Characterization of crack distribution: fabric analysis versus ultrasonic inversion

We analyse the relation between rock fabric, expressed by the preferred orientation of rock-forming minerals and microcracks, and elastic anisotropy of crystalline rock from the KTB pilot well. Detailed analyses of mineralogical composition, textures and microcrack fabrics were performed. In addition, ultrasonic velocity measurements of spherical samples in several directions were carried out at various confining pressures, and inverted in terms of the complete set of 21 elastic constants. By comparing the elastic tensors of the rocks at the final confining pressure (at which most of the microcracks are closed) with those at a lower pressure level, it is possible to separate the anisotropy induced by microcracks from that caused by mineral alignment. In contrast to previous work, no a priori knowledge of the type of anisotropy (triclinic, monoclinic, orthotropic etc.), or of the spatial orientation of the symmetry elements (planes, axes) of the cracked rock or of the intact rock is assumed. Furthermore, no restrictive assumptions on the orientation distribution function and the shape of the cracks are needed.
The results show that the elastic anisotropy characteristics, whether they are related to the microcracks or to the rock-forming minerals, are clearly correlated with the directly observed rock fabrics. We show that the symmetry directions of the mineral fabric and of microcrack fabric agree. A further result is that the microcrack-induced anisotropy dominates the other causes of anisotropy at confining pressures smaller than a few tens of megapascals, the situation being reversed at higher pressures. The laboratory data are quantitatively compared with sonic log data from the KTB well, showing the influence of pore fluids, effective pressure and crack density reduction on the anisotropy in situ .     

Seismic anisotropy as measured under high-pressure, high-temperature conditions

Summary. Four types of crustal and upper-mantle rocks have been used for the investigation of seismic P -wave velocities in three mutually perpendicular directions. Hydrostatic pressure, up to 6 kbar and temperatures up to 500°C were applied to the samples. Measurements of the ultrasonic P -wave travel times and velocities were carried out along two geotherms. All rock types show an anisotropic behaviour which is caused by the orientation of certain minerals. The anisotropy is not dependent on temperature and pressure. Gneiss and peridotite have 5–6 per cent anisotropy whereas granite and a metagabbro show values of only 2–3 per cent. The smallest velocity is always in the z direction, perpendicular to a schistocity or foliation. It is shown that the data agree with those of field observation. We conclude that anisotropy caused by preferred orientation of minerals must be expected in the whole lithosphere. Additional effects of layering, of cracks, and of nonhydrostatic stresses are estimated.     

P-wave velocities in rocks at high pressure and temperature and the constitution of the central California crust

Summary. Velocities of compressional waves are determined for central California rocks at pressures up to 0.7 GPa (7 kb) and temperatures up to 450°C. These data are used to interpret the seismic velocity structure of the crust in the California Coast Ranges. The seismic data on both sides of the San Andreas fault are consistent with the following model; besides some patches of surface sediments the upper 10—15 km of the crust on the northeast side consists predominantly of sedimentary and metasedimentary rocks of the Franciscan assemblage; the lower crust, of a thickness of 15—20 km, may be composed of gabbroic or other mafic rocks. Across the fault on the south-west side, the entire crustal section is probably a granitic complex similar to that exposed on the surface. The proposed model is shown to be consistent with the observed gravity anomaly.     

Global observation of off-great-circle propagation of Long-Period surface waves

In the current generation of global dispersion maps of surface waves, the long-wavelength structure seems to be very well determined. There is general agreement in the patterns of global phase velocity anomalies up to harmonic degree 16. However, the shorter-wavelength structure varies significantly between published maps, and it appears that this part of the models depends strongly on the inversion technique and on the data set of surface-wave dispersion (usually phase measurements). Polarization data depend on the lateral gradient of phase velocity and hence are more sensitive to shorter-wavelength structure than phase data; thus, including these data should enhance resolution. In this paper, I demonstrate that polarization data of long-period surface waves (80 s), as a function of frequency, can be reliably measured using a multitaper technique. the resulting off-great-circle arrival angles of the surface-wave packets are relatively easy to interpret within a ray-theoretical framework. Our data base of three-component recordings is now large enough to provide useful constraints on global dispersion maps, particularly on the shorter-wavelength parts. Apart from the phase velocity model itself, a possible misorientation of the horizontal components at each station is included in a non-linear inversion as an additional independent model parameter. This gives a significant improvement in the fit to the data. Misorientations of more than 3° are probable for at least four of the 37 stations investigated.     

Convective and conductive heat transfer in sedimentary basins

In the Earth's crust the temperature is largely controlled by heat conduction. However, under some circumstances, the thermal state is disturbed by advection of heat associated with groundwater flow. The corresponding thermal disturbance depends on the water flow velocity (modulus and direction) and therefore thermal data may be used to constrain the pattern of natural fluid flow. In this paper, some models of thermal disturbance induced by convective heat transfer are presented. They are based on the assumption that the water flow is concentrated in thin permeable structures such as aquifer or fault zones. The steady-state and transient thermal effects associated with such scenarios are computed using a somewhat idealized model which depends on a small number of parameters: flow rate, time, aquifer geometry and thermal parameters of surrounding rocks. In order to extract the conductive and convective components of heat transfer from temperature data and to estimate the corresponding fluid flow rate, it is first necessary to estimate the thermal conductivity field. The problem of the estimation of thermal conductivity in clay-rich rocks, based on laboratory and in-situ measurements, is emphasized. Then a method is proposed for the inversion of temperature data in terms of fluid flow. Vertical and lateral variations of thermal conductivity are taken into account and the fluid flow is assumed to be concentrated on a specified surface (2-D quasi-horizontal pattern). Thermal effects of the flow are simulated by a distribution of surface heat production which can be calculated and then inverted in terms of horizontal fluid flow pattern.     

Geological and geophysical investigation of the Mid-Cayman Spreading Centre: seismic velocity measurements and implications for the constitution of layer 3

Summary . In this paper we present laboratory measurements of compressional and shear wave velocities of a diverse suite of gabbroic rocks collected from the walls of the Mid-Cayman Spreading Centre with DSRV Alvin. The degree of deformation and alteration affecting these gabbros is quite variable, and we believe that they are typical of plutonic rocks emplaced at shallow levels (upper portion of seismic layer 3 and shallower) of the oceanic crust. The compositional and textural variations are reflected in the wide range of laboratory velocities which span most of the range of seismic velocities reported for oceanic and ophiolite rock samples including basalts, gabbros, ultramafics, and their altered derivatives. Based upon the laboratory velocities and the geological setting of the Mid-Cayman gabbros, it is argued that no unique lithology, except anhydrous peridotite, can be unequivocally identified in the oceanic lithosphere from seismic velocity data alone. Furthermore, these data allow for the possibility of considerable lithologic heterogeneity within portions of the oceanic crust at the scale of a few centimetres to a few hundred metres. Such heterogeneities would go unrecognized because seismic refraction studies mask these variations resulting in a picture of apparent uniformity.     

Simultaneous inversion of seismic data

Summary. The resolving power of different data sets, consisting of surface-wave dispersion measurements and S travel times, are compared for a continental structure. The shear velocity in the low-velocity zone can be resolved in some detail with higher-mode phase-velocity data. Sufficient resolution for small density contrasts (0.03 g cm⁻³) until depths of ∼ 300 km can be reached if higher-mode group velocities are available as well, even at a precision as low as 0.10 km/s. At greater depths the density is not resolved, and here travel-time data are superior to higher modes in resolving the shear velocity.     

Seal capacity estimation from subsurface pore pressures

A cap rock's capacity to seal hydrocarbons depends on its wettability and the sizes of the pore throats within the interconnected pore system that the leaking hydrocarbons must penetrate. These critical pore throat sizes are often poorly constrained in hydrocarbon exploration, partly because measurements of pore throat sizes have not been performed, and partly because pore throat measurements on a few individual samples in the cap rock may not be representative for the seal capacity of the top seal as a whole. To the contrary, the presence of formation overpressure can normally be estimated in drilled exploration targets. The presence of overpressure in reservoirs testifies to small pore throats in the cap rocks, as large pore throats will result in sufficiently high cap rock permeability to bleed off the overpressure. We suggest a stepwise procedure that enables quantification of top seal capacities of overpressured traps, based on subsurface pressure information. This procedure starts with the estimation of cap rock permeabilities, which are consistent with observed overpressure gradients across the top seals. Knowledge of burial histories is essential for these estimations. Relationships between pore throat size and permeability from laboratory experiments are then applied to estimate critical pore throat diameters in cap rocks. These critical pore throat diameters, combined with information of the physical properties of the pore fluids, are then used to calculate membrane seal capacity of cap rocks. Estimates of top seal capacity based on this procedure are rather sensitive to the vertical fluid velocity, but they are also to some extent sensitive to inaccuracies of the pore throat/permeability relationship, overpressure gradient, interfacial tensions between pore fluids, hydrocarbon density and water viscosity values. Despite these uncertainties, applications of the above‐mentioned procedure demonstrated that the mere presence of reservoir overpressures testifies to sufficient membrane seal capacity of cap rocks for most geological histories. Exempt from this statement are basins with rapid and substantial sediment compaction in the recent past.     

Scattering of waves in elastic media containing passive and active cracks

Scattering of wavefields in a 3-D medium that includes passive and/or active structures, is numerically solved by using the boundary integral equation method (BIEM). The passive structures are velocity anomalies that generate scattered waves upon incidence, and the active structures contain endogenous fracture sources, which are dynamically triggered by the dynamic load due to the incident waves. Simple models are adopted to represent these structures: passive cracks act as scatterers and active cracks as fracture sources. We form cracks using circular boundaries, which consist of many boundary elements. Scattering of elastic waves by the boundaries of passive cracks is treated as an exterior problem in BIEM. In the case of active cracks, both the exterior and interior problems need to be solved, because elastic waves are generated by fracturing with stress drop, and the growing crack boundaries scatter the incident waves from the outside of the cracks. The passive cracks and/or active cracks are randomly distributed in an infinite homogeneous elastic medium. Calculations of the complete waveform considering a single scatter show that the active crack has weak influence on the attenuation of first arrivals but strong influence on the amplitudes of coda waves, as compared with those due to the passive crack. In the active structures, multiple scattering between cracks and the waves triggered by fracturing strongly affect the amplitudes of first arrivals and coda waves. Compared to the case of the passive structures, the attenuation of initial phase is weak and the coda amplitudes decrease slowly.     

The relationship between seismic velocity and density in the continental crust — a useful constraint?

Summary . Plots of seismic velocity and density of rock samples show that a range of densities is possible for rocks of each seismic velocity and vice versa. although a single linear relationship is often assumed in crustal gravity calculations. Because of the scatter, whenever rocks of known seismic velocity are converted to density using this relationship, a reduction is made to the resolving power of the resulting gravity calculation. If these rocks reach thicknesses of more than a few kilometres, then the uncertainties become significant when compared with the size of commonly observed gravity anomalies. Examples are considered from the North Sea, Mississippi and Carolina Trough. It is concluded that the use of a seismic velocity measurement as the only indication of rock density does not provide a useful constraint when attempting to reproduce observed gravity variations. An appropriate model for isostatic compensation is probably the most important factor for successful predictions of crustal structure on the basis of gravity data.     

Inversion of P-wave velocity anisotropy

Summary. We examine the way in which measurements of velocity anisotropy can add to our understanding of upper mantle structure. Measurements of P -wave velocity anisotropy in a single plane contain very little direct information about the anisotropic structure. A promising technique is to fit the observed velocity variation with a mixture of an assumed anisotropic constituent and a proportion of isotropic material. Using this technique, mixtures of orthorhombic and transversely isotropic olivine are obtained, which are in excellent agreement with observed velocity variations in the Pacific.     

Forward modelling receiver functions for crustal structure beneath station TBZ (Trabzon, Turkey)

We use teleseismic three-component digital data from the Trabzon, Turkey broadband seismic station TBZ to model the crustal structure by the receiver function method. The station is located at a structural transition from continental northeastern Anatolia to the oceanic Black Sea basin. Rocks in the region are of volcanic origin covered by young sediments. By forward modelling the radial receiver functions, we construct 1-D crustal shear velocity models that include a lower crustal low-velocity zone, indicating a partial melt mechanism which may be the source of surfacing magmatic rocks and regional volcanism. Within the top 5 km, velocities increase sharply from about 1.5 to 3.5 km s⁻¹. Such near-surface low velocities are caused by sedimentation, extending from the Black Sea basin. Velocities at around 20 km depth have mantle-like values (about 4.25 km s⁻¹ ), which easily correlate to magmatic rocks cropping out on the surface. At 25 km depth there is a thin low-velocity layer of about 4.0 km s⁻¹. The average Moho velocity is about 4.6 km s⁻¹, and its depth changes from 32 to 40 km. Arrivals on the tangential components indicate that the Moho discontinuity dips approximately southwards, in agreement with the crustal thickening to the south. We searched for the solution of receiver functions around the regional surface wave group velocity inversion results, which helped alleviate the multiple solution problem frequently encountered in receiver function modelling.
Station TBZ is a recently deployed broadband seismic station, and the aim of this study is to report on the analysis of new receiver function data. The analysis of new data in such a structurally complex region provides constraining starting models for future structural studies in the region.     

Shear-wave polarization anisotropy in the Pacific Basin

Summary. Inversion of 295. Love- and Rayleigh-wave phase travel times across the Pacific Basin has yielded a structure which has a channel that is anisotropic with respect to the polarization of shear waves. The velocity of SH waves is approximately 4.24 km/s, and the velocity of SV waves is approximately 4.10 km/s in the low-velocity channel. The lid to the channel is isotropic with respect to the polarization of S waves and the velocity is approximately 4.60 km/s. The lid to the low-velocity channel increases in thickness with lithospheric age at the expense of the channel, and its thickness is apparently still increasing at a sea-floor age of 150 Myr.
These results can be explained in terms of a model with both randomly-and preferentially-oriented, liquid-filled cracks in the channel. In the model, it is assumed that the liquid-filled cracks are due to partial melting in the channel, and that any preferred orientation is caused by a shear-flow gradient resulting from differential motion between the lid and the deeper parts of the mantle.     

P-wave velocity anisotropy in crystalline rocks

Summary. Compressional wave velocities and anisotropy coefficients determined at high hydrostatic pressures are compiled from the data published for the main types of crystalline rocks. The crack-free elastic anisotropy of igneous crustal rocks is generally very low, between 1 and 3 per cent on average. The anisotropy of metamorphic rocks is higher (up to 22 per cent), but very variable. The average anisotropy coefficients in schists and amphibolites are about 10 per cent, in gneisses between 3 and 7 per cent, and in granulites less than 3 per cent. The average anisotropy of olivine ultramafites is between 7 and 12 per cent, whereas in pyroxenites and eclogites it is usually less than 4 per cent. A comparison of ranges of average velocities and average anisotropies for the individual rock groups suggests that, whereas in the crust the lateral velocity variations are mainly due to compositional changes, in the olivine of the uppermost mantle the velocity variations due to anisotropic structures could be of the same magnitude as the variations due to inhomogeneities.     

Lithoprobe seismic reflection profiling across Vancouver Island: results from reprocessing

Summary. Multichannel seismic reflection sections recorded across Vancouver Island have revealed two extensive zones of deep seismic reflections that dip gently to the northeast, and a number of moderate northeasterly dipping reflections that can be traced to the surface where major faults are exposed. Based on an integrated interpretation of these data with information from gravity, heat flow, seismicity, seismic refraction, magnetotelluric and geological studies it is concluded that the lower zone of gently dipping reflections is due to underplated oceanic sediments and igneous rocks associated with the current subduction of the Juan de Fuca plate, and that the upper zone represents a similar sequence of accreted rocks associated with an earlier episode of subduction. The high density/high velocity material between the two reflection zones is either an underplated slab of oceanic lithosphere or an imbricated package of mafic rocks. Reprocessing of data from two of the seismic lines has produced a remarkable image of the terrane bounding Leech River fault, with its dip undulating from >60° near the surface to 20° at 3 km depth and ∼38° at 6 km depth.     

Mode-Converted (P-SV) Waves for Imaging Thin Intercalated Basaltic Layers and Their Possible Geologic Implications

The subsurface imaging using conventional seismic reflection technique is challenging in areas where high velocity rocks such as basalts are underlain by low velocity rocks. The seismic image quality worsens in the presence of intercalated sediments within the basaltic layers. In the recent years, the multicomponent seismic exploration technique has drawn great attention because it reduces the ambiguity in seismic imaging, enlarges the S-wave information, and improves the prediction and identification of reservoir fluids. Improvements in sub-basalt imaging techniques could hold highly significant geologic implications such as resource exploration and identifying permanent geochemical trapping potential (such as for carbon sequestration studies). In this article, we examine the possibility of utilizing mode-converted (P-SV) waves for sub-basalt imaging as well as likely complicacies one may expect in such processes.     

Effects of topography and crustal heterogeneities on the source estimation of LP event at Kilauea volcano

The main goal of this study is to improve the modelling of the source mechanism associated with the generation of long period (LP) signals in volcanic areas. Our intent is to evaluate the effects that detailed structural features of the volcanic models play in the generation of LP signal and the consequent retrieval of LP source characteristics. In particular, effects associated with the presence of topography and crustal heterogeneities are here studied in detail. We focus our study on a LP event observed at Kilauea volcano, Hawaii, in 2001 May. A detailed analysis of this event and its source modelling is accompanied by a set of synthetic tests, which aim to evaluate the effects of topography and the presence of low velocity shallow layers in the source region. The forward problem of Green's function generation is solved numerically following a pseudo-spectral approach, assuming different 3-D models. The inversion is done in the frequency domain and the resulting source mechanism is represented by the sum of two time-dependent terms: a full moment tensor and a single force. Synthetic tests show how characteristic velocity structures, associated with shallow sources, may be partially responsible for the generation of the observed long-lasting ringing waveforms. When applying the inversion technique to Kilauea LP data set, inversions carried out for different crustal models led to very similar source geometries, indicating a subhorizontal cracks. On the other hand, the source time function and its duration are significantly different for different models. These results support the indication of a strong influence of crustal layering on the generation of the LP signal, while the assumption of homogeneous velocity model may bring to misleading results.     

A theoretical and experimental comparison of the anisotropies of magnetic susceptibility and remanence in rocks and minerals

Summary. Susceptibility, thermo-remanent magnetization (TRM) and isothermal remanent magnetization (IRM) anisotropy ellipsoids have been determined for several rock samples. The results indicate that the ellipsoid of initial susceptibility is less anisotropic than the TRM and low field IRM ellipsoids which are found experimentally to be of identical shape. This suggests that palaeomagnetic data for anisotropic rocks may be corrected by using the anisotropy ellipsoid determined from magnetically non-destructive low field IRM measurements. Such IRM measurements can also be used to obtain anisotropy axes of samples which are inherently anisotropic but which have a susceptibility which is too weak to be accurately measured. The results for a series of artificial anisotropic samples containing magnetite particles of different sizes (in the range 0.2–90 μm) were very similar to those for the rocks. In contrast, a comparison of the susceptibility and IRM ellipsoids for anisotropic samples containing particles from a magnetic tape gave very different results in accordance with theory. Such results imply that susceptibility and IRM ellipsoids could be used to determine whether anisotropic rocks contain uniaxial single-domain particles (magnetization confined to the easy axis) or whether the particles are essentially multidomain.     

On large-scale flow structures in a gravel-bed river

Previous studies have suggested the presence of large-scale flow structures in gravel-bed rivers. These structures are pictured as intermittent high-speed wedges separated by regions of lower velocity. However, the characteristics of these structures have not been examined in detail through either visualisation techniques or detailed field measurements. This paper confirms the presence of large-scale flow structures in gravel bed rivers, pictures their sequence and patterns and characterises their mean and individual properties. The analysis relies on a new technique for displaying velocity fluctuations in a space–time matrix that allows one to see the structures as they pass an array of current meters. Streamwise and vertical velocities were measured simultaneously with an array of three electromagnetic current meters. The sampling frequency was 20 Hz. Five velocity profiles of up to 13 1-min series of measurements and one profile of three 20-min measurements were sampled. These data suggest the presence of large wedges of faster fluid joined by regions of slower fluid. Space–time correlation analysis confirmed the presence of vertical coherence of the flow. The average angle of the front of the wedges is 36°. Although individual structures are variable in size and shape, a new detection technique using all three velocity signals simultaneously showed that their average frequency is nine events per minute and their duration is more than 2 s. The high-speed wedges display a complex organisation and do not show a preferred sequence of events as was postulated by previous studies. Because of their duration and size, these high-speed wedges are likely to play a major role in bedload sediment transport.     

Seismic-wave propagation through a cracked solid: polarization as a possible dilatancy diagnostic

summary . A new technique is presented for modelling the elastic constants of cracked structures with application to systems with weak concentrations of parallel cracks, and of simple biplanar and triplanar cracks. The velocities and V_p/V_s ratios of these anisotropic structures are used to provide quantitative models for some earthquake precursors. These results indicate the great importance of crack geometry to the behaviour of precursors. The velocities of saturated cracks appear to favour the dilatancy-diffusion model of precursory phenomena. Synthetic seismograms are calculated for propagation through possible dilatancy zones. The seismograms show some characteristic features which may be useful for the investigation of earthquake dilatancy.