bluuebunny/crossref_metadata_2025 huggingface dataset:
Magnetic and mineral fabric development in the Ordovician Martinsburg Formation in the Central Appalachian Fold and Thrust Belt, Pennsylvania
Authors: Hirt A. M. , Lowrie W. , Lüneburg C. , Lebit H. , Engelder T.
Published: April 2008

The Martinsburg Formation at Lehigh Gap, Pennsylvania, undergoes a transition from shales to slates, reflecting local progressive deformation on an outcrop scale. The anisotropy of magnetic susceptibility (AMS) was measured in low and high fields. The high-field measurements show that the magnetic susceptibility is controlled by the paramagnetic minerals. X-ray goniometry was used to define the mineral fabrics of chlorite and mica. The phyllosilicates are initially oriented preferentially in the bedding plane and are gradually reoriented into the cleavage plane through rotation, microfolding and recrystallization. The AMS fabric mirrors this change in mineral fabric. The magnetic fabric is originally oblate in the least deformed site, with the plane of flattening parallel to bedding, and becomes prolate with increasing deformation, reflecting the development of pencil structure in the shales. In the most deformed site, shortening results in a tectonic cleavage fabric, which controls the magnetic fabric. A similar pattern of fabric development can be observed on a regional scale at other sites across the central Appalachian fold and thrust belt. The AMS and mineral fabric from the Martinsburg Formation has undergone bedding compaction in the foreland near the Allegheny Front. The AMS and textural analysis both show that, as the deformation increases towards the hinterland, prolate fabrics develop and in the most deformed sites slaty cleavage controls both the mineral and magnetic fabrics.
An integrated AMS, structural, palaeo- and rock-magnetic study of Eocene marine marls from the Jaca-Pamplona basin (Pyrenees, N Spain); new insights into the timing of magnetic fabric acquisition in weakly deformed mudrocks
Authors: Larrasoaña Juan C. , Pueyo Emilio L. , Parés Josep M.
Published: April 2008

In this paper, we present a combined magnetic anisotropy (AMS), structural, palaeo- and rock-magnetic study of Eocene marine mudrocks from the western sector of the Jaca-Pamplona Basin (southwestern Pyrenees, north Spain). Comparison of structural, AMS and palaeomagnetic data reveals a subtle, but evident tectonic overprint affecting the phyllosilicate matrix of the mudrocks and the remanence carriers. The particular structural setting of the studied rocks has allowed us to combine palaeomagnetic and structural data with the AMS results in order to establish a relative chronology between sedimentation, blocking of magnetic fabrics, acquisition of magnetic remanence and deformation. Our data suggest that the blocking of the magnetic fabrics and the lock-in of the remanence occurred simultaneously during the early stages of gentle warping that affected the Jaca-Pamplona basin throughout the Mid-Late Eocene. According to the origin of the remanence carriers and the synsedimentary nature of the Mid-Late Eocene warping, the blocking of the magnetic fabrics and the lock-in of the remanence can be bracketed to a very short time span, of a few (<15?) ka, after sediment deposition. Our findings confirm previous interpretations claiming a very early origin of the magnetic fabric blocking in mudrocks undergoing weak deformation. They therefore validate the use of magnetic fabrics as palaeostress indicators and suggest that AMS data might also provide a useful tool for detecting preferred paths for the migration of geofluids during the early stages of deformation.
Magnetic properties and magnetic fabrics of Pleistocene loess/palaeosol deposits along west-central Siberian transect and their palaeoclimatic implications
Authors: Matasova Galina G. , Kazansky Alexey Yu.
Published: April 2008

Rock-magnetic properties, in particular anisotropy of magnetic susceptibility (AMS) were investigated in detail for eight sections transecting southwestern and central parts of Siberia. The results obtained indicate that magnetic properties and magnetic fabrics of loess/palaeosol deposits in western and southwestern Siberia depend on superposition of two different mechanisms namely a pedogenic mechanism proposed for Chinese loess and a wind-vigour mechanism for Alaskan loess. The wind-vigour mechanism is predominant in loess deposits and this allows palaeowind directions during glacial periods to be determined. In palaeosols, the balance between both models strongly depends on the geographical position of the section and thus reflects the palaeoclimate. In western Siberia, palaeosols corresponding to OIS 3 have sedimentary magnetic fabric, while the magnetic fabric of palaeosols corresponding to OIS 5 is completely reworked by pedogenesis. Such differences indicate a warmer climate during OIS 5. In central Siberia, separated from the west by the Kuznetsk Ala-Tau mountain ridge, the magnetic properties and AMS of loess/palaeosol sequences agree with ‘Alaskan’ type of loess, suggesting a colder and windier climate during the Late Pleistocene. Therefore, the Siberian subaerial realm may be subdivided into two provinces based on the palaeoclimate conditions prevailing during the Late Pleistocene. These climatic provinces remain in the modern climate.
The puzzle of axis-normal magnetic lineations in folded low-grade sediments (Bude Formation, SW England)
Authors: Anderson Mark W. , Morris Antony
Published: April 2008

A single upright, open anticline from sub-greenschist facies sedimentary rocks of the Bude Formation (Cornwall, UK) was sampled in order to investigate the kinematic relationships between fold development and anisotropy of magnetic susceptibility (AMS). The mean magnetic susceptibility of these samples is 0.25 × 10
−3
SI, suggesting low concentrations of ferromagnetic phases. AMS ellipsoids have a mean corrected anisotropy degree of 1.03 and a mean shape parameter of −0.54 (prolate).
K
min
and
K
int
define a girdle distribution striking sub-parallel to the fold axial plane, with
K
min
tending to cluster around the fold axis.
K
max
axes from both limbs of the fold define a cluster with a mean azimuth perpendicular to the fold axis. This arrangement of
K
max
and
K
min
could represent an inverse magnetic fabric of composite primary/tectonic origin. This is discounted, however, on the basis of broad correlation between the orientation of AMS and AIRM (anisotropy of isothermal remanence) ellipsoids. The prolate shapes and axis-normal orientation of
K
max
axes contrast markedly with the widely observed relationship of AMS ellipsoids in folds, which are typically oblate and have
K
max
parallel to the fold axis. This relationship is interpreted to represent progressive overprinting of primary depositional/compactional fabrics (
K
min
perpendicular to bedding) by a tectonic fabric (
K
min
perpendicular to cleavage). Consistency of
K
max
orientations irrespective of position within the fold clearly points to a fabric of tectonic origin. Prolate ellipsoids with long axes perpendicular to the fold hinge line are indicative of superimposed sub-horizontal stretching at a late stage or post-dating fold formation. Such a situation is not inconsistent with superimposed southward-directed thrusting simple shear that has been suggested in this area to account for variations in fold attitude on a regional scale. It is more likely, however, that the fabric reflects post-orogenic extension, with the fold occupying a position in the immediate hanging-wall of a major northward dipping normal fault. In either case, the AMS fabrics around the fold record only the last increments of deformation in this area, with earlier primary and fold-related fabrics being entirely obliterated.
How deformed are weakly deformed mudrocks? Insights from magnetic anisotropy
Authors: Parés Josep M.
Published: April 2008

For over thirty years the anisotropy of magnetic susceptibility (AMS) has been exploited in rock fabric studies. Our knowledge of the mechanisms leading to magnetic fabric development in deformed rocks has grown substantially, even though some details are still under debate. This paper reviews recent developments in AMS studies applied to the characterization of early deformation stages in mudrocks. From the current data set of AMS in rocks deformed at low temperature and low pressure it is possible to revisit the concept of weakly deformed rocks. We contend that weak deformation in mudrocks and the appearance of magnetic cryptofabric are concatenated. Furthermore, magnetic anisotropy studies in weakly deformed rocks suggest that cleavage fabric, the most common planar fabric in deformed mudrocks, builds up gradually from the earliest stages of deformation rather than suddenly at some strain threshold.
Anisotropy of magnetic susceptibility (AMS): magnetic petrofabrics of deformed rocks
Authors: Borradaile Graham J. , Jackson Mike
Published: April 2008

For 40 years magnetic anisotropy has provided successful geological interpretations of magnetic ellipsoid orientations; in contrast the interpretation of anisotropy magnitudes is far more convoluted. This is due to complexities at various levels within rocks, including different physical magnetic responses of different minerals, grain-scale magnetic anisotropy, the anisotropy of interacting ensembles, the mineralogical constitution of rocks and the processes and mechanisms that align minerals in nature. The chief factors determining the magnetic fabrics of tectonized rocks include: mineral-physics properties, crystal symmetry, mineral-abundances, tectonic symmetry and crystal orientation-distribution, strain
or
stress, kinematic history and certain tectono-metamorphic processes (e.g. diffusion, crystal plasticity, dynamic recrystallization, particulate flow, neomineralization). AMS ultimately provides an integrated record of some combination of these factors. Subfabrics due to distinct processes or events may be expressed in different mineral and/or grain-size fractions, and are superposed in the conventionally observed AMS. Their discrimination may be achieved by various laboratory techniques such as magnetization and torque measurements in weak and strong applied fields, anisotropy of ARM and IRM, gyroremanence, Rayleigh magnetization, chemical leaching. However, under limited circumstances, statistical approaches such as differential analysis, tensor standardization, symmetry of confidence regions for the principal axes may partly isolate different subfabric orientations.
Anisotropy of magnetic susceptibility in the Montes de Toledo area (Hercynian Iberian Belt, Spain) and its petrostructural significance
Authors: Gil-Imaz A. , Barbero L.
Published: April 2008

Anisotropy of magnetic susceptibility data are presented for samples from the anatectic complex of Toledo and the Mora batholith. The units located at the axial part of the Central Hercynian belt are separated by an important listric fault. Anatectic granites (Layos granite), epizonal granites (Mora granite) and, to a lesser extent, high-grade metamorphic rocks and metasediments are considered in this work in order to characterize their magnetic fabric and determine their structural significance in the framework of a large-scale extensional deformation of Late Variscan age. Samples from the Layos granites (footwall) show a magnetic lineation compatible with the stretching related to ductile shear deformation of the Toledo shear zone, which was responsible for the exhumation of the anatectic complex. During the exhumation, the folding of a previous metamorphic foliation in the anatectic complex took place, which is also recognized from the magnetic pattern of the minimum susceptibility axes of Layos granite. On the contrary, the magnetic fabric of the epizonal Mora granites (hanging wall), which intruded at the beginning of the extensional Late Variscan tectonics, mainly reflects magmatic processes.
Relationships between magnetic and structural fabrics revealed by Variscan basement rocks subjected to heterogeneous deformation—a case study from the Kłodzko Metamorphic Complex, Central Sudetes, Poland
Authors: Kądziałko-Hofmokl M. , Mazur S. , Werner T. , Kruczyk J.
Published: April 2008

The Kłodzko Metamorphic Complex comprises a number of thrust units, consisting of meta-igneous and metasedimentary rocks of the Variscan basement of the Sudetes, NE Bohemian Massif. The thrust sheet stack rests upon unmetamorphosed Nowa Ruda ophiolite and is unconformably overlain by Frasnian-Fammenian sediments. The studied rocks underwent six deformation events, scattered in time between the Middle Devonian and Late Carboniferous. The multiphase deformation produced a steep foliation that consistently trends WNW-ESE and a mineral lineation plunging to the ESE at a shallow to moderate angle. The results of the AMS study show that, despite the complex structural evolution, all the studied rocks bear a similar magnetic fabric mirroring the principal structural directions. This relationship between magnetic and structural fabrics is apparent in all tectonic units irrespective of the considerable variations in strain rate and metamorphic grade. This suggests a rather low dependence of magnetic anisotropy on the changing conditions and intensity of deformation.
Development of magnetic fabrics during hydrothermal alteration in the Soultz-sous-Forêts granite from the EPS-1 borehole, Upper Rhine Graben
Authors: Just Jana , Kontny Agnes , De Wall Helga , Hirt Ann M. , Martín-Hernández Fatíma
Published: April 2008

The Variscan, magnetite-bearing Soultz-sous-Forêts granite is found between 1420 and 2230 m of the EPS-1 borehole situated in the Upper Rhine Graben (France). Our study focuses on the changes of magnetic properties that occur during the progressive hydrothermal alteration and fracturing of the Soultz granite after emplacement. The magnetic susceptibility (κ) of the granite is between 10 and 80 × 10
−3
SI, and suggests that ferrimagnetic minerals are the primary carrier. During cooling and later tectonic and hydrothermal overprints, including the formation of the Rhine Graben, the granite was deformed under brittle conditions and partially altered by hydrothermal fluids. Along with this fluid activity, oxidation of magnetite to hematite occurred and reduced κ (< 1 × 10
−3
SI). AMS analysis on oriented samples documents the history of progressive transformation from primary magmatic fabric to tectonic fabric during hydrothermal alteration and faulting. The fresh granite with multidomain magnetite grains shows sub-horizontal magnetic foliations and randomly oriented magnetic lineations within the foliation plane. This fabric is similar to the magmatic fabric reflected by biotite. Transformation of the magnetic fabric started with localized magnetite oxidation along NW-SE oriented micro-cracks, which are probably associated with a late-magmatic alteration (stage I). Elongated and co-aligned magnetite relics within the newly formed hematite caused a well-defined NW-SE trending magnetic lineation and steeper magnetic foliation. Later alteration associated with intense brittle deformation (stage II) initially adopted this magnetic fabric, but intense cataclasis destroyed it. The geometry and orientation of magnetic fabric clearly indicate a hydrothermal alteration, which relates to the acting tectonic stresses in the post-emplacement history of the Soultz granite.
An introduction to prior information derived from probabilistic judgements: elicitation of knowledge, cognitive bias and herding
Authors: Baddeley Michelle C. , Curtis Andrew , Wood Rachel
Published: April 2008

Opinion of geological experts is often formed despite a paucity of data and is usually based on prior experience. In such situations humans employ heuristics (rules of thumb) to aid analysis and interpretation of data. As a result, future judgements are bootstrapped from, and hence biased by, both the heuristics employed and prior opinion.

This paper reviews the causes of bias and error inherent in prior information derived from the probabilistic judgements of people. Parallels are developed between the evolution of scientific opinion on one hand and the limits on rational behaviour on the other. We show that the combination of data paucity and commonly employed heuristics can lead to herding behaviour within groups of experts. Elicitation theory mitigates the effects of such behaviour, but a method to estimate reliable uncertainties on expert judgements remains elusive.

We have also identified several key directions in which future research is likely to lead to methods that reduce such emergent group behaviour, thereby increasing the probability that the stock of common knowledge will converge in a stable manner towards facts about the Earth as it really is. These include: (1) measuring the frequency with which different heuristics tend to be employed by experts within the geosciences; (2) developing geoscience-specific methods to reduce biases originating from the use of such heuristics; (3) creating methods to detect scientific herding behaviour; and (4) researching how best to reconcile opinions from multiple experts in order to obtain the best probabilistic description of an unknown, objective reality (in cases where one exists).
Sensitive dependence, divergence and unpredictable behaviour in a stratigraphic forward model of a carbonate system
Authors: Burgess Peter M. , Emery David J.
Published: April 2008

Although conceptual models of carbonate systems typically assume a dominance of external forcing and linear behaviour to generate metre-scale carbonate parasequences, there is no reason to preclude autocyclic and non-linear behaviour in such systems. Component parts of the carbonate system represented in this numerical forward model are entirely deterministic, but several parts are non-linear and exhibit complex interactions. Onshore sediment transport during relative sea-level rise generates autocyclic quasi-periodic shallowing upward parasequences but model behaviour is sufficiently complex that water depth evolution and parasequence thickness distributions are not predictable in any detail. The model shows sensitive dependence on initial conditions, resulting in divergence of two model cases, despite only a small difference in starting topography. Divergence in water-depth history at one point takes ∼ 10 ka, and for the whole model grid takes ∼ 100 ka. Fischer plots from the two cases show that divergence leads to entirely different parasequence thickness evolution in each case. Chaotic behaviour is a specific type of sensitive dependence, and calculation of trajectory divergence in a 3-D pseudo-phase space indicates that water depth evolution is not truly chaotic. If sensitive dependence, divergence and complex processes generating random products turn out to be common in real carbonate systems, predictions should be limited to elements of the system unaffected by these phenomena, or limited to cases where an element of periodic external forcing over-rides their affects. These results also suggest that increasingly complex and sophisticated stratigraphic forward models are not necessarily going to lead directly to more deterministic predictive power, although they may well be useful sources of statistical data on carbonate strata.
Input uncertainty and conditioning in siliciclastic process modelling
Authors: Tetzlaff Daniel M.
Published: April 2008

Deterministic forward sedimentary process models enhance our quantitative understanding of sedimentary systems. They are also being used increasingly to assist in the reconstruction of the geological past and the inference of the present configuration of sedimentary deposits. Such usage presents the challenge of having to establish the initial and boundary conditions that will cause the model’s output to match present-day observations. This constitutes an inversion problem that can sometimes be handled by traditional optimization methods. Clastic sedimentation models, however, often incorporate complex non-linear relationships that preclude the use of these techniques. The problem must then be handled statistically by relaxing the requirement of honouring exact observations and matching only the spatial variability of the observed deposits. Recent advances in control of non-linear dynamic systems may also shed light on possible solutions to the inversion problem in siliciclastic models. This paper reviews known approaches to problems related to input uncertainty and conditioning, and presents original preliminary results on control of sedimentation models.
Classical logic and the problem of uncertainty
Authors: Pshenichny Cyril A.
Published: April 2008

The uncertainty of knowledge, in contrast to that of data, can be assessed by its probability in the logical sense. The logical concept of probability has been developed since the 1930s but, to date, no complete and accepted framework has been found. This paper approaches this problem from the point of view of logical entailment and natural sequential calculus of Classical logic. It is shown herein that probability can be comprehended in terms of a set of formal theories built in similar language. This measure is compliant with general understanding of probability, can be both conditional and unconditional, accounts for learning new evidence and complements Bayes’ rule. The approach suggested is practically infeasible at present and requires further theoretical research in the domain of geoscience. Nevertheless, even within the framework of existing methods of expert judgement processing, there is a way of implementing logic that will improve the quality of judgements. Also, to reach the state of formalization necessary to use logical probability, techniques of knowledge engineering are required; this paper explains how logical probabilistic methods relate to such techniques, and shows that the perfect formalization of a domain of knowledge requires them. Hence, the lines for future research should be: (1) the development of a strategy of co-application of existing expert judgement-processing techniques, knowledge engineering and classical logic; and (2) further research into logic enabling the development of formal languages and theories in geoscience.
Scale dependence of hydraulic and structural parameters in fractured rock, from borehole data (KTB and HSDP)
Authors: Zimmermann G. , Burkhardt H. , Engelhard L.
Published: April 2008

Fundamental understanding of the origin, geometry, extension and scale dependence of fluid pathways in fractured rock is still incomplete. We analysed fracture networks on different scales, based on data from fluorescent thin sections and borehole televiewer (BHTV) images, to obtain geometrical network parameters and to estimate fracture permeability in the vicinity of a mantle plume (Hawaii Scientific Drilling Project, HSDP).

In the depth interval between 814 and 1088 m below sea-level, we observed microfractures in the fluorescent thin sections, and macroscopic fractures in the corresponding BHTV data from the same depth range. Initial modelling of the microscopic network from the fluorescent thin section taken at 1088 m below sea-level gives a clear indication that in this particular case the preferential hydraulic pathways on the microscopic scale are the microfractures in the olivine crystals. This is the only plausible explanation of high porosity (16.6%, based on core measurement), due to the observed vesicles and the corresponding low permeability of 10 µdarcy. Modelling hydraulic flow and calculation of permeability leads to similar values of permeability of 12.3 µdarcy, assuming a mean fracture aperture of 1 µm and an exponential distribution function of the fractures.

Detected structures from BHTV measurements were used to construct a macroscopic stochastic network to simulate the hydraulic flow. We found 337 fractures in the depth section from 783.5 to 1147.5 m below sea-level, which result in a linear frequency of 0.927 m
−1
. Assuming horizontal layers and constant fracture apertures of 100 µm for all structures, leads to a first estimate of permeability of 77 mdarcy (7.7 × 10
−14
m
2
) in this depth section.

In a recent work, we showed that for data from the Continental Deep Drilling Project (KTB), the fracture density versus fracture length follows a power law. First results from the Hawaiian data suggest a similar relationship, despite all of the differences in the lithological conditions between both sites.
Distribution and properties of fractures in and around the Nojima Fault in the Hirabayashi GSJ borehole
Authors: Ito H. , Kiguchi T.
Published: April 2008

A borehole penetrating the Nojima Fault was drilled in 1996, one year after the magnitude 7.2 1995 Kobe earthquake. The major fracture distribution detected by a borehole imaging tool (FMI) in the borehole was compared with the S-anisotropy from aftershocks. The fractures far from the Nojima Fault (more than 50 m) are aligned in an east—west direction, that is, parallel to the tectonic stress direction, whereas the fractures close to the Nojima Fault are normal to the fault. In the fault zone, a fault-parallel fault system was observed over short depth intervals, otherwise the fracture system in the fault zone is random. The fault-normal fracture suggests the complete reduction of the shear stress of the Nojima Fault.
Petrofabric-derived seismic properties of a mylonitic quartz simple shear zone: implications for seismic reflection profiling
Authors: Lloyd G. E. , Kendall J. M.
Published: April 2008

The link between petrofabric (LPO) and seismic properties of an amphibolite-facies quartzo-feldspathic shear zone is explored using SEM/EBSD. The shear-zone LPO develops by a combination of slip systems and dauphine twinning, with
a
-maximum parallel to lineation (
X
) and
c
-maximum normal to mylonitic foliation (
XY
). The LPO are used to predict elastic parameters, from which the three-dimensional seismic properties of different shear-zone regions are derived. Results suggest that LPO evolution is reflected in the seismic properties but the precise impact is not simple. In general, the P-wave velocity (
V
P
) minimum is parallel to the
a
-axis maximum; the direction of maximum shear-wave splitting (
AV
S
) is parallel to mylonitic foliation; and the
V
P
maximum and
AV
S
minimum are parallel to the
c
-axis maximum. The seismic anisotropy predicted is significant and increases from shear zone wallrock to mature mylonite. The P-wave anisotropy ranges from 11 to 13%, fast and slow shear waves’ anisotropies range from 6 to 15% and the magnitude of shear-wave splitting ranges from 9 to 16%. Nevertheless, such anisotropy requires a considerable thickness of rock with this LPO before it becomes seismically visible (i.e. 100s of m for local earthquakes; 10s of m for controlled source experiments). However, reflections and mode conversions provide much better resolution, particularly across tectonic boundaries. The low symmetry and strong anisotropy due to the LPO suggest that multi-azimuth wide-angle reflection data may be useful in the determination of the deformation characteristics of deep shear zones.
Fractured reservoir analysis using modern geophysical well techniques: application to basement reservoirs in Vietnam
Authors: Luthi S. M.
Published: April 2008

Recent geophysical well techniques have significantly improved the analysis of fractured reservoirs. These methods include electrical and ultrasonic scans and, in some cases, optical video images, that provide azimuthal high-resolution images of the borehole wall on which fractures are prominently visible. Additionally, fractures produce reflections and attenuations of the Stoneley wave, a borehole mode recorded by the array sonic wireline tool. A fracture identified with these methods can be individually probed with a new wireline formation tester featuring a dual-packer module that hydraulically isolates it from the surrounding formation. The combination of these techniques can provide information on fracture locations, dip, azimuth, aperture, permeability and fluid content. Seismic data can be used to extrapolate this information away from the wells. A case study on basement reservoirs from offshore Vietnam exhibits foliations, borehole breakouts, hydraulic and tectonic fracturing. Oil production comes from a small number of point entries that correspond to fractures, most of which produce more than 1000 barrels of oil per day. Two intersecting fracture sets were found, which may explain the high sustained production. Properly planned horizontal wells may increase production and decrease the chance of water breakthrough.
Fracture mapping with electrical core images
Authors: Lovell M. , Jackson P. , Flint R. , Harvey P. K.
Published: April 2008

Naturally fractured reservoirs often contain a range of different fracture types and networks; fractures that are relatively permeable and relatively impermeable, unconnected and connected to the part of the fracture network that carries fluid flow, and naturally occurring or drilling induced. Consequently, in terms of their fluid connectivity, fractures may be open or closed, while individual fractures may be isolated or well connected.

We have adapted our approach to imaging sedimentary fabric in the laboratory, where we related electrical core images to properties such as porosity, permeability, grain size and cementation, to enable electrical imaging of fractures in core. Our approach uses similar principles to those employed in down-hole electrical imaging. The results demonstrate an ability to image conductive fractures in fully saturated low-porosity water-bearing core: these fractures being electrically connected from the flat measurement surface through to the outer surface of the core.

Published results for numerical modelling of down-hole electrical imaging tools show the electrical response is related to fracture depth and fracture aperture. Our experimental results on fractured core in the laboratory support these numerical observations, increased current flowing into the fracture as the aperture increases. The finite size of the electrode, however, means that this technique cannot distinguish between a single fracture and smaller groups of fractures intersecting the electrode.
Reactive flow and permeability prediction — numerical simulation of complex hydrogeothermal problems
Authors: Bartels J. , Clauser C. , Kühn M. , Pape H. , Schneider W.
Published: April 2008

Simulating complex flow situations in hydrogeothermal reservoirs requires coupling of flow, heat transfer, transport of dissolved species, and heterogeneous geochemistry. We present results of simulations for typical applications using the numerical simulator SHEMAT/Processing SHEMAT. Heat transfer is non-linear, since all thermal fluid and rock properties depend on temperature. Due to the coupling of fluid density with both temperature and concentrations of dissolved species, the model is well suited to simulate density-driven flow.

Dissolution and precipitation of minerals are calculated with an improved version of the geochemical modelling code PHRQPITZ, which accurately calculates geochemical reactions in brines of low to high ionic strength and temperatures of 0–150°C. Changes in pore space structure and porosity are taken into account by updating permeability with respect to porosity changes due to precipitation and dissolution of minerals. This is based on a novel relationship between porosity and permeability, derived from a fractal model of the pore space structure and its changes due to chemical water — rock interaction.

A selection of model studies performed with SHEMAT completes the review. Examples highlight both density-driven and reactive flow with permeability feedback. With respect to the former, the thermohaline free convection Elder’s problem, and density-driven free convection in a coastal aquifer with geothermal exploitation, are considered. Mineral redistribution and associated permeability change during a core flooding experiment; reaction front fingering in reservoir sandstone; and long-term changes in reservoir properties during the operation of a geothermal installation, are all considered in relation to reactive flow with permeability feedback.
Permeability of rock samples from the Kola and KTB superdeep boreholes at high
P-T
parameters as related to the problem of underground disposal of radioactive waste
Authors: Zharikov A. V. , Malkovsky V. I. , Shmonov V. M. , Vitovtova V. M.
Published: April 2008

An experimental study of the samples collected from a depth of 3.8–11.4 km in the Kola and KTB superdeep boreholes, and from the Earth’s surface at the Kola drilling site was carried out at temperatures up to 600°C and pressures up to 150 MPa. The study was focused on the estimation of
in situ
permeability of the deep-seated rocks, their palaeopermeability during metamorphic transformations, and their protective properties for HLW disposal. Permeability dependencies on pressure and temperature were obtained. An increase in confining pressure leads to a decrease in rock sample permeability. The temperature trends obtained are of different types: permeability may decrease within the entire temperature range, or it may firstly decrease, reach its minimum and then decrease. It was found that this permeability behaviour is due to rock microstructure transformations caused by the competing effects of temperature and effective pressure. A possible
in situ
permeability trend for the superdeep section was proposed. A numerical simulation of convective transport was performed in order to determine a safe depth for the HLW well repository. The estimates obtained show that HLW well repositories can be used safely at relatively shallow depths.
On the neutron absorption properties of basic and ultrabasic rocks: the significance of minor and trace elements
Authors: Harvey P. K. , Brewer T. S.
Published: April 2008

The neutron absorption macroscopic cross-section, Σ, is measured routinely by neutron porosity tools and, although rarely presented as a logging curve in its own right, is used indirectly for the estimation of (neutron) porosity. One of the reasons that this primary measurement is not often employed directly in petrophysical analysis is the difficulty of interpretation. In particular, little is known about the range of Σ values for common lithologies, or exactly what information the measurement is providing.

In this contribution we demonstrate that excellent estimates of Σ can be calculated, provided that the chemistry of a sample is known in sufficient detail. When applied to a range of geochemical reference materials, it becomes apparent that the minor and trace elements present may have a profound effect on the Σ value of a sample, and, in turn, on the interpretation of neutron porosity measurements. Using this approach we present Σ data for basaltic and ultrabasic rocks, and model the change in Σ with alteration.

Alteration is considered in these models as an increase in alteration minerals (which are mainly clays, but also carbonates and zeolites in basic rock alteration) and changes in the trace-element chemistry of the rocks. Of the trace elements, boron and some of the rare-earth elements are of particular importance. Modelling the variation in Σ with these mineralogical and compositional changes indicates that increases in boron are the most important of these factors in increasing Σ; this is enhanced by the alteration, particularly to clay phases, which generally accompanies an increase in boron.

These models suggest that a Σ log should be able to act as a proxy for alteration trends in basic and ultrabasic crystalline rocks, and a quantitative model for such alteration is described.
Effect of compositional variations on log responses of igneous and metamorphic rocks. II: acid and intermediate rocks
Authors: Pechnig R. , Delius H. , Bartetzko A.
Published: April 2008

An extensive data-set of petrophysical down-hole measurements exists for boreholes drilled into continental crystalline crust. We selected boreholes covering a range of different types of plutonic rocks and gneisses in amphibolite or high-grade metamorphic rocks. According to Serra’s concept of electrofacies, a specific set of log responses should characterize one rock type. Here, we concentrate on the detection of compositional variations between rock types. Bulk composition of the protoliths influences the mineralogical composition of the metamorphic rock, and we demonstrate how this impacts on the down-hole measurements. Integration of logging data with geochemical core data and mineralogical descriptions allows the calibration of the log responses to rock types. The relationship of the log responses with core data shows a remarkably good correlation, and diagnostic trends are detected. From the logs, potassium and neutron porosity are particularly helpful in distinguishing different types of gneisses and igneous rocks with respect to their protoliths. The proportions of amphibole/pyroxene, mica + K-feldspar and feldspar + quartz in the rocks seem to control the direction of correlation in a cross-plot, i.e. positive or negative, depending on increasing or decreasing mineral proportions. This is true for all boreholes, and a generalized classification scheme could be developed for these crystalline rocks.
Laboratory investigations for the evaluation of
in situ
geophysical measurements in a salt mine
Authors: Kulenkampff J. , Just A. , Aschmann L. , Jacobs F.
Published: April 2008

Problem zones in a sylvinite mine were explored by underground measurements using a set of geophysical methods (geosonar, seismics, georadar, EM methods and geoelectrics), which yield parameter sets for disturbed and undisturbed rock. Underground geo-electrical methods are most suitable for examining humidity content and structures for the transport of brines, although they have to be constrained by wave-propagation methods. Additional laboratory experiments on rock salt samples were carried out in order to provide quantitative evaluation methods. A titration method was used to determine the amount of water. This was applied simultaneously with a four-electrode resistivity measuring system for samples with very high resistivity. These measurements were made on samples at different scales, yielding relationships between water content and resistivity, as well as information about the parameter distribution in larger core samples. Together, the field and laboratory results show that geo-electrics is a suitable method for the detection and evaluation of problem zones of the geological barrier of hazardous waste repositories, and a criterion for the risk assessment based upon resistivity measurements is defined.
Deformation of metavolcanics in the Karachay Lake area, Southern Urals: petrophysical and mineral-chemical aspects
Authors: Petrov V. A. , Poluektov V. V. , Zharikov A. V. , Velichkin V. I. , Nasimov R. M. , Diaur N. I. , Terentiev V. A. , Shmonov V. M. , Vitovtova V. M.
Published: April 2008

This paper is the result of laboratory tests aimed at comparative assessment of the filtration, elastic and mechanical properties of metavolcanics, as well as a preliminary investigation of the nature of characteristic variations in
P–T
parameters, simulating the conditions in the vicinity of an underground storage facility for heat-generating radioactive waste. The properties have been studied in cores collected from the deepest (1200 m) hole in the metavolcanics, focusing on intervals subjected to different strain intensities. Interpretation of the data obtained suggests that the identification of the most probable pathways for fluid flow and radionuclide transport depends on estimates of the retardation characteristics of zones of disjunctive dislocation. However, the widespread occurrence of linear zones of schistosity, mylonitization and brecciation may not be a decisive negative factor in the long-term performance assessment of the underground storage facility, because these minerals, which concentrate radionuclides such as epidote, chlorite, sericite, and Fe oxyhydroxides are widely distributed in these zones. Moreover, open microfractures and pores affected by heat will be filled with neoformed minerals (epidote, chlorite, carbonate) characterized by high sorption capacity with respect to radionuclides, due to release of intergranular water, as well as carbonatization reactions. Simultaneously, the system of open macro- and microfractures that resulted from stress affecting relatively undisturbed blocks may be one of the factors dramatically reducing country rock retention capability. Therefore, the results of petrophysical and mineral-chemical investigations must be taken into account in feasibility studies of designs for the construction of underground systems for final disposal of high-level wastes and/or long-term storage of spent nuclear fuel in the PA Mayak area.
Melt rates at calving termini: a study at Glaciar León, Chilean Patagonia
Authors: Haresign Eleanor , Warren Charles R.
Published: April 2008

Glaciar León, a temperate, grounded outlet of the North Patagonian Icefield, terminates at an active but stable calving margin in Lago Leones. Glaciological and limnological data gathered during 2001 and 2002 are used to examine the relative contributions of calving and melting to mass loss at the terminus, and the interplay between glacier and lake processes. The calving rate of 880 m/a in a mean water depth of 65 m is high for lake-calving glaciers. Subaerial melt rates at the terminus are small compared with calving rates, but melting at the waterline facilitates calving by undercutting the subaerial calving cliff. Ice-proximal surface water temperatures of 6–7°C allow waterline melt notches to grow at rates of
c.
0.8 m/day, suggesting that melt-driven calving accounts for
c.
23% of ice loss at the terminus. The significance of melting at calving termini decreases with increasing calving speeds, but is greater than simple calculations of melt losses suggest because of the process-linkage with calving.
Actual paraglacial progradation of the coastal zone in the Kongsfjorden area, western Spitsbergen (Svalbard)
Authors: Mercier Denis , Laffly Dominique
Published: April 2008

This research was carried out on the Brøgger Peninsula, northwest Spitsbergen, Svalbard (79°N 12°E). In the western part of Spitsbergen, cold-based valley glaciers have retreated more than 1 km from their Little Ice Age limits, and glacial meltwater has extensively reworked glacigenic sediments on exposed glacier forelands. In such areas, a paraglacial sediment transport regime has become predominant, with runoff as the dominant process. A combination of GIS, DEM, aerial photographic and field data was employed to estimate shoreline progradation at sandur outflows. Average shoreline progradation is estimated to amount to 3 m/annum over the last 30 years, a period of uninterrupted sediment provision from the glacial runoff system.
Holocene permafrost aggradation in Svalbard
Authors: Humlum Ole
Published: April 2008

The distribution and dynamics of permafrost represent a complex problem, confounded by a short research history and a limited number of deep vertical temperature profiles. This lack of knowledge is pronounced for the High Arctic, where most permafrost is found and where amplified responses to various climatic forcing mechanisms are expected. Within the High Arctic, the Svalbard region displays a unique climatic sensitivity and knowledge of Holocene, and modern permafrost dynamics in this region therefore have special interest. This paper reviews knowledge on Holocene permafrost development in Svalbard and the climatic background for this. In Svalbard, modern permafrost thickness ranges from less than 100 m near the coasts to more than 500 m in the highlands. Ground ice is present as rock glaciers, as ice-cored moraines, buried glacial ice, and in pingos and ice wedges in major valleys. Svalbard is characterized by ongoing local-scale twentieth-century permafrost aggradation, even though a distinct temperature increase around 1920 introduced relatively unfavourable climatic conditions for permafrost in Svalbard. Modern permafrost aggradation is to a large extent controlled by wind, solid precipitation and avalanche activity, and exemplifies the complexity of relating climate and permafrost dynamics.
Constraining kinematic rotation axes in high-strain zones: a potential microstructural method?
Authors: Reddy Steven M. , Buchan Craig
Published: April 2008

The correct determination of the kinematic rotation axis in high-strain zones is essential to the study of the tectonic evolution of the Earth’s crust. However, the common assumption that the kinematic rotation axis lies orthogonal to the XZ plane of the finite strain ellipse may be invalid in the case of general shear. Orientation data obtained by electron backscatter diffraction from calcite, deformed in the high-strain Gressoney Shear Zone of the Western Alps, has been investigated using orientation maps, bulk sample crystallographic orientation and misorientation analyses, and detailed intragrain misorientation and crystallographic dispersion analysis. The results demonstrate a strong geometrical coincidence amongst (1) the bulk macroscopic kinematic rotation axis, (2) the orientation of misorientation axes associated with low-angle boundaries, and (3) rotation axes associated with crystallographic dispersion at the intragrain scale. This coincidence is interpreted to reflect a geometric control of the kinematic framework of the high-strain zone on the activity of crystal slip systems. It is proposed that this relationship may be exploited as a new microstructural tool to determine the orientation of bulk kinematic rotation axes in high-strain zones without assuming a geometric link between kinematic rotation and XZ sections. Although further testing is required, application of the approach may lead to a significant advance in our understanding of natural general shear deformation.
Crack-seal patterns: records of uncorrelated stress release variations in crustal rocks
Authors: Renard François , Andréani Muriel , Boullier Anne-Marie , Labaume Pierre
Published: April 2008

Statistical properties of crack-seal veins are investigated with a view to assessing stress release fluctuations in crustal rocks. Crack-seal patterns correspond to sets of successive parallel fractures that are assumed to have propagated by a subcritical crack mechanism in the presence of a reactive fluid. They represent a time-sequence record of an aseismic and anelastic process of rock deformation. The statistical characteristics of several crack-seal patterns containing several hundreds of successive cracks have been studied. Samples were collected in three different areas, gold-bearing quartz veins from Abitibi in Canada, serpentine veins from the San Andreas system in California and calcite veins from the Apennine Mountains in Italy. Digitized pictures acquired from thin sections allow accurate measurement of crack-seal growth increments. All the samples show the same statistical behaviour regardless of their geological origin. The crack-seal statistical properties are described by an exponential distribution with a characteristic length scale and do not show any spatial correlation. They differ from other fracture patterns, such as earthquake data, which exhibit power-law correlations (Gutenberg-Richter relationship). Crack-seal series represent a natural fossil record of stress release variations (less than 50 bars) in the crust that show a characteristic length scale, associated with the resistance of rock to effective tension, and no correlation in time.