near-surface velocity measurements were calculated from common midpoint (CMP) surveys. Results from the present study demonstrate higher resolution from the 200-MHz antennae for the top 5–6 m, whereas the 25- and 50-MHz antennae show deeper penetration to >10 m. For the study site, 100-MHz antennae provided acceptable resolution, continuity of reflections, and penetration. The dip profiles show a shingle-like accretionary depositional pattern, whereas strike profiles show a horizontal and subhorizontal, nearly continuous reflection pattern. Results from the GPR experiment reveal upper shoreface reflections with dip towards the ocean at about 1–2°. The loss of signal from below a depth of 6–8 m indicates a lithofacies change because of the storm wave base. The parallel broadside and perpendicular broadside antennae orientation tests show detailed stratigraphy, continuity, and depth of penetration. The cross-polarization test exhibits reduced continuity of reflections and less depth of penetration, but dipping reflections are apparent. The grid pattern data provided a detailed view of 3-D geometry of individual reflections. High quality data were obtained, processed, and directly exported into a LANDMARK™ workstation for interpretation. The resulting interpretations of the upper shoreface beds from the test cube (50×50 m; total 2600 traces) are shown as vertical sections (slices), horizontal sections (time slices), contour maps, 3-D representations of individual beds, and an isopach map. The 3-D depositional framework allows a more detailed interpretation than widely spaced 2-D profiles.