bluuebunny/crossref_metadata_2025_split huggingface dataset:
Building and unfaulting fault-horizon networks
Authors: Rutten K. W. , Verschuren M. A. J. 
Published: April 2008

Abstract
A fault-horizon network is a set of horizon and fault surfaces that is geologically consistent and topologically complete. It is the missing link between seismic interpretation and reservoir modelling. Building the fault-horizon network from a seismic interpretation can be a fast and automatic process. The fault slip vector field embedded in the network is used to construct tip points and fault branches, and as a powerful tool to validate network quality.
A novel, very fast method is introduced to unfault the network, i.e. to remove fault-related strain. It allows further validation of the geological consistency of the network, and it delivers an unfaulted framework for reservoir modelling. The unfaulting method approximates the three-dimensional kinematics related to faulting with the simplest possible analytical description that can be calculated from the geometry of the fault-horizon network and its fault slip vector field.
The construction of ‘fault swallows’ at branching faults decomposes discontinuous slip and strain at branch lines into continuous segments. With this decomposition, the unfaulting method can handle complex branching fault situations.
The methods in this paper have been developed for normal faults in deltaic areas, but they can easily be extended to other geological settings.
A four-year study of shear-wave splitting in Iceland: 2. Temporal changes before earthquakes and volcanic eruptions
Authors: Volti T. , Crampin S. 
Published: April 2008

Abstract

This chapter reports temporal variations in the time-delays between split shear-waves before both earthquakes and volcanic eruptions in Iceland. The hypothesis is that during a build-up of stress, crack distributions in a large volume surrounding the immediate source zone are modified until the level of cracking reaches fracture criticality, when shear strength is lost, rocks fracture, and earthquakes, or some types of eruptions occur. In one two-year period, when volcanic and magmatic activity appeared to be low, changes in shear-wave splitting in SW Iceland were observed routinely before earthquakes with magnitudes between
M
3.5 and
M
5.1. Assuming a linear relationship between earthquake magnitude and the rate of increasing crack aspect-ratio in this comparatively narrow amplitude range, the time and magnitude of a
M
5 earthquake was successfully stress-forecast. These results confirm a new understanding of pre-fracturing deformation of
in situ
rock that has implications over a wide range of situations where the crust undergoes changes at low levels of deformation below those at which rocks fracture. Potential applications of this new understanding include monitoring hydrocarbon production, as well as stress-forecasting earthquakes and some volcanic eruptions.
Segment linkage during evolution of intracontinental rift systems: insights from analogue modelling
Authors: Tentler T. , Temperley S. 
Published: April 2008

Abstract
On the basis of the effective scale-independence of brittle structures, from microcracks to regional fault systems, we have used analogue centrifuge models to provide insights into the initiation and evolution of continental active rifts, with a single dilational fracture segment representing a prototype rift segment. In the models, which are physically and dynamically scaled, a semi-brittle compound material, with flexural rigidity, was devised to simulate the lithosphere as a single layer. This is justified by the fact that, at the largest scale, the lithosphere behaves as a single viscoelastic, flexurally rigid unit. Silicon polymers of two different viscosities and densities represent the asthenosphere. A parallelepiped of lower viscosity and lower density material is embedded within and near the base of the second polymer that fills up most of the model box. The former is activated as a plume-like diapir that rises, spreads laterally and ultimately generates extensional stress in the semi-brittle layer on top. In terms of model fracture distributions, both narrow and wide failure modes were achieved, analogous to narrow and wide modes of rifting. The processes of fracture initiation, propagation and coalescence are the same for each mode. During the early stages of model runs, fracture initiation is more important than the propagation of existing segments in relieving stress, although the latter dominates during later stages. The key parameters of overlap, offset, obliquity and propagation angle, defined and illustrated, are used to distinguish different ways in which pairs of fractures coalesce. We recognize three distinct types of coalescence (type 1, type 2 and type 3) involving both tip-to-tip and tip-to-sidewall linkages. These are described and both graphically and statistically discriminated. Whether narrow or wide mode, an intracontinental rift system consists of a number of discrete fault-bound rift segments. Similar types of interactions to those in the models have been identified between pairs of rift segments from the Cenozoic Baikal and East African rift systems, and the Mesozoic to early Tertiary Central African rift system. The factors that control the type of coalescence between natural rift segments would appear to be precisely those that govern linkages of fracture segments in the models.
Erosional forcing of basin dynamics: new aspects of syn- and post-rift evolution
Authors: Burov E. , Poliakov A. 
Published: April 2008

Abstract
We revisit a number of important topics associated with the problem of interactions between surface and subsurface processes during syn- and post-rift evolution. To demonstrate the importance of these interactions and to verify a number of earlier ideas on rift evolution, we use a fully coupled three-fold mechanical behaviour, surface processes, heat transport numerical model, which combines brittle-elastic-ductile rheology, fault localization, erosion and sedimentation mechanisms. The model simulates fault formation causing brittle strain localization. Fault distribution and evolution are thus outputs of the model, allowing for new, geologically sensible constraints on the results. The numerical algorithm accounts for ‘true’ surface erosion/sedimentation, that is, the numerical elements are eliminated (eroded) and created (sedimented) with respective changes in properties. The results show that sedimentation in the basin and erosion on the rift flanks strongly control the mode of extension. In particular, active erosion/sedimentation on the synrift phase results in more pronounced thinning and widening of the basin, so that the apparent coefficients of extension increase by a factor of 1.5–2. Surface loading/unloading results in lithospheric flexure. Flexural stresses in places of maximum bending exceed lithospheric strength and create zones of localized weakening that partly or completely compensate strengthening due to cooling in the post-rift phase, when the subsidence rates also accelerate. Erosional unloading on the rift shoulders has the opposite effect, producing local strengthening and flexural rebound. Pressure gradients induced by subsidence/rebound result in lower crustal flow that controls 20–30% of subsidence rates, stability of the rift shoulders and drives some post-rift extension or compression. By taking account for the intermediate and lower crustal rheology, new explanations for some synrift phase effects such as polyphase subsidence of the basement provoked by crustal flow and ‘switching’ of the level of necking from one competent lithospheric level to another are suggested. Syn- and post-rift stagnation, upward and downward accelerations find a natural explanation within our model without the necessity to invoke external mechanisms.
Modelling and processing of 3D seismic data collected over the overlapping spreading centre on the East Pacific Rise at 9° 03′ N
Authors: Hobbs R. , Tong C. H. , Pye J. 
Published: April 2008

Abstract
Oceanic ridges are important as the locus for the generation of new crust associated with the movement of plates on the Earth’s surface. The geometry of the axial magma chamber (AMC) under these ridges is poorly constrained. This is especially true for the AMC under an overlapping spreading centre (OSC) where the ridge undergoes a small lateral offset, typically less than 8 km. One such OSC exists on the East Pacific Rise at 9° 03′ N. Both 2D and more recently 3D seismic reflection surveys have been conducted over the area in an attempt to establish the relationship of the AMC with the two overlapping limbs of the axial rise. A major problem for imaging the AMC is caused by the seafloor, which has a large velocity contrast and has rapid changes in topography. This presents a major challenge for the processing of data. This paper aims to show that the understanding the effects of acquisition and processing can be facilitated by accurate 3D modelling of the seismic wavefield. In this paper we use a modelling method based on complex-screens, which is both robust and fast for large 3D models with deep water and diffractive interfaces. We compare synthetics with real data and explore the sensitivity of processing 3D data from shot gather to final 3D depth-migrated image. We show that the combination of severe seabed topography and 3D velocity variation within the oceanic crust causes distortion and complex focusing of the reflected energy. Also, for data with only sparse reflectivity it is not possible to construct an adequate velocity model for depth imaging from the reflection data alone. This problem may be overcome if there is an alternative method to determine velocity, e.g. refraction data, but care must be exercised during interpretation given the different resolutions of the two datasets. Lessons from the modelling exercise can be used to aid the interpretation of the real data and examples are shown for comparison; however, the geological implications of the results are outside the scope of this paper.
Integrated 3D geomechanical modelling for deep subsurface deformation: a case study of tectonic and human-induced deformation in the eastern Netherlands
Authors: Van Wees J. D. , Orlic B. , Van Eijs R. , Zijl W. , Jongerius P. , Schreppers G. J. , Hendriks M. , Cornu T. 
Published: April 2008

Abstract
Current advances in finite element computer codes and increase in computer power theoretically allow quantitative modelling of the geomechanical effects of hydrocarbon depletion from reservoirs. Here we show that it is technically possible to incorporate the full complexity of the 3D geological structure of a reservoir including faults into geomechanical models. In the workflow GOCAD is used for the structural modelling, integrated with DIANA for the geomechanical calculations.
A case study on the Roswinkel gas field in the eastern Netherlands illustrates the working methodology. The case study clearly shows the strong dependency of gas depletion deformation effects on the prevailing tectonic stress field. Our models for gas depletion predict a stabilization of the stress field (further away from failure) for reservoirs in compressive and strike-slip regimes. On the other hand extensional stress regimes will result in failure of the reservoir, in agreement with observed earthquakes, provided that (a) the reservoir material or existing faults are weak and (b) the state of stress is close to failure of the material.

The Roswinkel field, which is marked by a high abundance of earthquakes occurring after gas depletion started, is therefore most likely marked by an extensional tectonic regime and by geomechanically weak rock or pre-existing faults. According to base Tertiary fault displacements and the
World Stress Map
, the extension (minimum horizontal principal stress) is most likely NE-SW oriented.
Dynamics of magma degassing
Authors: Sparks R. S. J. 
Published: April 2008

Abstract
Gas exsolution and segregation are fundamental controls on eruption dynamics and magma genesis. Basaltic magma loses gas relatively easily because of its low viscosity. However, bubbles grown by decompression and diffusion during magma ascent are too small to segregate. Coalescence, however, can create bubbles big enough for gas to escape from the rising basalt magma. In evolved magmas, such as andesite and rhyolite, high viscosity prevents bubbles rising independently through the magma. The original gas content of magma erupted as lava is commonly the same as that erupted explosively, so that a gas separation mechanism is required. A permeable magma foam can form to allow gas escape once bubbles become interconnected. Magma permeabilities can be much higher than wall-rock permeabilities, and so vertical gas loss can be an important escape path, in addition to gas loss through the conduit walls. This inference is consistent with observations from the Soufrière Hills Volcano, Montserrat, where gas escapes directly from the dome, and particularly along shear zones (faults) related to the conduit wall. Dynamical models of magma ascent have been developed which incorporate gas escape. The magma ascent rate is sensitive to gas escape, as the volume proportion of gas affects density, magma compressibility and rheology, resulting in both horizontal and vertical pressure gradients in the magma column to allow gas escape. Slight changes in gas loss can make the difference between explosive and effusive eruption, and multiple steady-state flow states can exist. In certain circumstances, there can be abrupt jumps between effusive and explosive activity. Overpressures develop in the ascending magma, caused primarily by the rheological stiffening of magma as gas exsolves and crystals grow. A maximum overpressure develops in the upper parts of volcanic conduits. The overpressure is typically several MPa and increases as permeability decreases. Thus, the possibility of reaching conditions for explosions increases as permeability decreases, both due to overpressure increase and the retention of more gas. Models of magma ascent from an elastic magma chamber, combined with concepts of permeability and overpressure linked to degassing, provide an explanation for the periodic patterns of dome growth with short-lived explosive activity, as in the 1980–1986 activity of Mount St Helens. Degassing of magma in conduits can also cause strong convective circulation between deep magma reservoirs and the Earth’s surface. Such circulation not only allows degassing to occur from deep reservoirs, but may also be a significant driving force for crystal differentiation.
Water diffusion in natural potassic melts
Authors: Freda C. , Baker D. R. , Romano C. , Scarlato P. 
Published: April 2008

Abstract
Water diffusion experiments were performed on a trachytic melt from the Agnano-Monte Spina explosive eruption (Phlegrean Fields, South Italy). Experiments were run in a piston cylinder apparatus at 1 GPa pressure, at temperatures from 1373 to 1673 K and for durations of 0 to 255 s, using the diffusion-couple technique. Water concentration profiles were measured by Fourier transform infrared spectrometry. Water diffusion coefficients at different temperatures and water concentrations were calculated from the total water profiles, using the Boltzmann-Matano technique.

Over the investigated range of temperatures and water concentrations, the diffusivity of water in potassic melts (
D
water
), m
2
/s can be described by Arrhenius equations that can be generalized for water concentrations between 0.25 and 2 wt% as follows:

D
water

=
exp
(
−
11.924
−
1.003
ln

C

H
2

O

)
exp
(

−
(
exp
(
11.836
−
0.139
ln

C

H
2

O

)
)

RT

)

where
C

H
2
O

is the water concentration in wt%,
R
is 8.3145 (J K
−1
mol.
−1
) and
T
is the temperature in Kelvin. Water diffusivities in trachytic melts were compared with water diffusivities in rhyolitic and basaltic melts. The activation energies for water diffusivity in trachyte and basalt are comparable, and higher than the haplogranitic melt. This results in a convergence of water diffusion coefficients in all melts at lower (magmatic) temperatures.
A model for the saturation of C-O-H-S fluids in silicate melts
Authors: Moretti R. , Papale P. , Ottonello G. 
Published: April 2008

Abstract

The behaviour of volatile components in magmas is crucial for magmatic and volcanic processes, from the deep regions of magma generation and storage to the shallow regions of magma eruption and emplacement. Water, carbon dioxide, and sulphur compounds are the main volatile components in natural magmas, generally comprising more than 99% of the volcanic gases released before, during, and after eruption. We have set up a method to calculate the chemical equilibrium between a fluid phase in the C-O-H-S system and a silicate liquid with a composition defined by ten major oxides. The method is based on previous models for the saturation of H
2
O-CO
2
fluids (Papale 1997, 1999) and sulphur solubility (Moretti 2002) in silicate liquids, and for the fugacities of components in fluids with complex composition (Belonoshko
et al.
1992). The model calculations provide estimates of the partitioning of H
2
O, CO
2
and S between the silicate liquid and the coexisting fluid, and the composition of the fluid phase in terms of H
2
O, CO
2
, SO
2
, and H
2
S, as a function of pressure, temperature, volatile-free liquid composition, oxygen fugacity, and total amount of each volatile component in the system. Model calculations are presented for silicate liquids with tholeiitic and rhyolitic composition, oxygen fugacities in the NNO ± 2 range, and pressures from a few hundred MPa to atmospheric, with the simplifying assumption that no reduced or oxidized sulphur-bearing solid or liquid phases nucleate or separate from the liquid-gas system. The results are in good agreement with the bulk of experimental data from the literature, and show the well-known minima in sulphur saturation contents as a function of oxygen fugacity, the mutual effects of volatiles on their saturation contents, and the complex relationships between the saturation surface of multi-component fluids and the liquid composition, volatile abundance, and pressure-temperature-oxygen fugacity conditions. The new model therefore represents a powerful tool for the prediction of multi-component gas-liquid equilibria in natural magmatic systems, for the simulation of magmatic and volcanic processes, and for the interpretation of data on the degassing of magma bodies and composition of volcanic plumes, provided that the assumptions on which the model rests, first of all that no separation of additional S-bearing phases occurs, are satisfied.
Particles from the plume of Popocatépetl volcano, Mexico — the FESEM/EDS approach
Authors: Obenholzner J. H. , Schroettner H. , Golob P. , Delgado H. 
Published: April 2008

Abstract
Magma-wall-rock interaction contributes gases to evolving magmatic systems, and removes volatiles into the country rock. These processes happen at depth, far away from direct observation. Micro-analysis of particles collected from volcanic plumes can provide information about these processes. For Popocatépetl volcano, scanning electron microscope (SEM) and field emission gun SEM (FESEM/EDS) analysis of contact-metamorphosed particles from fallout ash reveal the presence of wollastonite, hercynite and glass of non-volcanic, contact-metamorphic origin. Condensates from the passively degassing plume show a wide variety of chemical elements and are rich in phosphorus, indicating a possible non-magmatic source for this element.
Elevation effects in volcano applications of the COSPEC
Authors: Gerlach T. M. 
Published: April 2008

Abstract

Volcano applications commonly involve sizeable departures from the reference pressure and temperature of COSPEC calibration cells. Analysis shows that COSPEC SO
2
column abundances and derived mass emission rates are independent of pressure and temperature, and thus unaffected by elevation effects related to deviations from calibration cell reference state. However, path-length concentrations are pressure and temperature dependent. Since COSPEC path-length concentration data assume the reference pressure and temperature of calibration cells, they can lead to large errors when used to calculate SO
2
mixing ratios of volcanic plumes. Correction factors for COSPEC path-length concentrations become significant (
c
. 10%) at elevations of about 1 km (e.g. Kilauea volcano) and rise rapidly to
c
. 80% at 6 km (e.g. Cotopaxi volcano). Calculating SO
2
mixing ratios for volcanic plumes directly from COSPEC path-length concentrations always gives low results. Corrections can substantially increase mixing ratios; for example, corrections increase SO
2
ppm concentrations reported for the Mount St Helens, Colima, and Erebus plumes by 25–50%. Several arguments suggest it would be advantageous to calibrate COSPEC measurements in column abundance units rather than path-length concentration units.
Carbon dioxide emissions from fumarolic ice towers, Mount Erebus volcano, Antarctica
Authors: Wardell L. J. , Kyle P. R. , Campbell A. R. 
Published: April 2008

Abstract

Degassing at Mount Erebus occurs as a plume from a persistent convecting anorthoclase phonolite lava lake, and by flank degassing through warm ground and fumarolic ice towers within the summit caldera. The fumarolic ice towers offer a unique and simple approach to quantifying the flank CO
2
emissions. Carbon dioxide effluxes were determined at openings in the ice towers by measuring the CO
2
concentration, air-flow velocity, and size of the exit orifice. Fluxes ranged from <0.0001 to 0.034 kg s
−1
at 43 actively degassing ice towers. Small patches of steaming warm ground contributed 0.010 kg s
−1
. The δ
13
C isotopic compositions of the CO
2
samples ranged from −2.1 to −4.7‰, suggesting a magmatic origin for the CO
2
. Fumarolic ice towers allow diffuse degassing to be visually identified, providing a strong advantage in determining the total flux rate of these passive emissions. The estimated output of flank CO
2
degassing is 0.46 kg s
−1
(40 Mg d
−1
). Compared with direct airborne measurements of the volcanic plume, passive flank emissions constitute less than 2% of the total volcanic CO
2
budget emitted from Mount Erebus.