Ground-penetrating Radar Transects Research Articles

Postglacial evolution of the Lake Constance shelf and its relation to a 10-km long chain of submerged Neolithic mounds

Postglacial evolution of the Lake Constance shelf and its relation to a 10-km long chain of submerged Neolithic mounds

Ground-penetration radar detection of Root-mass in a Tree Groove in Southwestern Nigeria

In this study, we present the results of a ground-penetrating radar (GPR) survey aimed at imaging the lateral root systems of a section of a tree groove lined by the Tectona Grandis (Teak) species. This was to set the basis for deploying a multi-frequency GPR system for the non-invasive monitoring of rootmass development and evaluation of plant development and health. The survey involved a single GPR Transect with a total length of 60.1 m established along the middle of a line of matured Teak trees. Data was collected at 250 MHz, 500 MHz and 1000 MHz, simultaneously using a wheel-triggered Utsi Trivue system. Data processing involved a dewow, static corrections, gain application and background removal. A root zone consisting of two layers was delineated and a total of 159 hyperbolae were interpreted as roots. The roots were limited to the shallow subsurface, the upper 1.1 m of a potential root zone 1.6 m deep. Imaged roots generally occur in clusters that create much disturbance of the otherwise continuous reflections in the root zone. Often, roots may be laterally offset from the location of tree bases on the Transect. We further identified a potential set of laterally migrating roots in a zone of undisturbed ground outside the main groove indicating the lateral reach of the usually shallow roots of the Teak tree. The results set the basis for further work in root-mass estimation and monitoring tree health within our team.

A Time Series of Snow Density and Snow Water Equivalent Observations Derived From the Integration of GPR and UAV SfM Observations

Snow depth can be mapped from airborne platforms and measured in situ rapidly, but manual snow density and snow water equivalent (SWE) measurements are time consuming to obtain using traditional survey methods. As a result, the limited number of point observations are likely insufficient to capture the true spatial complexity of snow density and SWE in many settings, highlighting the value of distributed observations. Here, we combine measured two-way travel time from repeat ground-penetrating radar (GPR) surveys along a ∼150 m transect with snow depth estimates from UAV-based Structure from Motion Multi-View Stereo (SfM-MVS) surveys to estimate snow density and SWE. These estimates were successfully calculated on eleven dates between January and May during the NASA SnowEx21 campaign at Cameron Pass, CO. GPR measurements were made with a surface-coupled Sensors and Software PulseEkko Pro 1 GHz system, while UAV flights were completed using a DJI Mavic 2 Pro platform and consisted of two orthogonal flights at ∼60 m elevation above ground level. SfM-MVS derived dense point clouds (DPCs) were georeferenced using eight ground control points and evaluated using three checkpoints, which were distributed across the ∼3.5 ha study plot containing the GPR transect. The DPCs were classified to identify the snow surface and then rasterized to produce snow-on digital surface models (DSMs) at 1 m resolution. Snow depths on each survey date were calculated by differencing these snow-on DSMs from a nearly snow-off DSM collected near the end of the melt season. SfM-derived snow depths were evaluated with independent snow depth measurements from manual probing (mean r2 = 0.67, NMAD = 0.11 m and RMSE = 0.12 m). The GPR-SfM derived snow densities were compared to snow density measurements made in snowpits (r2 = 0.42, NMAD = 39 kg m−3 and RMSE = 68 kg m−3). The integration of SfM and GPR observations provides an accurate, efficient, and a relatively non-destructive approach for measuring snow density and SWE at intermediate spatial scales and over seasonal timescales. Ongoing developments in snow depth retrieval technologies could be leveraged in the future to extend the spatial extent of this method.

Delineation and description of soil horizons using ground-penetrating radar for soils under boreal forest in Central Karelia (Russia)

This article focused on studying the landscape-scale spatial variation of boreal forest soils in data-scarce areas using ground-penetrating radar (GPR). An important goal of the research was to discuss correlations between attributes of the soil profile under boreal forest and GPR data. We detected the soil horizons of Entic Podzols and determined their attributes based on a GPR transect with a 400 MHz antenna unit. GPR cross-section showed the soil thickness variation over the transect being 18.8 cm to 44.7 cm. The mean depth for the soil's lower boundary from sounding was 23.1 cm versus the probing of 24 cm. The fieldwork research has revealed GPR patterns for different ecological settings: typical forest, paludified forest, and felling area. We investigated the GPR applicability for delineating organic layers in soils and considered quantifying soil organic carbon content (SOC) relying on GPR attributes. To achieve this purpose, we have surveyed Albic Podzols with a 1,700 MHz antenna unit considering vegetation cover. The resolution of GPR was ±1 cm, which made to determine relatively thin horizons. GPR reflected signal amplitudes demonstrated sensitivity to SOC variations in the organic layers. The obtained relationship between SOC and GPR attributes indicates the possibility of determining the carbon storage variations in situ. However, the problem is to substantiate the mechanism of the SOC influence on the signature of the GPR traces. The findings prove that GPR is a tool for predicting the state and scope of the transformation of sandy forest soils. GPR cross-sections can be the basement for a novel approach for forest soil mapping on a landscape scale. In prospect, digital soil mapping can include a sequence of methods: remote sensing – GPR – manual probing. Admittedly, alignments between soil characteristics and GPR attributes are needed to identify the capabilities and limitations of the method.

Relating regional and point measurements of accumulation in southwest Greenland

Abstract. In recent decades, the Greenland ice sheet (GrIS) has frequently experienced record melt events, which have significantly affected surface mass balance (SMB) and estimates thereof. SMB data are derived from remote sensing, regional climate models (RCMs), firn cores and automatic weather stations (AWSs). While remote sensing and RCMs cover regional scales with extents ranging from 1 to 10 km, AWS data and firn cores are point observations. To link regional scales with point measurements, we investigate the spatial variability of snow accumulation (bs) within areas of approximately 1–4 km2 and its temporal changes within 2 years of measurements. At three different sites on the southwestern GrIS (Swiss Camp, KAN-U, DYE-2), we performed extensive ground-penetrating radar (GPR) transects and recorded multiple snow pits. If the density is known and the snowpack dry, radar-measured two-way travel time can be converted to snow depth and bs. We spatially filtered GPR transect data to remove small-scale noise related to surface characteristics. The combined uncertainty of bs from density variations and spatial filtering of radar transects is at 7 %–8 % per regional scale of 1–4 km2. Snow accumulation from a randomly selected snow pit is very likely representative of the regional scale of 1–4 km2 (with probability p=0.8 for a value within 10 % of the regional mean for KAN-U, and p>0.95 for Swiss Camp and DYE-2). However, to achieve such high representativeness of snow pits, it is required to determine the average snow depth within the vicinity of the pits. At DYE-2, the spatial pattern of snow accumulation was very similar for 2 consecutive years. Using target reflectors placed at respective end-of-summer-melt horizons, we additionally investigated the occurrences of lateral redistribution within one melt season. We found no evidence of lateral flow of meltwater in the current climate at DYE-2. Such studies of spatial representativeness and temporal changes in accumulation are necessary to assess uncertainties of the linkages of point measurements and regional-scale data, which are used for validation and calibration of remote-sensing data and RCM outputs.

Formation and evolution of an extensive blue ice moraine in central Transantarctic Mountains, Antarctica

AbstractMount Achernar moraine is a terrestrial sediment archive that preserves a record of ice-sheet dynamics and climate over multiple glacial cycles. Similar records exist in other blue ice moraines elsewhere on the continent, but an understanding of how these moraines form is limited. We propose a model to explain the formation of extensive, coherent blue ice moraine sequences based on the integration of ground-penetrating radar (GPR) data with ice velocity and surface exposure ages. GPR transects (100 and 25 MHz) both perpendicular and parallel to moraine ridges at Mount Achernar reveal an internal structure defined by alternating relatively clean ice and steeply dipping debris bands extending to depth, and where visible, to the underlying bedrock surface. Sediment is carried to the surface from depth along these debris bands, and sublimates out of the ice, accumulating over time (>300 ka). The internal pattern of dipping reflectors, combined with increasing surface exposure ages, suggest sequential exposure of the sediment where ice and debris accretes laterally to form the moraine. Subsurface structure varies across the moraine and can be linked to changes in basal entrainment conditions. We speculate that higher concentrations of debris may have been entrained in the ice during colder glacial periods or entrained more proximal to the moraine sequence.

Rapid Estimation of Volumetric Groundwater Recharge in the Vadose Zone via Ground Penetrating Radar

Saturation in the vadose zone is known to reduce the ground penetrating radar (GPR) electromagnetic wave velocities. We examine the impact of increased water content in highly-permeable beach and dune systems on GPR velocities along coastal margin of Perth, Western Australia. We acquire repeat GPR transects in May and August before and after annual high rainfall periods. The assumption of exceedingly flat water table reflector permits us to estimated change in GPR velocity at successive dates. This change in velocity can be translated to an estimated change in water saturation via the Topp relationship which is an imperial mapping of water saturation to dielectric permittivity.

Delineation of Agricultural Drainage Pipe Patterns Using Ground Penetrating Radar Integrated with a Real-Time Kinematic Global Navigation Satellite System

Better methods are needed for mapping agricultural drainage pipe systems. Prior research on small test plots indicates that ground penetrating radar (GPR) is oftentimes capable of detecting buried drainage pipes; however, the feasibility of employing this geophysical technique in larger field areas has not been adequately evaluated. Ground penetrating radar integrated with a Real-Time Kinematic (RTK) Global Navigation Satellite System (GNSS) may be an effective and efficient means of mapping drain lines within agricultural fields. Therefore, GPR-RTK/GNSS was tested in three agricultural settings; with Site 1 and Site 2 located in Beltsville, MD, USA and Site 3 near Columbus, OH, USA. Soils at the three sites ranged from silty clay loam to loamy sand. A GPR unit with 250 MHz antennas was used to detect drainage pipes, and at Sites 1 and 2, a physical GNSS base station was utilized, while a virtual base station was employed at Site 3. The GPR-RTK/GNSS configurations used in this study delineated a complex rectangular drainage pipe system at Site 1, with one set of drainage pipes oriented southwest-northeast and a second oriented southeast-northwest. At Site 2, a herringbone drain line pattern was outlined, and at Site 3, random drain lines were found. When integrated with RTK/GNSS, spiral or serpentine GPR transects (or spiral/serpentine segments of a GPR transects) were utilized to provide insight on drain line directional trends. Consequently, given suitable field conditions, GPR integrated with RTK/GNSS can be a valuable tool for farmers and drainage contractors needing to map subsurface drainage systems.

Mapping Freshwater Subaqueous Soil Resources: Examples from Southern New England

Core Ideas Soil series were developed for Humaquepts, Frasiwassists, and Psammentic and Aeric Frasiwassents. GPR technology provided accurate analysis of water depth, substrate type, and organic thickness. We identified five soil‐landscape units: lakebed, shoals, shoreline, coves, and deepwater. Subaqueous soils are those that occur under permanently flooded or ponded environments. Few studies have investigated freshwater subaqueous soil systems. In this study, we used traditional and modified soil survey techniques to investigate subaqueous soils in three created (impoundments) and three natural freshwater lakes in southern New England. The objectives were to test the effectiveness of bathymetric maps and ground‐penetrating radar (GPR) for establishing soil–landscape relationships in freshwater subaqueous systems, compare soil types between natural and impounded systems, and investigate the genesis of these soils. Bathymetric data were collected with a fathometer to create topographic maps of the lake bottoms to delineate subaqueous landscapes. Ground penetrating radar transects were run across landscape units to test the effectiveness of this technology to identify soil properties that could be used to separate mapping units. The primary landscape units identified included lakeshores, coves, lakebeds, and shoals. Lakebed and lakeshore units comprised 71 to 90% of the landscape regardless of whether the water bodies were natural or impounded. Ground penetrating radar was effective at identifying the presence of stones and boulders at the soil surface, stratified mineral layers, and the thickness of organic soil materials. Eighty‐one pedons were characterized and classified as Frasiwassists (34), Frasiwassents (32), and Humaquepts (18). All three great groups were identified in both natural and impounded systems with Frasiwassists the dominant soil type in all of the impounded sites. The primary soil processes included melanization resulting in umbric epipedons and organic matter accumulation resulting in histic epipedons in impounded sites with dense invasive vegetation. Frassiwassists were likely developed prior to submergence in both the natural and impounded systems.

Influence of the structural framework on peat bog distribution in the tropical highlands of Minas Gerais, Brazil

Peat bogs are ecosystems characterized by high levels of organic matter (OM), which accumulates in environments of low biological activity. The peat bogs present in the tropical highlands of Serra do Espinhaço Meridional (SdEM), Brazil, are associated with hydromorphic environments formed under specific conditions and the strong influence of the quartzite basement, whereby a complex pattern of faults, fractures and folds has deeply affected the drainage network and geomorphological framework. This study investigated the influence of the structural framework on the distribution and morphology of these ecosystems within a hydrological and geomorphological context. The physical features of the landscape were analyzed using geological maps, a digital elevation model and by mapping the main geomorphological feature patterns obtained from geographic information systems (GIS). Ground Penetrating Radar transects were taken in representative areas, as the best strategy for detection of buried peat bogs, as well as to analyze bedrock configuration and infer its relationship with water flow. Four peat bog typologies were identified: “entrenched bogs”, “subsurface bogs”, “structural bogs”, and “hanging bogs”. The distribution and morphology of these typologies display a strong relationship either with alluvial sediments and floodplains or with structural/lithological features which caused water retention.

Imaging and characterization of facies heterogeneity in an alluvial aquifer using GPR full-waveform inversion and cone penetration tests

Summary Spatially highly resolved mapping of aquifer heterogeneities is critical for the accurate prediction of groundwater flow and contaminant transport. Here, we demonstrate the value of using full-waveform inversion of crosshole ground penetrating radar (GPR) data for aquifer characterization. We analyze field data from the Krauthausen test site, where crosshole GPR data were acquired along a transect of 20 m length and 10 m depth. Densely spaced cone penetration tests (CPT), located close to the GPR transect, were used to validate and interpret the tomographic images obtained from GPR. A strong correlation was observed between CPT porosity logs and porosity estimates derived from GPR using the Complex Refractive Index Model (CRIM). A less pronounced correlation was observed between electrical conductivity data derived from GPR and CPT. Cluster analysis of the GPR data defined three different subsurface facies, which were found to correspond to sediments with different grain size and porosity. In conclusion, our study suggests that full-waveform inversion of crosshole GPR data followed by cluster analysis is an applicable approach to identify hydrogeological facies in alluvial aquifers and to map their architecture and connectivity. Such facies maps provide valuable information about the subsurface heterogeneity and can be used to construct geologically realistic subsurface models for numerical flow and transport prediction.

Characterization of the porosity distribution in the upper part of the karst Biscayne aquifer using common offset ground penetrating radar, Everglades National Park, Florida

• We use common offset GPR to rapidly characterize porosity in karst limestone. • We demonstrate the ability of GPR to characterize subsurface heterogeneities. • The CRIM petrophysical model is used to build field scale GPR-based porosity models. The karst Biscayne aquifer is characterized by a heterogeneous spatial arrangement of porosity and hydraulic conductivity, making conceptualization difficult. The Biscayne aquifer is the primary source of drinking water for millions of people in south Florida; thus, information concerning the distribution of karst features that concentrate the groundwater flow and affect contaminant transport is critical. The principal purpose of the study was to investigate the ability of two-dimensional ground penetrating radar (GPR) to rapidly characterize porosity variability in the karst Biscayne aquifer in south Florida. An 800-m-long GPR transect of a previously investigated area at the Long Pine Key Nature Trail in Everglades National Park, collected in fast acquisition common offset mode, shows hundreds of diffraction hyperbolae. The distribution of diffraction hyperbolae was used to estimate electromagnetic (EM) wave velocity at each diffraction location and to assess both horizontal and vertical changes in velocity within the transect. A petrophysical model (complex refractive index model or CRIM) was used to estimate total bulk porosity. A set of common midpoint surveys at selected locations distributed along the common-offset transect also were collected for comparison with the common offsets and were used to constrain one-dimensional (1-D) distributions of porosity with depth. Porosity values for the saturated Miami Limestone ranged between 25% and 41% for common offset GPR surveys, and between 23% and 39% for common midpoint GPR surveys. Laboratory measurements of porosity in five whole-core samples from the saturated part of the aquifer in the study area ranged between 7.1% and 41.8%. GPR estimates of porosity were found to be valid only under saturated conditions; other limitations are related to the vertical resolution of the GPR signal and the volume of the material considered by the measurement methodology. Overall, good correspondence between GPR estimates and the direct porosity values from the whole-core samples confirms the ability of GPR common offset surveys to provide rapid characterization of porosity variability in the Biscayne aquifer. The common offset survey method has several advantages: (1) improved time efficiency in comparison to other GPR acquisition modes such as common midpoints; and (2) enhanced lateral continuity of porosity estimates, particularly when compared to porosity measurements on 1-D samples such as rock cores. The results also support the presence of areas of low EM wave velocity or high porosity under saturated conditions, causing velocity pull-down areas and apparent sag features in the reflection record. This study shows that GPR can be a useful tool for improving understanding of the petrophysical properties of highly heterogeneous systems such as karst aquifers, and thus may assist with the development of more accurate groundwater flow models, such as those used for restoration efforts in the Everglades.

Inverse estimation of snow accumulation along a radar transect on Nordenskiöldbreen, Svalbard

We present an inverse modeling approach to reconstruct annual accumulation patterns from ground‐penetrating radar (GPR) data. A coupled surface energy balance‐snow model simulates surface melt and the evolution of subsurface density, temperature, and water content. The inverse problem consists of iteratively calibrating accumulation, serving as input for the model, by finding a match between modeled and observed radar travel times. The inverse method is applied to a 16 km GPR transect on Nordenskiöldbreen, Svalbard, yielding annual accumulation patterns for 2007–2012. Accumulation patterns with a mean of 0.75 meter water equivalent (mwe) a−1contain substantial spatial variability, with a mean annual standard deviation of 0.17 mwe a−1, and show only partial consistency from year to year. In contrast to traditional methods, accounting for melt water percolation, refreezing, and runoff facilitates accurate accumulation reconstruction in areas with substantial melt. Additionally, accounting for horizontal density variability along the transect is shown to reduce spatial variability in reconstructed accumulation, whereas incorporating irreducible water storage lowers accumulation estimates. Correlating accumulation to terrain characteristics in the dominant wind direction indicates a strong preference of snow deposition on leeward slopes, whereas weaker correlations are found with terrain curvature. Sensitivity experiments reveal a nonlinear response of the mass balance to accumulation changes. The related negative impact of small‐scale accumulation variability on the mean net mass balance is quantified, yielding a negligible impact in the accumulation zone and a negative impact of −0.09 mwe a−1in the ablation area.

Stratigraphic Architecture of a Regressive Strand Plain, Flinders Beach, North Stradbroke Island, Queensland, Australia

Gontz, A.M.; Moss, P.T., and Wagenknecht, E.K., 2014. Stratigraphic architecture of a regressive strand plain, Flinders Beach, North Stradbroke Island, Queensland, Australia. Regressive beaches are commonly composed of extensive and complex beach ridge systems. The physical and stratigraphic relationships of the beach ridges can help decipher the interactions between sea-level changes, variation in storm periodicity and intensity, changes in current directions, and alterations in sediment sources. Similar systems have been identified on the SE coast of Queensland and the spatial and temporal relationships between the various depositional environments are poorly understood. A small-strand plain system was identified on the NE coast of North Stradbroke Island, Queensland, Australia, and targeted for study with ground-penetrating radar (GPR). A series of GPR transects were acquired to elucidate the subsurface architecture of the system and to establish a geologic and geomorphic framework for the system through the application of stratigraphic analysis and decipher the spatial relationship. The GPR survey was concentrated in the center of the system and acquired approximately 3 km of 250-MHz data. The survey shows the stratigraphic relationship between modern sand dunes, beach ridge sequences, wetlands, and the much older Pleistocene sand dunes and identifies a potential Holocene storm record. We established a stratigraphic link between environments and propose a model driven by sea-level change, which proposes beach-ridge development during the early/mid-Holocene regression.

Sedimentary architecture of a spit-end (Salisbury Beach, Massachusetts): The imprints of sea-level rise and inlet dynamics

In the vicinity of tidal inlets, the facies architecture of barrier islands is largely controlled by inlet channel shifting and the transfer of sand between the ebb-tidal delta and the adjacent shore, including processes of inlet sediment bypas sing. Merrimack River Inlet is situated in a mixed-energy setting and is characterized by episodic shifting of the ebb channel prior to being stabilized in 1881. Here, we delineate the internal structure and evolution of the updrift barrier-spit terminus by integrating data obtained from ground-penetrating radar transects and sediment cores to document the effects of inlet-associated sedimentation processes. Our results document complex radar and sedimentological facies, which define two distinctive associations of facies, the inlet-distal and -proximal, within the core of the present barrier. These facies are associated with three phases of barrier-terminus evolution: (i) barrier retrogradation, characterized by the preservation of washover and backbarrier marsh facies deposited during sea-level rise, (ii) downdrift barrier progradation, characterized by channel fill and swash platform deposits overlain by foreshore deposits generated by long shore currents, and swash bars migrating to the foreshore from the swash platform, and (iii) foreshore seaward progradation, characterized by cross-beds. This study illustrates the highly complex nature of inlet associated spit-ends of barriers. Likewise, our results shed light on the mechanisms of spit elongation and recurving in a setting where the estuary opening is narrowing.

Field Testing of an Integrated Surface/Subsurface Modeling Technique for Planetary Exploration

While there has been much interest in developing ground-penetrating radar (GPR) technology for rover-based planetary exploration, relatively little work has been done on the data collection process. Starting from the manual method, we fully automate GPR data collection using only sensors typically found on a rover. Further, we produce two novel data products: (1) a three-dimensional, photorealistic surface model coupled with a ribbon of GPR data, and (2) a two-dimensional, topography-corrected GPR radargram with the surface topography plotted above. Each result is derived from only the onboard sensors of the rover, as would be required in a planetary exploration setting. These techniques were tested using data collected in a Mars analogue environment on Devon Island in the Canadian High Arctic. GPR transects were gathered over polygonal patterned ground similar to that seen on Mars by the Phoenix Lander. Using the techniques developed here, scientists may remotely explore the interaction of the surface topography and subsurface structure as if they were on site.

Using integrated near‐surface geophysical surveys to aid mapping and interpretation of geology in an alluvial landscape within a 3D soil‐geology framework

ABSTRACTAn integrated geological, geophysical and remote sensing survey was undertaken as part of the construction of a high‐resolution 3D model of the shallow subsurface geology of part of the Trent Valley in Nottinghamshire, UK. The 3D model was created using the GSI3D software package and the geophysical techniques used included ground‐penetrating radar (GPR), electrical resistivity tomography (ERT) and automated resistivity profiling. In addition, the remote sensing techniques of light detection and ranging (LIDAR) and airborne thematic mapping (ATM) were used. The objective of the study was to assess the contribution of these techniques to improve the geological mapping and interpretation of terrace deposits and other geological features. The study site had an area of ~2 km2 and consisted of a Triassic mudstone escarpment, overlain first by a sand and gravel river terrace that extended to the modern floodplain of the River Trent. Automated resistivity profiling mapping proved to be the central tool in identifying and positioning geological features at a greater resolution than would be obtained through traditional geological mapping and borehole observation. These features included i) a buried cliff delineating the south‐eastern limits of the incised Trent valley, ii) siltstone beds within the Gunthorpe Member of the Mercia Mudstone Group and iii) the variability of the sediments within the river terrace. A long ERT transect across the site successfully imaged the buried cliff and outcropping siltstone beds on the escarpment. Combined ERT and GPR transects revealed the depth of the sand and gravel deposits (Holme Pierrepont sands and gravels), whilst the GPR provided information about the depositional environment. Remote sensing using light detection and ranging proved essential in the original geological survey because it confirmed the absence of a second river terrace that had been previously thought to exist. This case study demonstrates the importance of combining geophysical techniques with traditional geological survey and borehole analysis, in order to create high‐resolution 3D geological models, which are increasingly being used as a platform to understand and solve environmental problems.

History, Complex Hunter-gatherers, and the Mounds and Monuments of Crystal River, Florida, USA: A Geophysical Perspective

ABSTRACT Crystal River (8CI1) is one of Florida's most famous archaeological sites. Yet, after over a century of investigations, its place in the history of Florida and the southeastern United States is not well understood. Crystal River is an important example, in terms of world archaeology, of a monumental landscape constructed by complex hunter-gatherer-fishers along the coast of the southeastern United States. Here, we present the results of our remote sensing program at the site. This research includes topographic mapping, a resistance survey, and ground-penetrating radar transects over various architectural components at the site. These data lend insight into the scale and rapidity of landscape modification at the site, as well as information on the location of previous archaeological excavations and modern disturbances. Further, the data illustrate the potential of shallow geophysical survey to the investigations of shell architecture.

Large-scale washover sedimentation in a freshwater lagoon from the southeast Australian coast: sea-level change, tsunami or exceptionally large storm?

A distinct lens of marine sand, up to 90 cm thick, confined vertically by peat, is found in the upper fill of a closed freshwater back-barrier lagoon on the southeast Australian coast. Coring of the deposit suggests it extends continuously up to 600 m inland and tapers landward rising to ~1.6 m above principle datum. In places the sand is overlain by accumulations of organic-rich silt that contain charophytes, indicating re-establishment of lagoon conditions. Hypotheses considered for the deposition of the sandsheet are higher Holocene sea level, storms and tsunami. Ground-penetrating radar transects of the seaward dune system suggest a penecontemporaneous erosional contact between a series of truncated pre-event dunes and several small overlying post-event dunes. Dating the sandsheet was problematic but it is confined to the last 800 years. The young age combined with a lack of associated beach deposits and evidence of wave scouring suggest that a higher sea-level hypothesis is unlikely. This sand lens is attributed to a large-scale washover event from the southeast. Based on comparisons with modern storm deposits from the same coast and sedimentological diagnostic criteria derived from studies of modern storm- and tsunami-deposited sandsheets, it is concluded that this sand deposit is the product of a short-lived, large-scale overwash event attributed to a late-Holocene tsunami.

Ground-penetrating radar profiling on embanked floodplains

AbstractManagement of the Dutch embanked floodplains is of crucial interest in the light of a likely increase of extreme floods. One of the issues is a gradual decrease of floodwater accommodation space as a result of overbank deposition of mud and sand during floods. To address this issue, sediment deposits of an undisturbed embanked floodplain near Winssen along the river Waal were studied using ground-penetrating radar (GPR). A number of radar facies units were recognized. Boreholes were used to relate radar facies units to sedimentary facies and to determine radar velocity. The GPR groundwave is affected by differences in moisture and texture of the top layer and probably interferes with the first subsurface reflector. The architectural elements recognized in the GPR transects confirm earlier reported insights on human-influenced river behaviour. This is testified in the development of sand bars during flood regimes that are probably more widespread than previously established.