Effect Of Lithology Research Articles

Effects of lithology and bulk chemistry on phyllosilicate reaction progress in the low-T metamorphic Graz Paleozoic, Eastern Alps, Austria

Structural and chemical evolution paths of white K-mica and chlorite from low-temperature metamorphic rocks are studied by X-ray powder diffractometry and electron probe micro-analysis in order to determine the effects of lithology expressed by bulk rock major element chemistry on the phyllosilicate reaction progress. The sample set representing three main lithotypes, namely pelitic and marly slates, metatuffites and massive meta-igneous rocks of basic to intermediate compositions, derived from the Graz and Sausal Paleozoic of the Upper Austroalpine unit, Eastern Alps, Austria. Reaction progress, modelled in terms of proportion of swelling mixed-layers, crystallinity indices, mean crystallite size, lattice strain and mineral chemistry, shows grain-size dependent variations in all lithologies. Referring to disequilibrium conditions, the larger authigenic grains reflect more evolved stages than the

The environmental impact of the Minoan eruption of Santorini (Thera): statistical analysis of palaeoecological data from Golbisar, southwest Turkey

A tephra layer originating from the mid-second millennium BC (3300 14C yr BP) ‘Minoan’ eruption of Santorini (or Thera) in the Aegean has been found in lake sediments at G6lhisar in southwest Turkey. Microstratigraphic analyses of tephra shard concentration (TSC), pollen, diatoms, sponge spicules and nonsiliceous microfossils in sediments from Gtlhisar permit the impact of this major volcanic eruption on terrestrial and aquatic biota to be investigated quantitatively. Partial redundancy analysis and associated Monte Carlo permutation tests suggest that TSC alone cannot be shown to have had a demonstrable independent and statistically significant effect on terrestrial pollen, non-siliceous microfossil or diatom assemblages. The lack of any clear, discernible change in the terrestrial pollen composition following tephra deposition suggests that there was minimal impact on regional vegetation over decadal-to-century timescales. However, evidence that the deposition of Santorini tephra may have had an impact on the lake system comes from the combined effect of lithology and TSC (which significantly covary) that explains a significant amount of variance in the aquatic data sets. In particular, diatoms and non-siliceous algae show increases in concentration following tephra deposition, exhibiting what appear to be decadal response times to perturbation. These imply enhanced lake productivity due to accelerated input of silica and other nutrients following tephra dissolution.

Effects of lithology on geothermal gradient on the southeast Nigeria Delta, Nigeria

A study of the effects of lithologic formations on geothermal gradients is carried out in the south-east Niger Delta, Nigeria, using continuous temperature and lithologic log data from closely-spaced petroleum wells. The gradient profiles obtained for the deep wells, logged to depths between 6500 ft (1981m) and 8500ft (2591m), were observed to have markedly different geothermal gradients with corresponding vertical lithologic units. For the upper sandy lithology, the Benin Formation, the temperature gradient ranges between 0.80 oF/100 ft (1.456 oC/100m) and 1.22 oF/100 ft (2.220 oC/100m) with an average of 1.03 oF/100 ft (1.875 oC/100m). For the paralic shaly Agbada Formation segment, the temperature gradient ranges between 0.92 oF/100 ft (1.674 oC/100m) and 1.87 oF/100ft (3.403 oC/100m) with an average of 1.40 oF/100ft (2.548 oC/100m). The depths of Benin Formation were observed to vary between 4200ft (1280 m) and 5400ft (1879 m). Geothermal gradients are thus observed to increase with depth and shale units. Key Words: Subsurface temperature, lithology, temperature gradient, thermal. (Global Journal of Pure and Applied Sciences: 2002 8(3): 325-338)

Hydrate Composition from Seismic Data: An Inverse Procedure

Hydrate existence conditions and the associated thermal gradients are examined using equations of state for mixes of gases. Three examples are developed to illustrate the resolution possible in determining the composition of a hydrate and the required range of thermal gradient using seismic information concerning the depth and thickness of a hydrate-bearing body. The examples use shallow (70m), medium (250m) and deep (600m) water as archetypical conditions. In general, it is shown that one cannot determine a unique composition for the hydrate due to incomplete knowledge of parameters in the hydrate existence equations of state, including the lack of knowledge concerning salinity and sedimentary lithological effects on parameters in the equations of state, and also due to such parameters being massively influenced by the types and mixing fractions of gases that compose the hydrate. What is shown is that it is possible to determine the cumulative probability that a hydrate has greater than a given fraction of methane and at the same time determine the probability that the associated thermal gradient lies within geologically required constraints. In association with this determination, one can also identify the relative contributions of each uncertain parameter to the resolution of the methane fraction and the thermal gradient, including geological parameters (such as sediment density) and “kinetic” parameters in the hydrate equation of state. In this way one can determine where to place effort in any further attempt to narrow down the uncertainty in the methane fraction for a given hydrate, and one can also identify when it is necessary to do so. These aspects are developed in the paper. There is also the point that, for deep water hydrates, the attempt to use the top of the hydrate body as the shallowest depth for hydrate existence can fail and recourse to a more appropriate strategy, but one that is not so statistically sharp, is required. This aspect, too, is developed in the paper. The main point to make is that the quantitative procedure developed here allows one to use seismic data directly to assess a hydrate body, to determine what can be deduced concerning hydrate composition, the sort of uncertainty one has to contend with, and also to determine the worth of proceeding further with more information collection to help resolve better the uncertainties.

韓国ソウル郊外の花こう岩と片麻岩山地における土層構造と表層崩壊形状に与える基盤岩質の影響

韓国ソウル郊外の花こう岩と片麻岩山地における土層構造と表層崩壊形状に与える基盤岩質の影響

Aeolian erosional lineations in the Libyan Desert, Dakhla Region, Egypt

This paper presents results of a study of aeolian erosion at the landscape scale. There have been few such studies in desert geomorphology compared to those focused on individual landforms and on rock surface sculpture. The present study area lies in the southern part of the Libyan Desert in south-central Egypt, between 25° and 27°N, 29° and 30°E. Bedrock comprises Paleogene limestone of various lithologies. Climate is hyperarid. The following topics are treated. (i) Correspondences between rock outcrop belts and belts of terrain lineated by aeolian erosion and unlineated terrain. (ii) The effect of lithology on aeolian lineation, which works through the presence or absence of chert in the limestones. Lineation is produced by aeolian erosion in chert-free/poor rocks, whereas erosion of chert-rich rocks produces a desert pavement that armors the surface, suppressing erosion. (iii) The effect of large valleys eroded upwind of lineated terrain, which deflect winds and trap sand, ending erosion, so that downwind lineations are reduced and finally erased by weathering. (iv) Sample lineated landscapes in the area show stages of evolution, arranged in the time domain into a proposed cycle of aeolian erosion; the cycle progresses from initial smooth plain to grooves separating long, blade-shaped ridges, to segmentation of ridges into shorter blades along diagonal joints, to streamlining of shorter blades and size reduction, to final planation. In any one wind-parallel swath of lineated terrain, stages in the cycle progress downwind in the space domain, so that upwind landscapes are more advanced in the cycle. (v) The structure of air flow responsible for lineation is still uncertain, but initial grooving may respond to stable longitudinal horseshoe vortices, or self-organized regularity of erosion by random vortices; subsequently, the erosion pattern is fixed by evolving relief. (vi) Large, smooth basins within the lineated terrain were carved by aeolian erosion before Oxygen Isotope Stage (OIS) 5 (70–130 ka), as were similar basins along the Dakhla piedmont below the scarp; little geomorphic change has occurred since, so in this field of aeolian erosional lineations (AELs) the aeolian erosion cycle may have occupied 10 times as long. Since continental scale aridity set in at ca. 2.4 Ma, there have been only two cycles in this area. A completed cycle earlier than the current incomplete one is indicated by rare small yardangs riding ‘piggyback’ on larger ones.

Groundwater phosphate dynamics in a river riparian zone: effects of hydrologic flowpaths, lithology and redox chemistry

This study examines the influence of riparian zone hydrology, lithology and redox chemistry on groundwater phosphate dynamics. Patterns of soluble reactive phosphorus (SRP), dissolved oxygen (DO) and ferrous iron (Fe 2+) in combination with hydrologic data and sediment characteristics were studied in a forested floodplain connected to a large upland sand aquifer in an agricultural region of southern Ontario, Canada. Groundwater discharge from the upland aquifer flowed laterally beneath peat in a 2–4 m thick zone of permeable sands across the floodplain to the river. Within the sands, low SRP concentrations (<25 μg L −1) occurred in a plume of groundwater with DO concentrations >3 mg L −1 and Fe 2+ concentrations <0.2 mg L −1 which extended for a horizontal distance of 100–140 m across the riparian zone. High SRP concentrations (50–950 μg L −1) were associated with low DO and high Fe 2+ concentrations which exceeded 1 mg L −1 in buried channel sediments near the river bank. Sediment P fractionation indicated that the buried channel sediments contained a much higher pool of total P and Fe+Al–P than the sands. Groundwater SRP concentrations at the river bank were 25–80 μg L −1 compared to <10 μg L −1 in river water indicating that the floodplain was a source of SRP to the river. Areas of elevated SRP and Fe 2+ within the floodplain expanded in August when DO concentrations in groundwater were lower than in late spring or autumn. These data suggest that the microbial reduction of Fe 3+ to soluble Fe 2+ in anaerobic conditions influences groundwater SRP concentrations in the riparian zone. This study shows that well-organized patterns of groundwater SRP concentrations occur in riparian zones which reflect the interaction of hydrologic flowpaths and environments of different redox state. Internal sources of P associated with buried channel sediments can also influence subsurface SRP transport and release to streams.

Exponential approximations to compacted sediment porosity profiles

Sediment compaction and corresponding porosity variations can be modeled by a simple exponential with depth. The porosity solution is derived analytically as a complicated function of pore water pressure, but the underlying form is shown to approximate an exponential except near the surface where the behavior is linear. Even though the analytical simplifications ignore some of the detailed effects of sediment lithology, a large world-wide collection of porosity data from compacted shales, silts and sandstones supports the general exponential trend. For comparison, a numerical compaction routine (with fewer assumptions than the analytical solution) is used in conjunction with a previously developed process-based model of marine sedimentation. Together, the compaction and sedimentation models show that in a realistic depositional environment, porosity varies exponentially with depth, in agreement with both the empirical data and the analytical approximation.

A compressional wave seismic model for the Ashrafi-1 well

Despite disappointing drilling results, the South Caspian basin has yielded scientific results of significant value to future exploration. This paper describes work that utilized data from the offshore Ashrafi-1 well, but we believe it has applications to exploration elsewhere in Azerbaijan. The North Absheron Operating Company (NAOC) drilled the Ashrafi-1 well between November 30, 1997 and February 6, 1998 to a total depth of 3668 m (sub Caspian sea level). The Balakhany and Pereryva Suite (SP) Formations were wet but the Upper Kirmaku Sandy (NKP) Formation tested oil and the Lower Kirmaku (PK) Formation tested gas, both at high flow rates. Anomalously high stack amplitude responses, generally conforming to structural closure, had been observed in the 3D seismic data at the NKP and PK Formations prior to drilling of the well. An offset synthetic modeling study was undertaken, post-drill, to determine the in-situ amplitude response (stack and amplitude variation with offset, or AVO) for both pay and wet zones and to model the changes in AVO and stacked amplitudes in response to changes in variables such as thickness, fluid fill and porosity. As well as the 3D seismic data set, the principal data sources used were a dipole sonic log for compressional wave velocity ( Vp) and shear wave velocity ( Vs); checkshots for time-to depth ( T– D) conversion; a density log; log interpretations of porosity; fluid properties based on pressure–volume–temperature (PVT) analysis of fluids from drill stem tests (DSTs) and pressure/temperature data. In this simple case, despite the highly permeable reservoir rocks, the Gassmann assumption of immobile fluids within the pore space was used. Although the effects of partial gas saturation are known to be a problem elsewhere in this basin, density modeling derived from borehole-guided long offset AVO was not attempted. Despite this, normal incident amplitudes, derived from pre-stack data, cross-plotted against amplitude gradients suggest a methodology for distinguishing wet sandstones from gas and oil in this basin in the absence of recent leakage. Despite the difference in response for oil and gas (oil gave a stronger response than gas), a statistically meaningful number of wells is not yet available and this result may be influenced by local lithologic effects. Thus, the ability to distinguish oil from gas ahead of the drill bit — which is of vital importance in evaluating commerciality — is not proven.

Wedge equilibrium in fold-and-thrust belts: prediction of out-of-sequence thrusting based on sandbox experiments and natural examples

AbstractThrust tectonics are dealt with on the basis of primarily experiments focusing on the dynamics of a developing thrust belt and on understanding and predicting normal-sequence and out-of-sequence thrusting. Field examples are presented in addition to the examples of sandbox-model experiments. The results have improved the insight into thrust-belt-forming mechanisms; the validity of the conclusions is supported by natural examples.The experimental program was aimed at examining the effect of changes in a selection of key parameters in thrust tectonics on the geometry and the successive phases in the development of thrust sheets. Sandbox experiments were used to analyse the effects of basal friction, detachment lithology, basement relief and syntectonic sedimentation. Multilayer experiments were performed to simulate the effects of ductile detachment lithologies.It was found that a thrust belt’s cross-sectional geometry is formed in a dynamic process during which the wedge may develop from subcritical through critical to supercritical and back to critical again. The process is illustrated with sandbox experiments, analysed by time-lapse computed tomography scans and in-situ stress measurements. On the basis of the sandbox-model results and the natural examples, we conclude that a critical examination of the boundary conditions of a fold-and-thrust belt and of changes in these conditions during the deformation process enables predictions about the geometry and kinematics of the thrust belt.

Effects of lithology on geometry and scaling of small faults in Triassic sandstones, East Greenland

A study of the Lower Triassic sandstones exposed in the Månedalen Fault Zone on Traill Ö, East Greenland, reveals how diagenetic carbonate cement affects the deformational behaviour of sandstones. A siliciclastic sequence of beds ( ≂300 m thick) is variably cemented because of the precipitation of carbonate close to stromatolitic interbeds. Displacements or throws of minor faults ( <1.8 m ) were sampled along lines through damage zones of major faults (throws >9 m) in three different lithologies: (1) Porous sandstones with little carbonate cement, (2) low-porosity sandstones rich in carbonate cement, and (3) thin beds of carbonate-cemented sandstone in mudstone. The latter shows a strong anisotropy causing fault refraction with syn-kinematic calcite growth in the sandstone beds. Viewed in a microscope, porous sandstones show disaggregated zones with porosity reduced by 40–60%, and cataclastic bands. The development of groups of cataclastic bands was governed by strain-hardening. The low-porous sandstones show cataclastic bands with minor textural change towards the bands. The apparent strength of these sandstones suggests that strain-hardening was much less significant for the development of the faults. Logarithmic plots of N vs D, where N is the cumulative number of faults with throw greater or equal to D, appear to follow linear trends. The least-square regression method gives the following values for the power-law exponent; C=1.01 for 993 faults in the porous sandstones, C=0.70 for 457 faults in carbonate-cemented sandstones, and C=0.38 for 166 faults in the carbonate-cemented sandstones interbedded with mudstones. The estimated confidence intervals for C indicate a true difference in scaling relationships of the samples. The different scaling relationships cannot be linked to strain differences encountered along the traverses. Thus, the lithological heterogeneity arising from local calcite precipitation is thought to be the cause for the differences in deformational style and scaling behaviour.

Lithological control on relief and hypsometry in the Hérault drainage basin (France)

Relief and hypsometry have been calculated for three drainage basins along the Hérault river, all of them in the same tectonic context but with different rock types. Basin hypsometry clearly depends on lithology, whereas lithological effect is minor on relief amplitude at various spatial scales. In addition, relief amplitude is a specific characteristic of a drainage basin, and consequently could be a relevant morphometric indicator of differential vertical surface motions.

Criteria for incipient slaty and crenulation cleavage development in Tertiary flysch of the Helvetic zone of the Swiss Alps

Two parameters of incipient cleavage development are compared in a series of Tertiary flysch from the Helvetic zone of the Swiss Alps: (1) the overall relative phyllosilicate orientation as obtained from the X-ray diffraction intensity ratio of their 00l reflections in cleavage- and bedding-parallel slabs (I 10Å (0.5°) C/B) and (2) the dimensional aspect ratios of fissility fragments, the ‘cleavage/bedding fissility ratio’ ( c/b). These two parameters show a roughly linear power relationship with a wide scatter. Appreciably better fits are obtained if populations showing different cleavage morphology (closely vs widely spaced cleavage; crenulation vs non-crenulation cleavage) and lithology (silt and fine-sand size quartz content; carbonate content) are plotted separately. In contrast, there is no clear link between the two criteria of cleavage intensity and the degree of incipient metamorphism as expressed by illite ‘crystallinity'. Samples with crenulation cleavage show both lower ( I 10Å (0.5°) C/B) and c/b cleavage/bedding fissility ratios than most of those with non-crenulation domainal cleavages with closely spaced cleavage domains; these lower ratios reflect the presence of an earlier, largely bedding-parallel, cleavage fabric. This difference in the ratios between these groups of cleavage morphologies proves that the cleavage/bedding fissility ratio is not only dependent on the overall phyllosilicate orientation, but also on the cleavage microstructure, in particular the morphology and spacing of the cleavage domains and the persistence of a bedding-parallel cleavage fabric in the microlithons of the crenulated samples. The relationship between the c/b cleavage/bedding fissility ratio and the I 10Å (0.5°) C/B ratio can provide a useful indication of the type of incipient cleavage fabric developed, and help in detecting the presence of an earlier slaty cleavage. Additional measurement of the intersection/bedding fissility ratio i/b on the fissility fragments is likely to help in assessing the effect of lithology, particularly the effect of the initial bedding anisotropy. The differences in the illite/chlorite ratios in the cleavage and bedding planes, as determined from the I 10Å/I 7Å C/B ratio, provides information regarding the relative formation of illite and chlorite in the cleavage domains and the chlorite–mica ‘stacks’ during development of the cleavage fabric.

Sediment supply from landslide‐dominated catchments: implications for basin‐margin fans

The sediment flux from a mountainous catchment can be expressed as a function of a landslide rate constant κ which accounts for the vigour of hillslope erosion. Since the incising drainage network flushes all or a portion of the products of hillslope erosion to a range front where fan deposition takes place, a conservation of solid sediment volume allows the fan area and progradation distance to be calculated. These parameters are related primarily to the discharge of sediment from the catchment and to local tectonic subsidence.A survey of modern alluvial fans in a wide range of climatic and tectonic settings shows that the effects of climate and bedrock lithology cannot be discriminated in the scatter of data of catchment area vs. fan area. However, by focusing on over 100 fans in the arid and semiarid zone of SW USA, the impact of tectonic subsidence rate is unambiguous. Although further quantitative data on local tectonic subsidence rates are urgently required, our preliminary analysis suggests considerable potential for reconstructing palaeocatchments where basin tectonic subsidence rates can be estimated. The progradation distances of fans from the northern and southern margins of the Middle Devonian Hornelen Basin of Norway, and the western and north‐eastern margins of the Mio‐Pliocene Ridge Basin, California, allow catchment sizes and denudation rates to be approximated. Although unique solution sets are not possible, an iteration of parameter values allows plausible parameter combinations to be calculated which shed light on the tectonic and sedimentary history of the proximal basin and upland source regions. Model results suggest significant asymmetry in basin subsidence rates, catchment slopes and transport mechanics between the two margins.

The effects of source lithology, transport, deposition and sampling scale on the composition of southern California sand

ABSTRACTThe Transverse Ranges of southern California represent an uplifted and variably dissected Mesozoic magmatic arc, and Mesozoic to Holocene sedimentary and volcanic strata deposited in convergent and transform tectonic settings. Modern sand within part of the Western Transverse Ranges represents: first‐order sampling scale of the Santa Monica and the San Gabriel Mountains; second‐order sampling scale of the Santa Clara River draining both mountain ranges; and third‐order sampling scale of the beach system between the mouth of the Santa Clara River and the eastern Santa Monica Mountains, and turbidite sand of the Hueneme‐Mugu submarine fan.Source lithology includes plutonic and metamorphic rocks of the San Gabriel Mountains, and sedimentary and volcanic rocks of the Santa Monica Mountains. First‐order sands have large compositional variability. Sand from local coastal drainage of the Santa Monica Mountains ranges from basaltic feldspatholithic to quartzofeldspathic. Sand of the San Gabriel Mountains local drainages has three distinct petrofacies, ranging from metamorphiclastic feldspatholithic to mixed metamorphi/plutoniclastic and plutoniclastic quartzofeldspathic. Second‐order sand is represented by the main channel of the Santa Clara River; the sand has an abrupt downstream compositional change, from feldspathic to quartzofeldspathic. Third‐order sand (beaches and deep‐sea turbidite samples) of the Santa Monica Basin is quartzofeldspathic. Beach sand is more quartz‐rich than is Santa Clara river sand, whereas turbidite sand is more feldspar‐rich than is beach sand. Deep‐sea sand has intermediate composition with respect to second‐order samples of the Santa Clara River and third‐order samples of the beach system, suggesting that (1) the Santa Clara River is the main source of sediments to the marine environment; and (2) local entry points from canyons located near local drainages may generate turbidity currents during exceptional flood conditions. Petrologic data of modern sand of the study area are highly variable at first‐ and second‐order scale, whereas third‐order sand is homogenized. The homogenized composition of deep‐marine sand is similar to the composition of most ancient sandstone derived primarily from the Mesozoic dissected magmatic arc of southern California. This study of the Western Transverse Ranges illustrates the effects of source lithology, transport, depositional environment, and sampling scale on sand composition of a complex system, which provides insights regarding actualistic petrofacies models.

Total Porosity and Bound-Fluid Measurements From an NMR Tool

New measurements recently available from a pulse-echonuclearmagnetic-resonance (NMR) tool enhance the usefulness of this logging technique. These include total NMR porosity and bound-fluid measurements. TotalNMR porosity provides a complete analysis of a reservoir's fluid volumes, including what is trapped in the very smallest pores associated with clays and very-fine-grained rock. This leads to improved permeability, irreducible water saturation, and effective porosity answers and, consequently, improved estimates of hydrocarbon producibility. Bound-fluid measurements, taken at logging speeds of up to 548.6 m/hr, provide irreducible water saturation and permeability when used in conjunction with another porosity device. This logging mode is favored when total NMR porosity is not required and when fast logging speeds are needed because of long log intervals, expensive rig rates, or poor wellbore conditions. Fig, 1 shows sample NMR logs taken from the Schlumberger Combinable Magnetic Resonance (CMR) tool. NMR signal amplitude is proportional to the number of hydrogen nuclei present in pore fluids and calibrated to give porosity (free from radioactive sources)and lithology effects. However, signal decay during each measurement cycle -called the transverse relaxation time, t2 - excites more petrophysicists. Signal amplitude is the sum of all decaying t2 signals generated by hydrogennuclei measured by the tool. Separating out the distribution of t2 values by a mathematical inversion process produces a t2 distribution. Thus, permeability and bound- and free-fluid porosity can be determined. Water in a test tube has a long t2 relaxation time of 3,500 milliseconds at25°C. However, water in rock pores has shorter relaxation times. For example, relaxation times for a sandstone typically range from 3 milliseconds for small pores to 500 milliseconds for large pores. Laboratory studies and field data show that t2 distributions in shales, shaly sands, some carbonate rocks, and heavy-oil reservoirs can have significant porosity amounts associated with t2of less than 3 milliseconds.

Analysis of digital elevation data for the Scottish Highlands and recognition of pre-Quaternary elevated surfaces

Abstract Remnants of elevated palaeosurfaces in the Scottish Highlands, proposed in numerous earlier studies, have been re-evaluated using digital elevation data from the Ordnance Survey. Histograms of elevation frequency, for areas 5, 10 and 20 km 2 , display multi-modal distributions which can be interpreted in terms of plateau and base-level components. The effects of glaciation, lithology and faulting can be inferred from patterns in the elevation distribution. The histogram modes reveal a series of elevated surfaces which broadly confirm previous, qualitative models of inclined Tertiary palaeosurfaces, but indicate a much more complex pattern. Many of the elevated surfaces are seen to be inclined towards two major faults, the Great Glen Fault and the Ericht-Laidon Fault. However, the highest surfaces may have crossed these fault zones uninterrupted. A model in which an episode of tectonic reactivation imposes a major change on pre-Quaternary geomorphological evolution in this area is proposed.

A simple but powerful model for simulating elastic wave velocities in clastic Silicate rocks

Abstract We describe a practical velocity model for clastic silicate rocks developed from the Kuster and Toksöz, differential effective medium and Gassmann theories. The model divides the total pore space into two parts, one associated with sand grains and the other associated with clays (including bound water). The difference in pore geometry makes the clay fraction more compliant with increasing porosity than the sand fraction. The model accurately simulates the combined effect of lithology, porosity, clay content, water saturation and fluid type on laboratory and logging P- and S-wave velocities. Velocity dispersion is modelled by considering the relaxed and unrelaxed extremes of fluid flow. Of three possible schemes for predicting S-wave logs from other logs, prediction from the P-wave sonic and porosity logs is generally the most accurate; comparisons of blind test predictions and S-wave logs at two Norwegian North Sea wells demonstrate the accuracy and robustness of the method. The predictions were further improved once geological information was provided on the formations encountered in the well. Examples of application to the prediction of velocity dispersion and dry frame moduli, the detection of hydrocarbons from crossplotting the P-wave and S-wave sonic velocities, and sonic log editing confirm the flexibility, accuracy and reliability of the model. The model has proved invaluable in constructing accurate models of the seismic reflection response in the vicinity of wells.

Tectonic control of fan size: the importance of spatially variable subsidence rates

We study the geophysical controls on the size of alluvial fans. Simple relationships between catchment characteristics, sediment yield, subsidence patterns and fan size are developed. As predicting fan size is essentially a conservation of mass problem, our analysis is general, applying to all types of fan landform. The importance of spatially variable subsidence rates has gone largely unrecognized in previous studies of modern fans. Here we stress that the distribution of subsidence rates in the depositional basin is a primary control on relative fan size. Both free coefficients in the oft‐cited power‐law correlation of fan area and catchment area can be shown to be set primarily by the tectonic setting, taken to include source area uplift rate and the subsidence distribution in the depositional basin. In the case of a steady‐state landscape, relative fan size is shown to be independent of both climate and source lithology; only during times of significant departure from steady state can relative fan size be expected to vary with either climate or source lithology. Transients associated with (1) a sudden increase in rock uplift rate, (2) a sudden change in climate and (3) the unroofing of strata with greatly differing erodibilities may produce variation of relative fan areas with both climate and source lithology. Variation of relative fan size with climate or lithology, however, requires that catchment–fan system response to perturbations away from steady state is sensitive to climate and lithology. Neither the strength of transient system responses nor their sensitivity to climate or lithology are known at present. Furthermore, internal feedbacks can significantly dampen any climatic or lithological effect. Thus theoretical considerations of the importance of climatic and lithological variables are inconclusive, but suggest that climatic and lithological effects are probably of secondary importance to tectonic effects. Field data from an unsteady landscape in Owens Valley, California, support and illustrate theoretical predictions regarding tectonic control of fan size. Field data from Owens Valley allow, but do not prove, a secondary dependence on source lithology. In addition, the Owens Valley field data indicate no relationship between relative fan size and climate. Headward catchment growth and enhanced sediment bypassing of fans during times of increased sediment yield (glacial) are put forward as plausible explanations.

Statistical Evidence of the Geological Control over Radon Soil Gas Concentrations and its Implications for Mapping Radon Potential

Determining how radon varies spatially over a given area in the natural environment is important for defining high risk areas and has implications for building practice. To achieve the former, radon concentrations in three areas of the English Midlands were surveyed. The first area comprised a single rock formation near Hereford (described elsewhere). The others, reported here, were areas where elevated concentrations of radon were expected. One was an area comprising two major rock types near Buxton in Derbyshire where a multistage sampling design was used to determine the approximate spatial scale of variation in soil radon concentration. The third was an area of more complex geology near Nottingham, where sampling along a transect enabled the structure and scale of variation to be determined. In all of the areas radon concentrations varied considerably, both over large and small distances. The data were analysed using methods embodied in geostatistics. The results showed that structure in the spatial variation of radon for the Buxton and Nottingham surveys at the longer scale could be attributed to the effect of lithology. The latter appears to account for approximately 50% of the total variation in both surveys. These results have important implications for mapping radon and also for building programmes, insurance, etc. They also suggest that to estimate radon reliably at the local level by interpolation would generally require very intensive sampling, i.e. at a scale of metres rather than kilometres. However, stratification of an area based on geology, with sampling within the strata designed to estimate average radon concentrations optimally, would provide reasonable estimates in certain situations for somewhat less sampling effort. This could provide a quick and efficient means of assessing the general radon potential of a given area. The best estimate of radon concentration at an unsampled site would then be the mean for that particular rock formation. However, if exact values are needed then radon should be measured in situ.