Subsurface Interface Research Articles

Evidence for Microbial Mediated NO3− Cycling Within Floodplain Sediments During Groundwater Fluctuations

The capillary fringe is a subsurface terrestrial-aquatic interface that can be a significant hotspot for biogeochemical cycling of terrestrially derived organic matter and nutrients. However, pathways of nitrogen (N) cycling within this environment are poorly understood, and observations of temporal fluctuations in nitrate (NO3 􀀀) concentrations lack the necessary resolution to partition between biotic or abiotic mechanisms. At discrete sampling points we measured NO3 􀀀, nitrite (NO2 􀀀), ammonium (NH4 +), gaseous nitrous oxide (N2O) and nitrogen (N2), and the corresponding isotopic composition of NO3 􀀀 within floodplain sediments at Rifle, Colorado. Coincident with an annually reoccurring spring/ summer excursion in groundwater elevation driven by snowmelt, we observed a rapid decline in NO3 􀀀 followed by transient peaks in NO2 􀀀, at three depths (2, 2.5 and 3 m) below the ground surface. Isotopic measurements (15N and 18O of NO3 􀀀) suggest an immediate onset of biological N loss at 2 m. At 2.5 and 3 m, NO3 􀀀 concentrations declined initially with no observable isotopic response, indicating dilution of NO3 􀀀 as the NO3 􀀀-deficient groundwater rose, followed by denitrification after prolonged saturation. A simple Rayleigh model further supports this depth-dependent variability in the significance of actively fractionating mechanisms (i.e., NO3 􀀀 reduction) relative to non-fractionating mechanisms (mixing and dilution). NO3 􀀀 reduction was calculated to be responsible for 64 % of the NO3 􀀀 decline at 2 m, 28 % at 2.5 and 47 % at 3 m, respectively. Finally, by accounting for previous molecular and geochemical analysis at this site, and comparing the trajectories between
15N:
18O, we conclude that biological NO3 􀀀 consumption at the two deeper and frequently saturated depths (2.5 and 3 m) can be attributed to heterotrophic denitrification. However, the
15N:
18O trajectory at the shallower, irregularly saturated site at 2 m shows a more complicated relationship best explained by the cyclic production of NO3 􀀀 via aerobic oxidation, and consumption via NO3 􀀀 reduction.

The 3D modelling techniques of digital geological mapping

Digital geological mapping has been widely applied in regional geological surveying in China since 2004. Using geological mapping data obtained by the digital geological mapping technique to build 3D geological modelling is the trend of the future. Based on previous research, a new method of constructing 3D geological models of shallow surfaces by using digital geological mapping routes and topographic data is proposed in this paper. The key steps of this modelling method are described systematically in the paper. These steps include data pre-processing, construction of an original database, construction of geological boundaries, and model combination. In particular, we introduce in detail the model construction methodologies of the model boundary surface, digital elevation model (DEM) surface, Quaternary interface, fault plane, overlying residual stratum interface, layered geological interface, rock mass interface, and others. During the geological interface construction, we should follow two principles: (1) use the boundary and occurrence data as the main modelling data, and use the high-precision subsurface interface data as constraint data; (2) construct younger geological bodies first, and then construct older geological bodies. The next step is to compose sectionalised geological interfaces and use them to generate a geological interface of wider range. The interface construction progresses from small to large and is constantly constrained and continuously improved. Based on this new modelling method, we successfully constructed two 3D geological models, the 1:25,000 Xiaosuangou and the 1:50,000 Tuoshang models, and achieved good results. This modelling method has the advantages of simple realisation, high speed, high accuracy, and good applicability. Using this method to build 3D geological models can facilitate further development and utilisation of existing digital geological mapping data. The 3D geological models can be used as a new way of expressing the regional geological survey results, as well as the constraint data of deep 3D geological modelling in the future.

Seismic wavefield modelling in two‐phase media including undulating topography with the modified Biot/squirt model by a curvilinear‐grid finite difference method

ABSTRACTIn this paper, we deduced the corresponding first‐order velocity–stress equation for curvilinear coordinates from the first‐order velocity–stress equation based on the modified Biot/squirt model for a two‐dimensional two‐phase medium. The equations are then numerically solved by an optimized high‐order non‐staggered finite difference scheme, that is, the dispersion relation preserving/optimization MacCormack scheme. To implement undulating free‐surface topography, we derive an analytical relationship between the derivatives of the particle velocity components and use the compact finite‐difference scheme plus a traction‐image method. In the undulating free surface and the undulating subsurface interface of two‐phase medium, the complex reflected wave and transmitted wave can be clearly recognized in the numerical simulation results. The simulation results show that the curvilinear‐grid finite‐difference method, which uses a body‐conforming grid to describe the undulating surface, can accurately reduce the numerical scattering effect of seismic wave propagation caused by the use of ladder‐shaped grid to fit the surfaces when undulating topography is present in a two‐phase isotropic medium.

Reflection and transmission responses for layered transversely isotropic media with vertical and horizontal symmetry axes

Reflection and transmission (R/T) responses characterize the energy distributions for incident and generated waves across the subsurface interface. The R/T coefficients are considerably influenced by the local anisotropy, and this implies the significance of the R/T responses analysis for anisotropic media. We have considered the plane interface bounded by two transversely isotropic media with, respectively, vertical and horizontal symmetry axes, and R/T coefficients normalized by the vertical energy flux are obtained in the phase domain. We define two simple anisotropic layered models characterized by fewer independent model parameters. Under the assumption of weak contrast model parameters across the interface, the R/T coefficient approximations are obtained as the perturbations from the simple models’ counterparts. The isotropic background medium is also used to obtain the approximations under an additional weak anisotropy assumption. Compared with approximations degenerated from more general cases, our approximations rely on fewer independent parameters. Numerical tests are implemented to evaluate our approximations.

Functional traits of hyporheic and benthic invertebrates reveal importance of wood‐driven geomorphological processes in rivers

Abstract Large wood (LW) is a natural element of river environments and an integral component of many river restoration schemes to promote biodiversity. It is an important habitat in itself, but it also induces a wide range of hydraulic, hydrological, geomorphological and chemical conditions that influence the ecological community. However, the effects of hydro‐geomorphological processes induced by LW on local benthic and hyporheic invertebrates have not been well characterized. A functional approach was applied to invertebrate data collected in a field survey at sites with LW and without LW (control) to investigate the response of hyporheic and benthic invertebrates' trait profiles in response to local LW‐induced processes. We hypothesized LW sites to be associated with different trait modalities than control sites in relation to wood‐induced processes and conditions (i.e., hyporheic exchange flow, oxygen availability, temporal stability, organic matter, denitrification, hydraulic conductivity). Multivariate analyses and partial least squares (PLS) path modelling were used to detect the differences in trait profiles between LW and control sites and to study the variation of traits as a function of hydrological, sedimentological, physical and chemical variables. Biological (i.e., aquatic stages, reproduction), physiological (i.e., dispersal, feeding habits) and behavioural (i.e., substrate preferences) trait utilization by the hyporheic meiofauna differed between LW and control sites. At LW sites, the hyporheic meiofaunal assemblage was significantly associated with aquatic active dispersal, aquatic eggs and hard substrate preferences. This trait category selection was linked to changes in physical–sedimentological processes at LW sites when compared to control sites. Macrofaunal benthic and hyporheic functional traits did not differ significantly between wood and control sites, suggesting similar functioning of these assemblages at the surface–subsurface interface. This study found that LW affects invertebrate traits by altering fluvial processes to produce, locally, a mosaic of habitats. Hyporheic meiofauna trait responses to LW processes have suggested (a) the crucial role of LW in supporting river benthic zone functioning and thus (b) a possible benefit to river restoration by enhancing functional interactions among different ecological niches. A free Plain Language Summary can be found within the Supporting Information of this article.

An accessible approach for the site response analysis of quasi-horizontal layered deposits

This study focusses on site response analysis for sites that are neither strictly one-dimensional (with flat parallel soil layers) nor clearly two-dimensional (steep valleys, canyons and basins). Both these types of geometries are well studied in the literature. There is a lack of studies, however, for all those geometries that are in between these two worlds, such as sites with gently dipping layers. Theoretically, such sites should be studied with a two-dimensional dynamic approach because of the formation of surface waves due to the non-horizontal layering. In certain situations, however, the one-dimensional dynamic assumption leads to minor errors and it may save a lot of effort in terms of defining a two-dimensional model, computing the response and interpreting the results. As a result of these practical advantages, an accessible approach is presented here to determine when one-dimensional analysis can be used for geometries consisting of quasi-horizontal layers. The methodology is based on the construction of a chart, delimiting the applicability of the one-dimensional approach, using simple but valid variables, such as the slope of the critical subsurface interface and the impedance contrast at this interface. Indeed, we propose our guidance on the limits of the one-dimensional analysis in the form of this power law separating the one-dimensional and two-dimensional dynamic regimes: Iz = 6.95 γ−0.69, where Iz is the impedance contrast and γ is the angle in degrees of the sloping critical subsurface interface. Site response analysis for geometries with values of Iz below this critical value can be computed using a standard one-dimensional approach without large error whereas geometries with values of Iz above this threshold require two-dimensional calculations.

Anisotropic Crosshole+VSP Traveltime Tomography Through Triangular Cell Model with a Normalized Jacobian Matrix and Multistage Inversion Strategy

To overcome the difficulty in forward and inversion problem in complex geological model, including undulated topography and irregular subsurface interface, we in this paper realize a multistage triangular shortest-path method for multi-phase seismic ray tracing and combine a nonlinear inversion solver (damped minimum norm and constrained least squares problem solved by a conjugated gradient method) to simultaneously invert four elastic parameters (or four Thomsen parameters) in anisotropic media using combined direct and reflected arrival times. For trading-off between different parameter updating, we normalize the sensitivity functions (traveltimes derivatives with respect to different elastic (or Thomsen) parameter changes) to balance four updated elastic (or Thomsen) parameters in simultaneous inversion process, due to the magnitude and pattern of partial derivatives more sensitive to different phase slowness angles. In addition, in order to solve the multi-solution problems of qSV-wave inversion, a strategy of multistage inversion is proposed. For testing the efficiency and correctness of proposed inversion method, we use the picked traveltimes in synthetic seismograms as the observed traveltimes in the inversion. The results show that both inverted images by taking the picked traveltimes in synthetic seismograms as the observed data and by taking predicted traveltimes using ray tracing method as observed data have a similar imaging pictures, which verify the efficiency and correctness of the proposed inversion method. Furthermore, the inverted results by normalized kernel functions are better than that of inversion without applying the normalized kernel functions for both qP and qSV data, and the results of the multistage inversion are better than that of the simultaneous inversion without multistage inversion applied in qSV-wave inversion. In addition, both elastic and Thomsen parameter inversions for the same anisotropic model indicate that there are no big differences between the two inverted results even though the sensitivity kernels related to the Thomsen parameters have relatively large values.

Study of Scholte wave dispersion curves and modal energy distribution using a wavefield numerical simulation method

Surface waves travelling along the subsurface interface between water and sediment are called Scholte (or generalised Rayleigh) waves. In this paper, we examine the characteristics of Scholte wave dispersion curves and their modal energy distribution through synthetic simulation tests. Analytical solutions of the dispersion equation for a homogeneous isotropic layered medium describe the propagation characteristics of the fundamental and higher modes, but do not provide information about the relative energy distributions of those wave modes. In real situations, the dispersion curves with different modes have different energies corresponding to different frequency bands. It is instructive to study and analyse the behaviours of Scholte wave dispersion curves by using joint dispersion equation solutions and numerical simulations results. Dispersion equation solutions are used to validate the numerical results, which in turn are used to assess energy distribution for both the fundamental and the higher modes. We observe that the thickness of the overlying water layer and the elastic properties of the ocean bottom play important roles in the dispersion characteristics of Scholte waves. The dispersion of the soft ocean-bottom model (whose S-wave velocity of the first solid layer is lower than the P-wave velocity of water) is dominated by the fundamental mode and appears to be less dependent on the thickness of the overlying water layer. In contrast, the dispersion of the hard ocean-bottom model contains significant energy in the higher modes. Higher modes will shift to lower frequencies and will be spatially closer (in the frequency direction) when the water layer thickness increases. The dispersion characteristics of the real data are in a good agreement with the conclusion of a soft ocean-bottom model. Our modelling approach of combining analytical solutions of the dispersion equation with numerical modelling can facilitate Scholte wave exploration in water-covered areas.

Seismic waveform simulation for models with fluctuating interfaces

The contrast of elastic properties across a subsurface interface imposes a dominant influence on the seismic wavefield, which includes transmitted and reflected waves from the interface. Therefore, for an accurate waveform simulation, it is necessary to have an accurate representation of the subsurface interfaces within the numerical model. Accordingly, body-fitted gridding is used to partition subsurface models so that the grids coincide well with both the irregular surface and fluctuating interfaces of the Earth. However, non-rectangular meshes inevitably exist across fluctuating interfaces. This non-orthogonality degrades the accuracy of the waveform simulation when using a conventional finite-difference method. Here, we find that a summation-by-parts (SBP) finite-difference method can be used for models with non-rectangular meshes across fluctuating interfaces, and can achieve desirable simulation accuracy. The acute angle of non-rectangular meshes can be relaxed to as low as 47°. The cell size rate of change between neighbouring grids can be relaxed to as much as 30%. Because the non-orthogonality of grids has a much smaller impact on the waveform simulation accuracy, the model discretisation can be relatively flexible for fitting fluctuating boundaries within any complex problem. Consequently, seismic waveform inversion can explicitly include fluctuating interfaces within a subsurface velocity model.

Seismoelectric interface electromagnetic wave characteristics for the finite offset Vertical Seismoelectric Profiling configuration: Theoretical modeling and experiment verification.

The seismoelectric interface electromagnetic characteristics have been studied for the finite offset Vertical Seismoelectric Profiling (VSEP) configuration. The approach consists of theoretical modeling and laboratory verification. The results show that the wave variation characteristics along the finite offset measurement line are markedly different from those along the zero-offset line. More interestingly, the wave characteristics for both configurations can be satisfactorily explained by the electric dipole model for the seismoelectric interface wave radiation. Besides, the experiment confirms the modeling result based on the seismoelectric coupling theory and validates the VSEP technique as an effective method for subsurface interface delineation.

Simultaneous travel time tomography for updating both velocity and reflector geometry in triangular/tetrahedral cell model

To conduct forward and simultaneous inversion in a complex geological model, including an irregular topography (or irregular reflector or velocity anomaly), we in this paper combined our previous multiphase arrival tracking method (referred as triangular shortest-path method, TSPM) in triangular (2D) or tetrahedral (3D) cell model and a linearized inversion solver (referred to as damped minimum norms and constrained least squares problem solved using the conjugate gradient method, DMNCLS-CG) to formulate a simultaneous travel time inversion method for updating both velocity and reflector geometry by using multiphase arrival times. In the triangular/tetrahedral cells, we deduced the partial derivative of velocity variation with respective to the depth change of reflector. The numerical simulation results show that the computational accuracy can be tuned to a high precision in forward modeling and the irregular velocity anomaly and reflector geometry can be accurately captured in the simultaneous inversion, because the triangular/tetrahedral cell can be easily used to stitch the irregular topography or subsurface interface.

New insights into the differences between the dual node approach and the common node approach for coupling surface–subsurface flow

Abstract. The common node approach and the dual node approach are two widely applied approaches to coupling surface–subsurface flow. In this study both approaches are analyzed for cell-centered as well as vertex-centered finite difference schemes. It is shown that the dual node approach should be conceptualized and implemented as a one-sided first-order finite difference to approximate the vertical subsurface hydraulic gradient at the land surface. This results in a consistent dual node approach in which the coupling length is related to grid topology. In this coupling approach the coupling length is not to be interpreted as a nonphysical model parameter. Although this particular coupling approach is technically not new, the differences between this consistent dual node approach and the common node approach have not been studied in detail. In fact, this coupling scheme is often believed to be similar to the common node approach. In this study it is illustrated that in comparison to the common node approach, the head continuity at the surface–subsurface interface is formulated more correctly in the consistent dual node approach. Numerical experiments indicate that the consistent dual node approach is less sensitive to the vertical discretization when simulating excess infiltration. It is also found that the consistent dual node approach can be advantageous in terms of numerical efficiency.

Full-Wave Electromagnetic Scattering From Rough Surfaces With Buried Inhomogeneities

We develop a methodology for modeling coherent electromagnetic scattering from rough surfaces with buried inhomogeneities in three dimensions. The inhomogeneities considered in this paper include random spherical media, random cylindrical media, and root-like cylindrical clusters. They are used to simulate rocks, ice particles, and vegetation roots buried beneath the ground surface that can be seen by the low-frequency radars in Earth remote sensing applications. The approach we develop first calculates volumetric scattering from the media using coherent approaches, including both the conventional recursive transition matrix (T-matrix) method as well as a new generalized iterative extended boundary condition method we developed for tilted finite cylinders, and then transforms the T-matrix to the scattering matrix, which is then used to form the full scattered field of layered structures with rough surface and subsurfaces. We validate the methodology by comparing with other numerical solutions for special cases, and show sensitivity results for scattering from rough surface with buried random spherical media and random cylindrical media of different densities. We also construct a basic root model and calculate the scattering cross sections from single and multiple root clusters with or without a subsurface interface underneath. With the approach developed in this paper, we are able to study the sensitivity of radar signals to subsurface scatterers. For example, our simulations show that, depending on their density and water content, buried roots could enhance the backscatter from a single rough surface by as much as 5 dB in co-pol components, and substantially more in cross-pol components. The results of this model are expected to enable more accurate geophysical retrievals of soil moisture as well as soil organic content.

HVPE homoepitaxial growth of high quality bulk GaN using acid wet etching method and its mechanism analysis

In this paper, crack-free 2-inch bulk GaN wafer with the thickness up to 3mm was obtained by HVPE homoepitaxy. A new method of acid wet etching was used to pre-treat GaN substrate before re-growth. The formation of the mesh-like subsurface crack and interface layer were found to be suppressed between the re-growth layer and as-grown GaN substrate. EDS and time varied contact angle measurement proved that chemical etching would decrease the oxygen related surface adsorption and increase atoms diffusion length during HVPE homoepitaxial growth. Moreover, Morphology, Low temperature photoluminescence measurements indicated a reduction in stress of wet etching treated as-grown GaN substrate due to etching effect on its N face. High quality bulk GaN with the dislocation density of 1×106cm−2 was achieved by using wet etching and HVPE multiple re-growth. It would offer a simple method to obtain bulk GaN with thicker layer and high quality.

An exploration of coupled surface–subsurface solute transport in a fully integrated catchment model

Summary Coupling surface and subsurface water flow in fully integrated hydrological codes is becoming common in hydrological research; however, the coupling of surface–subsurface solute transport has received much less attention. Previous studies on fully integrated solute transport focus on small scales, simple geometric domains, and have not utilised many different field data sources. The objective of this study is to demonstrate the inclusion of both flow and solute transport in a 3D, fully integrated catchment model, utilising high resolution observations of dissolved organic carbon (DOC) export from a wetland complex during a rainfall event. A sensitivity analysis is performed to span a range of transport conditions for the surface–subsurface boundary (e.g. advective exchange only, advection plus diffusion, advection plus full mechanical dispersion) and subsurface dispersivities. The catchment model captures some aspects of observed catchment behaviour (e.g. solute discharge at the catchment outlet, increasing discharge from wetlands with increased stream discharge, and counter-clockwise concentration–discharge relationships), although other known behaviours are not well represented in the model (e.g. slope of concentration–discharge plots). Including surface–subsurface solute transport aids in evaluating internal model processes, however there are challenges related to the influence of dispersion across the surface–subsurface interface, and non-uniqueness of the solute transport solution. This highlights that obtaining solute field data is especially important for constraining integrated models of solute transport.

Yutu rover reveals subsurface structure of the Moon

Chinese lunar mission Chang’E-3 is the latest in-situ exploration to the Moon after the Apollo and Luna missions finished 40 years ago. It is composed of a lander and a rover (Yutu: named after the jade rabbit on the Moon in a Chinese fairy tale). Yutu rover is equipped with several scientific instruments including a lunar penetrating radar (LPR), whose measurement has never been carried out on a rover. After integrating the LPR data with other in-situ and geological informations, Xiao et al. [1] discovered several subsurface layers in northern Mare Imbrium, which reveals a much more complex geological history than previous thought. The surface of the landing site is within the Eratosthenian materials [2], which is younger than those of the Apollo and Luna landing sites. In the landing area, impact craters with different sizes and degradations were observed. Among the fresh ejecta deposits, a ~4 m long, ~1.5 m high boulder (Loong Rock) is of particular interest. It looks homogeneous with the ~1.5 to 2 cm light color crystals in high resolution images taken by Panoramic Camera, and it is distinct in texture from the samples obtained by the Apollo missions. The most significant advance is the subsurface structure revealed by LPR (Figure 1). LPR has two channels (Channel 1: 60 MHz and Channel 2: 500 MHz), which were designed to detect subsurface layers with different depths and resolutions [3]: Channel 1 radar can disclose subsurface interface deep to ~400 m, while Channel 2 radar can “see” depth shallower than 12 m but with greater detail. LPR observed the interface of Eratosthenian and Imbrian, which is consistent with the thickness of Eratosthenian determined by impact crater [4]. In addition, several layers of paleoregolith were also detected, and they demonstrated the existence of intervals of different volcanic eruptions and subsequent space weathering processes. These thicknesses of the paleoregolith layers could help us understand the rate variations of space weathering through time. Besides the paleoregolith layers, LPR discovered several distinct layers that could be composed of pyroclastic materials. They revealed the changes of eruption style during emplacement of the Imbrian volcanic materials. Several questions still need to be addressed in future study: 1) What were the conditions forming the coarsegrained Loong Rock? 2) What were the rates of space weathering in the Imbrian and what were the reasons for the rate variations? 3) What were the key elements controlling the changes of volcanic eruption and/or overflow style?

Studies of lava flows in the Tharsis region of Mars using SHARAD

AbstractThe Tharsis region of Mars is covered in volcanic flows that can stretch for tens to hundreds of kilometers. Radar measurements of the dielectric properties of these flows can provide information regarding their composition and density. SHARAD (shallow radar), a sounding radar on the Mars Reconnaissance Orbiter, detects basal interfaces beneath flows in some areas of Tharsis northwest and west of Ascraeus Mons, with additional detections south of Pavonis Mons. Comparisons with 12.6 cm ground‐based radar images suggest that SHARAD detects basal interfaces primarily in dust or regolith‐mantled regions. We use SHARAD data to estimate the real relative permittivity of the flows by comparing the measured time delay of returns from the subsurface with altimetry measurements of the flow heights relative to the surrounding plains. In cases where the subsurface interface is visible at different depths, spanning tens of meters, it is also possible to measure the loss tangent (tan δ) of the material. The permittivity values calculated range from 7.6 to 11.6, with an average of 9.6, while the mean loss tangent values range from 7.8 × 10−3 to 2.9 × 10−2 with an average of 1.0 × 10−2. These permittivity and loss tangent estimates for the flows northwest of Ascraeus Mons, west of Ascraeus Mons, and south of Pavonis Mons are consistent with the lab‐measured values for dense, low‐titanium basalt.

S-Zero Stack: A converted wave processing to extract subsurface density information

Analyzing the Aki-Richards equation for converted waves, I found that it is possible to decouple the effect of density contrast from that of shear velocity contrast. The two terms were mixed when the P-wave incident angle was less than 30°, but they started to separate at a middle angle range (approximately 40°). The term related to S-wave velocity contrast reached zero at an incident angle around 60°. However, the other term, which was related to the density contrast, did not reverse polarity until 90°. Furthermore, this density term reached almost the maximum (magnitude) around 60°. Based on those characteristics, I designed a new method called “S-Zero Stack” to capture the density contrast reliably at the subsurface interface without going to inversion. S-Zero Stack captured subsurface density anomalies using a special stacking method. It is simple but robust, even when there is noise in the common-conversion-point gathers. Combined with the traditional P-wave amplitude-variation-with-offset technique, S-Zero Stack of PS-waves may help discriminate commercial gas from fizz in gas sand and could be a useful tool in shale gas exploration to locate lower-density anomalies (sweet spots).

An AVO inversion method in the frequency domain based on a layered model—a Bakken Shale case study

Conventional AVO techniques are based on the linear approximation of Zoeppritz equations for the inversion of elastic properties across a subsurface interface. However, for a layered model consisting of internal multiple layers beyond seismic resolution, it can be difficult to use the traditional AVO inversion methods due to the practicalities of picking amplitudes, resolution problems, and thin-layer effects. We propose an improved AVO inversion method applied to a layered model for inversion of layer thickness and properties by incorporating the frequency content of the wavelet. The proposed method uses the propagator matrix method as a theoretical description of the frequency-dependent reflection coefficients for the layered model in an inversion scheme. We test the proposed method on a single layered model with synthetic data to produce an inversion for layer thickness and porosity, and then investigate the feasibility of the method for the characterization of the fracture zone in the Bakken formation. For the inversion of the reference crack density and layer thickness of the fracture zone in the Bakken formation, the minima of the objective functions generate inversion results that indicate a reasonable fit with the true model parameters. The inversion error may result from the intrinsic complexity of reflections from a layered model, in which several different combinations of layer thicknesses and associated properties may produce similar frequency-dependent coefficients. By contrast, in the multicomponent model data example, the inversion of converted PS-wave data seems to be less stable compared to PP-wave data. The potential of the proposed AVO inversion method may include applications to complex models, such as a sandstone/shale interbedded system, or a formation that presents internal heterogeneity.

Modeling of groundwater flow at depth in crystalline rock beneath a moving ice-sheet margin, exemplified by the Fennoscandian Shield, Sweden

On-going geological disposal programs for spent nuclear fuel have generated strong demands for investigation and characterization of deep-lying groundwater systems. Because of the long time scales for which radiological safety needs to be demonstrated in safety assessment applications, an analysis of the hydrogeological performance of the geosphere system during glacial climate conditions is needed. Groundwater flow at depth in crystalline rock during the passage of an ice-sheet margin is discussed based on performed groundwater-flow modeling of two bedrock sites, Forsmark and Laxemar, in the Fennoscandian Shield, Sweden. The modeled ice sheet mimics the Weichselian ice sheet during its last major advance and retreat over northern Europe. The paper elaborates and analyzes different choices of top boundary conditions at the ice sheet–subsurface interface (e.g. ice-sheet thickness and ice-margin velocity) and in the proglacial area (presence or lack of permafrost) and relates these choices to available groundwater-flow-model hydraulic output and prevailing conceptual hydrogeochemical models of the salinity evolution at the two sites. It is concluded that the choice of boundary conditions has a strong impact on results and that the studied sites behave differently for identical boundary conditions due to differences in their structural-hydraulic properties.