Common Midpoint Surveys Research Articles

GPR velocity correction method in transversely heterogeneous media based on CMP data

AbstractDetermining ground‐penetrating radar (GPR) velocity has always been a critical problem. The GPR velocity estimation method based on common midpoint (CMP) data has been widely used because of its simplicity. However, we found that in sediment investigation and soil assessment, transversal heterogeneity is universal, which violates the basic assumption of velocity estimation through CMP data. Due to the rapid change of underground media and the limitation of the scope of some surveying areas, the CMP survey line will inevitably be selected in the area where the velocity changes laterally, making it difficult to obtain accurate velocity. To address this problem, we propose a velocity correction method. First, we determined the characteristics of CMP data and the corresponding velocity spectra acquired in transversely heterogeneous media through numerical simulations. Subsequently, we found that the simulated CMP data could determine the location of changes in the underground medium, and that the velocity obtained from the semblance analysis could be corrected according to the location where the medium changes laterally. We then used models wherein the thickness, relative permittivity and proportion of abnormal parts varied independently or simultaneously to verify the proposed velocity correction method. The results show that our method can control the GPR velocity error within 3.44%, and the precision is about 0.002 m/ns. Finally, we conducted a sediment investigation experiment on a channel bar in the lower reaches of the Yarlung Zangbo River. We determined the interface at which the sediment changed transversely and obtained the corresponding electromagnetic velocity using the proposed method. This study provides a reliable method for determining the GPR velocity in transversal heterogeneous media, which is of great significance for various practical applications.

Estimation of Ice Thickness of the Satopanth Glacier, Central Himalaya Using Ground Penetrating Radar

Total volume of stored ice in the Himalayan glaciers is an important quantity for water resource management of the Himalayan catchments. However, direct measurement of glacier-ice thickness is rare in the Indian Himalaya. We have estimated the ice thickness of the debris-covered Satopanth Glacier (SPG) using a ground penetrating radar (GPR). Multiple bistatic, unshielded antennae with frequencies of 16, 20, 40 and 80 MHz were used for this purpose. We have done GPR surveys at various locations over the ablation zone of SPG. However, satisfactory results were obtained only on two transects. Near the glacier snout, a transverse GPR profile shows an ice thickness of 38 ± 3.5–50 ± 3.5 m. We have obtained 98 ± 7–112 ± 7 m ice thickness at a longitudinal transect in the upper ablation zone. To measure the speed of the radar waves in ice, a common midpoint survey was carried out. Our results for the speed of the electromagnetic waves are slightly lower than the standard values of such waves through pure ice.

Active Layer Stratigraphy and Organic Layer Thickness at a Thermokarst Site in Arctic Alaska Identified Using Ground Penetrating Radar

In permafrost terrains, the frozen-unfrozen boundary, located at the base of the active layer, is a prominent ground-penetrating radar (GPR) target and is typically used to retrieve active layer thickness. Less attention has been given to the capability of the GPR in detecting structures within the active layer. In this paper, using 500 MHz GPR data from a thermokarst site in the Arctic Coastal Plain, we demonstrate that GPR can retrieve, when present, the internal stratigraphy of the thawed layer. We recognized two types of thermokarst-related microtopographic units: dry-and-uniform peaty hummocks with a thin (∼30 cm) active layer and inter-hummock depressions with a thicker (∼60 cm) active layer characterized by two different layers—a surface peat layer on top of silt confirmed by test pits. Radar wave velocity analysis, done with a common-midpoint survey, suggests a contrast in volumetric water content (87% and 45% for the upper and lower layers, respectively). The subsurface radar wave velocity suggests that the porous peat layer contains more water (87% by volume) than the underlying silt layer (45% by volume), resulting in a strong dielectric contrast and a strong radar reflection. This study demonstrates the usefulness of GPR to measure the thickness and properties of the surface organic layer in permafrost regions.

Automated high-resolution GPR data collection for monitoring dynamic hydrologic processes in two and three dimensions

An automated GPR data-collection system was constructed to monitor dynamic hydrologic processes with high spatiotemporal resolution. The design of the system allows for fast acquisition of constant-offset profiles (COP) and common-midpoint surveys (CMP) to monitor unsaturated flow at multiple locations. A fast and accurate motion-control system enables the high-resolution monitoring of spatial patterns of reflectors through time while preserving important information about the normal moveout of reflectors for velocity analysis. In two experiments, infiltration was monitored using time-lapse ground-penetrating radar (GPR) measurements in two and three dimensions. The data from both experiments provide substantial qualitative insight about the dynamics of hydrologic events and a path forward for quantitative analysis of surface-based GPR monitoring data. Analysis shows (1) the advantages of collecting high-resolution time-lapse data, (2) the complexities of patterns associated with the wetting of the soil, and (3) evidence of nonuniform propagation of a wetting front through the soil column.

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.

Evaluating approaches for estimating peat depth

Estimates of peat depth are required to inform understanding of peatland development, functioning, and ecosystem services such as carbon storage. However, there is a considerable lack of peat depth data at local, national, and global scales. Recent studies have attempted to address this knowledge deficit by using manual probing and ground-penetrating radar (GPR) to estimate depth. Despite increasing application, little consideration has been given to the accuracy of either of these techniques. This study examines the accuracy of probing and GPR for measuring peat depth. Corresponding GPR and probing surveys were carried out at a catchment scale in a blanket peatland. GPR depth estimations, calibrated using common midpoint (CMP) surveys, were found to be on average 35% greater than probe measurements. The source of disagreement was found to be predominantly caused by depth probes becoming obstructed by artifacts within the peat body, although occasionally probing rods also penetrated sediments underlying the peat. Using the Complex Refractive Index Model, it was found that applying a single velocity of 0.036 m ns−1 across a single site may also result in −8 to +17% error in estimation of peat depth due to spatial variability in water content and porosity. It is suggested that GPR calibrated at each site using CMP surveys may provide a more accurate method for measuring peat depth.

Modeling and simulation of GPR wave propagation through wet snowpacks: Testing the sensitivity of a method for snow water equivalent estimation

Abstract Snow water equivalent (SWE) of a snowpack is an important input to distributed snow hydrological models used for runoff predictions in areas with annual snowpacks. Since the conventional method of manually measuring SWE is very time-consuming, more automated methods are being adopted, such as using ground penetrating radar operated from a snowmobile with SWE estimated from radar wave two-way travel time. However, this method suffers from significant errors when liquid water is present in the snow. In our previous work, a new method for estimating SWE of wet snowpacks from radar wave travel times and amplitudes was proposed, with both these parameters obtained from a common mid-point survey. Here we present a custom ray-based model of radar wave propagation through wet snowpacks and results of MATLAB simulations conducted to investigate the method's sensitivity to measurement errors and snowpack properties. In particular, for a single-layer snowpack up to 2.1 m deep and with liquid water content up to 4.5% (by volume), the simulations indicate that SWE can be estimated with an error of ± 5% or less if (a) the noise (measurement errors) in resulting amplitude has a standard deviation less than 15% and(b) the noise in two-way travel time has a standard deviation less than 0.075 ns (22.5% and 0.15 ns for a snowpack less than 1.3 m deep).

Variations in free‐phase gases in peat landforms determined by ground‐penetrating radar

The spatial variability of biogenic gas produced by methanogenic archaea is difficult to assess within saturated peat soils and is often poorly quantified. This study uses ground‐penetrating radar to noninvasively estimate the vertical distribution of biogenic free‐phase gas (FPG) in two distinct peat landform types in the Glacial Lake Agassiz Peatland, Minnesota: a near‐crest bog and a midslope lawn (i.e., both as defined by surface vegetation communities). Ground‐penetrating radar velocities retrieved from common midpoint surveys were modeled using the Complex Refractive Index Model to estimate free‐phase gas volumes along one‐dimensional vertical profiles. Near‐crest bog landforms are characterized by vertical variability in gas content with accumulations along the peat column up to 24% by volume of FPG localized within the deep peat (e.g., 2–4 m depth). Midslope lawn sites show lower total volumes (up to 12% free‐phase gas) and less variability that result in a more even free‐phase gas distribution throughout the vertical profile. High‐resolution data also suggests the presence of thinner layers (<1 m) containing up to 40% free‐phase gas at a near‐crest site. Our results suggest that spatial distribution of free‐phase gas along the peat column may vary in the Glacial Lake Agassiz Peatlands depending on peat landform. Statistical analysis supports vertical variability of ground‐penetrating radar velocity data with a 95% confidence limit. Our results could potentially be upscaled using surface vegetation communities to estimate landscape‐scale gas storage potential in peat landforms.

Water-Content of Glacier-Ice: Limitations on Estimates from Velocity Analysis of Surface Ground-Penetrating Radar Surveys

The rheology of ice is strongly controlled by its liquid water content. Since water content and its distribution also exert a strong control on radar propagation velocity and attenuation, this provides a potential remote technique for assessing ice-water content. A suite of surface ground-penetrating radar (GPR) surveys have been undertaken on two glaciers, Tsanfleuron Glacier in the European Alps and Bakaninbreen, Svalbard in order to determine their water content variation with depth. Common-offset surface radar profiling shows both glaciers have a two-layered structure, with a shallow layer characterised by low returned radar power and a deeper layer characterised by strong scattering. The thickness of these layers varies rapidly across the glaciers. In order to provide a robust interpretation of the properties of the layers within these glaciers we present a quantitative semblance analysis of two common midpoint surveys, making estimates of layer thickness and water content. This analysis includes a Monte Carlo estimate of the likely resolution of these estimates, as well as of the effects of the non-minimum-phase nature of the GPR wavelet. In Tsanfleuron Glacier, the shallow layer consists of relatively dry ice with an estimated water content of 1.18 [Formula: see text], whereas the deeper layer is interpreted as containing small water bodies, 3.90 [Formula: see text] by volume. At Bakaninbreen, the shallow layer contains no water and the deeper layer 1.29 [Formula: see text] water. At both glaciers the deeper layer will undoubtedly be rheologically softer, with implications for ice dynamics; furthermore, the layer will provide a store for a substantial water volume with important implications for the glacier’s water system. The uncertainty in the calculated water content is, however, significant in terms of ice dynamics, meaning that alternative methods of assessing water content, possibly using borehole radar, are required to provide the input for predictive models of glacier flow.

Radar determination of the spatial structure of hydraulic conductivity.

Spatial variability of hydraulic conductivity exerts a predominant control on the flow of fluid through porous media. Heterogeneities influence advective pathways, hydrodynamic dispersion, and density-dependent dispersion; they are, therefore, a key concern for studies of ground water resource development, contaminant transport, and reservoir engineering. Ground-penetrating radar contributes to the remote, geophysical characterization of the macroscale variability of natural porous media. On a controlled excavation of a glacial-fluvial sand and gravel deposit in the Fanshawe Delta area (Ontario, Canada), the hydraulic conductivity field of a 45 x 3 m vertical exposure was characterized using constant-head permeameter measurements performed on undisturbed horizontal sediment cores. Ground-penetrating radar data were collected along the excavation face in the form of both reflection and common midpoint surveys. Comparison of geostatistical analyses of the permeameter measurements and the radar data suggests thatthe horizontal correlation structure of radar stack velocity can be used to directly infer the horizontal correlation structure of hydraulic conductivity. The averaging nature of the common midpoint survey is manifest in the vertical correlation structure of stack velocity, making it less useful. Radar reflection data do not exhibit a spatial structure similar to that of hydraulic conductivity possibly because reflections are a result of material property contrasts rather than the material properties themselves.

Electromagnetic wave speed in polar ice: validation of the common-midpoint technique with high-resolution dielectric-profiling and γ-density measurements

AbstractThe accuracy of the travel-time–velocity and travel-time–depth profile derived from ground-penetrating radar (GPR) common-midpoint (CMP) surveys at different frequencies is investigated for the first time ever by direct comparison with the profile calculated from high-resolution dielectric-profiling (DEP) ice-core data. In addition, we compare two travel-time profiles calculated from ice-core density data by means of different dielectrical mixture models with the DEP-based profile. CMP surveys were carried out at frequencies of 25,50,100 and 200 MHz near the new European deep-drilling site DML05 in Dronning Maud Land, Antarctica, during the 1998/99 field season. An improved scanning capacitor for high-resolution DEP and a γ-densiometer for density measurements were used to determine the complex dielectric constant and the density at 5 mm increments along the ice core B32, retrieved in 1997/98 at DML05. The comparisons with DEP- and density-based velocity series show that the CMP velocity series are slightly higher but asymptotically approach the core-based velocities with depth. Root-mean-square differences of the DEP velocity series range between 8% for the 25 MHz CMP and 2% in the case of the 200 MHz survey. Density-based velocities differ from the DEP velocities by 51 %. The travel-time–depth series calculated from the interval velocities show a better agreement between all series than the velocity series. Differences are 5.7–1.4% for the 25 and 200 MHz CMP measurements, and <0.6% for the density data. Based on these comparisons, we evaluate the accuracy with which the depth of electromagnetic reflectors observed in common-offset profiles can be determined, and discuss reasons for the observed differences between CMP- and core-based profiles. Moreover, we compare the errors determined from the field measurements with those estimated from GPR system characteristics to provide a measure that can be used to estimate the accuracy of GPR analyses for the planning of GPR campaigns. Our results show that CMP surveys are a useful technique to determine the depth of radar reflectors in combination with common-offset measurements, especially on a region-wide basis.

Mapping porosity variation in a welded pyroclastic deposit with signal and velocity patterns from ground-penetrating radar surveys

Ground-penetrating radar (GPR) is used to both image and quantify porosity variations in a variably welded pyroclastic flow deposit. Characteristic radar signals for nonwelded (constant, high porosity) and welded (porosity lower and variable) zones are identified by comparison of radar signals to exposed stratigraphy. A moderate rate of change in porosity with depth generates abundant, unresolvable reflections. A relatively constant porosity results in a flat, zero-amplitude response. Lastly, a discrete jump or extremely high rate of change in porosity (abrupt at scale of radar wavelength) can produce a strong, distinct reflection. Common-midpoint (CMP) survey data are analyzed to determine relative radar velocity patterns in the pyroclastic flow. Changes in radar velocity are linked to changes in relative porosities that are attributed to differential welding. Our analysis shows that welding causes substantial reductions in radar velocity. Moisture in subsurface stratigraphy also strongly affects velocity. Therefore, we advocate the interpretation of radar data in terms of relative changes in porosity. Our results also suggest that, in areas of rapid facies changes, multiple CMP surveys are required for accurate conversions of travel time to depth.

Radar Structure of Earthquake-Induced, Coastal Landslides in Anchorage, Alaska

Ground-penetrating radar (GPR) was used to investigate the internal structure of two large landslides in Anchorage, Alaska that resulted from the great 1964 earthquake. The Government Hill and Turnagain Heights landslides occurred in similar stratigraphic and geographic settings, yet the style of ground deformation is different at each site. GPR data are compared with previous investigations and are shown, under certain conditions, to have utility in the identification of ancient landslides. Reflection surveys accurately reproduced the subsurface geometry of horst and graben structures and imaged finer scale features such as ground cracks and fissures. Where more complete disintegration of the bluff occurred, GPR reflections from within the slide mass are generally chaotic and include no recognizable evidence of the original stratigraphy. Common midpoint surveys estimated GPR velocity in the sediment and allowed the conversion of travel times to depths.

Super-deep reflection profiling: exploring the continental mantle lid

Reflections from the continental mantle lithosphere have now been reported from common mid-point (CMP) profiles at numerous sites around the world. These reflections include: (1) distinct dipping events that appear to originate in the lower crust and continue some distance into the upper mantle; (2) kilometer scale, discontinuous, diffusely distributed, or isolated reflections at varying depths; (3) continuous, sub-horizontal reflections tens of kilometers in extent, often interpreted as the ‘base of the lithosphere’ in spite of a wide variation in depth and; (4) very deep reflections in the mantle lithosphere (> 125 km). The first category of mantle reflections is most commonly observed in proximity to crustal sutures, lending credence to their interpretation as fossil subduction zones. Isolated, discontinuous mantle reflections are by far the most common and, though observed in both ancient and modern collisional environments, are difficult to interpret due to both their limited extent and a corresponding lack of three-dimensional control. Sub-horizontal mantle reflections appear in disparate tectonic environments in at least nine locations worldwide. Of special note are the various reports of specific reflections at ∼80–100 km (22–24 s twt) in the North Sea, eastern Europe, and the Urals which suggest a more ubiquitous mantle horizon. The deepest events imaged thus far in a controlled source CMP survey are the ‘super-deep’ mantle reflections of the URSEIS project that have been identified at ∼135–165 km (35–43 s) and ∼225 km (56 s). Of the many reported mantle reflections, these are the most likely to be associated with the base of the lithosphere although such an interpretation averts the question of why an essentially thermo-mechanical transition should be reflective at high frequencies (10–20 Hz).