bluuebunny/crossref_metadata_2025_split huggingface dataset:
Complementary functions reveal data hidden in your logs
Authors: Samworth J. R. 
Published: April 2008

Abstract
Many logging tools make multiple measurements of the same type that have more than one depth of penetration. Common examples are Compensated Density, Compensated Neutron and Array Induction logs. The purpose of the compensation is to reduce or remove the effects of a disturbance that distort the true measurement. Examples of this disturbance are the borehole size, mudcake and salinity.
A general technique can be derived based on a theory of Linear Perturbation which requires no prior knowledge of the nature of the perturbation, the only requirement being that it is approximately locally linear. Various interpretations can be made of the general equation depending on the particular circumstances.
The technique also produces a Complementary Parameter associated with the degree of correction. This parameter is usually discarded or paid scant regard, but can often be of some significant value and exposes surprising information. A number of examples can be used to illustrate these techniques, showing that they have wide applicability in situations ranging from difficult logging conditions (e.g. density through casing) to the apparently routine, where unusual and unexpected borehole fluids are revealed from neutron logs.
Acoustic wave anisotropy in sandstones with systems of aligned cracks
Authors: Shakeel A. , King M. S. 
Published: April 2008

Abstract
Seismic anisotropy has been studied on a number of dry cubic sandstone specimens, of 51 mm side, in which a system of aligned cracks has been first introduced progressively by the application of a polyaxial state of stress, and then closed by hydrostatic stress. One P- and two S-wave velocities polarized at right angles, along with the deformation, have been measured at each stress level in each of the three principal stress directions. Thomsen’s (1986) anisotropy parameters (ɛ, γ, δ) have been calculated at each stress level during the cracking and crack closing cycles using Nishizawa’s (1982) theory. Test results indicate that anisotropy in the P-wave velocity is greater and more sensitive to the presence of aligned cracks than that observed for S waves. Modelling studies show that the P-wave anisotropy parameter ɛ is always greater than that of anisotropy parameter δ, for low crack densities and for small aspect ratios. The reverse is true for high crack densities and low aspect ratios. The results of numerical studies indicate that S-wave anisotropy is independent of the nature of the saturating fluid and that it is possible to observe elliptical anisotropy in a medium containing aligned dry ellipsoidal inclusions.
In situ
stress prediction using differential strain analysis and ultrasonic shear-wave splitting
Authors: Widarsono B. , Marsden J. R. , King M. S. 
Published: April 2008

Abstract

Knowledge of the
in situ
states of stress in rock masses is of considerable importance to a number of subsurface engineering activities, including those involved in exploiting petroleum and geothermal energy reserves. In this paper, a comparison is made of two laboratory techniques, based upon stress-relief microcracks, for determining the
in situ
state of stress: differential strain analysis (DSA) and ultrasonic shear-wave splitting (USWS). Measurements on ten sandstone samples recovered from deep boreholes, made using the well-established technique of DSA, have been compared to those made by the comparatively new technique of USWS and to sleeve fracturing measurements of
in situ
stress made in the corresponding boreholes. The results obtained indicate that the USWS technique, with its ability to test a large number of samples quickly, provides a useful adjunct to DSA and sleeve fracturing in determining trends in
in situ
stresses. Used in combination, the two laboratory techniques have also proved useful for examining rock micro-structural features.
Measurement scale and formation heterogeneity: effects on the integration of resistivity data
Authors: Jackson P. D. , Harvey P. K. , Lovell M. A. , Gunn D. A. , Williams C. G. , Flint R. C. 
Published: April 2008

Abstract
Core and downhole logging resistivity data gathered during Leg 133 of the Ocean Drilling Program are used to illustrate the wide range of scales of resistivity data available for reservoir characterization. The differences in scale and sampling interval between quantitative log resistivity data and conventional core plug data is shown to be central to reconciling these two datasets. Resistivity images of fine scale sedimentary structures taken on half-round cores are presented (at the same resolution as the downhole borehole wall imaging tools) and these fine structures are shown to be ‘lost’ if investigated using conventional core plugs and downhole resistivity logging tools. The limitations of conventional measurements on core plugs are presented and contrasted with the benefits of logging all of the core in the laboratory at a resolution comparable to the borehole wall imaging tools. An example of integrating different scales of resistivity data using a modelling approach is presented and is shown to be applicable to both core and log data. Visualizing and comparing the scale content of different resistivity datasets has been achieved in an intuitive way using a spectral method which illustrates the ‘data gap’ in quantitative resistivities which exists between core and log data.
Aspects of core-log integration: an approach using high resolution images
Authors: Lofts J C. , Bristow J. F. 
Published: April 2008

Abstract
Core-log comparisons are often not considered routinely enough within the exploration environment. This may be for a number of reasons, such as problems with depth-matching the core and log datasets, the environment of acquisition of both datasets, the lack of understanding of log and core acquisition or a lack of confidence in laboratory or log measurements. These problems are discussed as a preliminary step to the development of a strategy aimed at improving core-log integration.
Using recent technological advances in the side-by-side presentation of core and high resolution image data from logging tools, a strategy is presented with the aim of making core-log integration more rigid and routine. Features in both core and images are correlated interactively—thus ensuring the best possible integration. This is a two stage process involving core-to-image matching, and then image-to-log matching. This strategy has the potential to make core-log integration more accurate and as a result enable the interpreter to realize the most from sub-surface data.
Multi-scalar structure at DSDP/ODP Site 504, Costa Rica Rift, I: stratigraphy of eruptive products and accretion processes
Authors: Ayadi M. , Pezard P. A. , Laverne C. , Bronner G. 
Published: April 2008

Abstract

Hole 504B is located about 200 km south of the Costa Rica Rift and constitutes the reference section for the structure of the upper oceanic crust. Compared to core, the continuous electrical resistivity (at m scale) and the high-resolution electrical images (at cm scale) recorded in Hole 504B, provide a continuous and detailed lithostratigraphic description of the effusive section at Site 504. Flow thicknesses measured from cm scale FMS images average 0.5 (±0.1) m. The massive units, known to bound fluid circulation at large scale into the crust, are constituted with a series of 20 to 50 individual flows. If Site 504 was created over two volcanic cycles, each volcanic cycle allows the emplacement of [0.60 (±0.30)] × 10
6
m
3
of magma per m along the ridge axis. This computation leads to an estimate of magma volume for a single eruption of [0.003 (± 0.001)] × 10
6
m
3
per m along the ridge axis, and eventually, a gradient in magma pressure within the magma chamber lens of 52 (±26) MPa, appropriate for one eruption.
Development of the Cote D’Ivoire-Ghana transform margin: evidence from the integration of core and wireline log data
Authors: Gonçalves C. A. , Ewert L. 
Published: April 2008

Abstract
The primary objective for drilling the Cote d’Ivoire-Ghana Transform Margin during ODP Leg 159 was to assess the sedimentary and deformation processes resulting from the different stages of continental break-up and related transform tectonism. In view of the structural importance of the leg, integration of logging and core data is important to help understand the main tectonic and deformation events that occurred.
The effect of the transform deformation can be seen in physical properties data, for instance the porosity data derived from index properties measurements. Major breaks in porosity are associated with the tectonized lower Cretaceous and Cenozoic boundary, a trend also reflected in the P-wave velocity measurements. At each site, core and well log data show the presence of a major unconformity between the Cretaceous and Cenozoic, marked by an offset in porosity, density and P-wave data. The physical properties of log data are also heterogeneous, reflecting variations in consolidation, age and lithology.
Another interesting aspect covered by core-log integration was the structural relationship within the sediments. As well as the direct measurements made on cores, in situ structural measurements have been obtained using the Formation MicroScanner (FMS; Mark of Schlumberger) logging tool in two of the holes. The measurements cover the Eocene to Turonian-upper Santonian limestones. Bedding planes dip predominantly towards NWNNW and show an increase with depth which can be interpreted to be the result of steady subsidence of the Deep Ivorian Basin. Break-outs and fracturing were also observed. Breakout occurrences depend on sediment type and their axes are perpendicular to the maximum compressive horizontal stress east-northeast west-southwest. Fracturing occurs as normal and reverse microfaults, with dispersion of dips and azimuth directions in these zones. The presence of fault zones are also correlated with changes in the physical properties of the sediments.
The stability of a mudflat in the Humber estuary, South Yorkshire, UK
Authors: Amos Carl L. , Brylinsky M. , Sutherland T. F. , O’Brien D. , Lee S. , Cramp A. 
Published: April 2008

Abstract

The stability of a mudflat in the Humber estuary, South Yorkshire, was investigated at seven sites along a shore-normal transect during early spring. This was carried out using the benthic flume Sea Carousel, and from the investigation of surface samples in a laboratory equivalent to the Sea Carousel, the Lab Carousel. A clear trend in erosion threshold [τ
c
(0)] was evident, showing two maxima: the greatest on the inner mudflat (0.78 Pa); and a second on the central mudflat (0.75 Pa). We ascribe these maxima to two causes: (1) desiccation of the inner mudflats; and (2) biostabilization by algae of the central flats. Our results suggest that of these factors biostabilization was the dominant one at the time of the study. Most of the variation in erosion rate is explained through the sediment bulk density (ρ
b
) and colloidal carbohydrate (DCHO): [τ
c
(0)] = 0.5[1.93log
10
(DCHO + 2.5 × 10
−4
ρ
b
] + 1.7;
r
2
= 0.63. Thus variations in DCHO had O(10
4
) greater impact on erosion threshold than equivalent variations ρ
b
. The relationship between mean erosion rate (
E
m
) and current speed (
U
y
) was similar for all sites and is defined by the exponential function:
E
m
= 2.47 × 10
−6
· 10

(3.749
U
y
)

kgm
−2
s
−1
.
In situ
mean still-water settling rates (
W
s
) were up to 2.46 × 10
−3
ms
−1
, which is up to an order of magnitude faster than was measured in other estuaries at similar suspended sediment concentrations,
S
. The decay constant,
k
, for still-water settling appeared to be a linear function of
S
, and compared favourably with values derived from five Canadian coastal mudflats. Results from Lab Carousel showed that the erosion thresholds were the same as those measured
in situ
, using Sea Carousel. Furthermore the spatial trends in erosion were the same in both studies with the exception of the innermost sites (A and A/B). Differences are explained by consolidation and desiccation caused by solar radiation which were not simulated in the lab. Mean erosion rates showed a similar exponential relationship with current speed to that determined from Sea Carousel. Lab-derived values of
k
were much higher than the Sea Carousel ones and indicated much faster sediment settling under laboratory conditions.
k
was a weak inverse function of applied stress. Values of τ
d
derived from Krone’s (1962) method yielded threshold values of between 0.03 and 0.32 Pa. These values were 38 ± 16% of the erosion threshold τ
c
(0).
Measurements of the turbid tidal edge over the Skeffling mudflats
Authors: Christie M. C. , Dyer K. R. 
Published: April 2008

Abstract

The effects of wave and current action on the shallow water’s edge are quantified, by describing the relationships between velocity, bed shear stress and suspended sediment concentration. Advective processes dominated the sediment fluxes. Wave effects were negligible and high concentrations were generally limited to the first few minutes of covering by the flood tide. Velocities over 0.5 m s
−1
and concentrations above 10 gl
−1
were measured in the advancing water’s edge at this time. For depths <1 m, there was a linear relationship between concentration and velocity. An apparent erosion threshold occurred at a velocity of 0.2 m s
−1
.
Ecological interaction and sediment transport on an intertidal mudflat I. Evidence for a biologically mediated sediment-water interface
Authors: Ruddy G. , Turley C. M. , Jones T. E. R. 
Published: April 2008

Abstract

Using biological, chemical and physical data, evidence of biologically mediated cohesive sediment dynamics is presented for an intertidal mudflat in the Humber Estuary, UK. The data suggest that in excess of 98% of sediment fluxes to the sediment bed on the mudflat are subject to resuspension, forming a bedload that is important in generating the shear stress required to erode the sediment bed. Benthic micro-algae photosynthesize and are grazed by macro-heterotrophs within this 300–3000 μm deep dynamic surface layer. The data suggest that the algal growth within this layer is limited by nitrogen fluxes, resulting in ‘excess’ carbon fixation by photosynthesis. A significant positive relationship is apparent between ‘excess’ carbon fixation rates and the sediment critical erosion stress. This indicates that nitrogen limitation and sediment stability were directly proportional to each other on the mudflat. Grazing of the algae by
Macoma balthica
(a burrowing bivalve) profoundly affects the carbon and nitrogen cycles in the dynamic surface layer, and hence also ‘excess’ carbon fixation. We propose that a highly interdependent community of benthic algae, bacteria and macro-heterotrophs, acts to regulate sediment dynamics via small-scale nitrogen cycling and the algal exudation of ‘excess’ carbon as sediment-binding polymers.
Ecological interaction and sediment transport on an intertidal mudflat II. An experimental dynamic model of the sediment-water interface
Authors: Ruddy G. , Turley C. M. , Jones T. E. R. 
Published: April 2008

Abstract
A one-dimensional model is presented which combines simplified biological, chemical and physical dynamics across the sediment-water interface of intertidal mudflat sediments. Interactions between ecology, biogeochemistry and sediment dynamics are simulated during iteration through a short-term sequence of light and tides. The model shows that, under ‘normal’ intertidal conditions, a biolayer composed of unicellular algae can develop at the sediment-water interface. Rapid algal growth generates regularly recurring nitrogen limited conditions in the biolayer. In the model, the algal carbohydrate polymers produced and exuded during nitrogen limitation profoundly influence model sediment dynamics by adjusting sediment cohesion. Stable, but dynamic, sediment surfaces result in the model from a combination of carbohydrate exuding algae, bacterial remineralization of exuded carbohydrate and grazing of the algae by bioturbating macroheterotrophs. Model predictions for sites on an intertidal mudflat in the Humber Estuary, UK show good agreement with the field observations for a range of different physical, chemical and biological variables. The results imply that sediment accretion may be more directly coupled to nitrogen supply than to sediment supply. The results are discussed in terms of an algal growth advantage to carbohydrate exudation.
Measurements and preliminary modelling of current velocity over an intertidal mudflat, Humber estuary, UK
Authors: Wood R. G. , Black K. S. , Jago C. F. 
Published: April 2008

Abstract
This paper presents tidal current velocity data from Station D on the LISP transect, Humber estuary, UK, collected during April 1995. Ten impeller-type current meters, aligned along the major flood-ebb tidal axis, were deployed in a vertical array at the site. Five of these measured flood speeds and five measured ebb speeds. First, depth-averaged flow is considered. Observed depth-averaged velocities are compared to results from three sets of modelled velocities: one set in the Hawke channel taken from a 2D model of the Humber estuary, one from a 1D hydrodynamic model of a flooding/drying transect, and one from a uniform elevation, volume-conservation model of a flooding/drying transect. The uniform elevation model is simpler, faster and gives a smoother response than the hydrodynamic model at this scale; and the observed velocities are closest in magnitude and direction to results from this model. The nature of the vertical velocity profile is then considered for both Spring and Neap tidal cycles. It is suggested that the vertical advection of horizontal velocities (due to the large ratio of water level change to water depth) has a significant effect on the velocity profile during the ebb, modifying the expected deviation in profile due to the flow acceleration.
Considerations on wave-induced fluid mud streaming at open coasts
Authors: Rodriguez Hugo N. , Mehta Ashish J. 
Published: April 2008

Abstract
Wave-induced streaming of fluid mud at open coasts under damped, non-breaking waves is examined as a likely mechanism to explain nearshore bottom evolution in such an environment. For a two-layered, water-fluid mud domain in which mud is considered to be a viscous continuum, an expression for the streaming velocity is obtained. Preliminary data on alongshore streaming of fluid mud in a wave basin show an order of magnitude agreement with theory. Previous observations on the rates of mud movement along the coasts of Surinam, Guyana and Louisiana are shown to yield alongshore velocities that are commensurate with those obtained by considering streaming to be the prevalent cause of mud transport. These velocities are considerably lower than those which would occur due to breaking waves. Off the southwestern coast of India, shoreward streaming of mud under monsoonal waves results in the formation of shore-fast mudbanks. It is argued that among transport mechanisms which govern the dynamics of microtidal muddy coasts, streaming may be an important means by which fluid mud influences the formation, migration and dissipation mudbanks in areas where wave breaking is significantly attenuated by viscous damping.
Comparison of flocculated and dispersed suspended sediment in the Dollard estuary
Authors: Mikkelsen Ole , Pejrup Morten 
Published: April 2008

Abstract

The purpose of this paper is to give a Lagrangian description of the changes in field settling diameter over a tidal period, while carrying out a tidal excursion with a research vessel. Settling tube analyses were carried out in the Dollard estuary, North-East Netherlands, by use of the Braystoke SK110 settling tube. From each tube settling velocity and grain size grading curves for the suspended material were computed. Subsequently, the sediment was dispersed from the filters and analysed in a laser diffraction analyser, a Malvern Mastersizer/E. The resulting grain size distribution of the primary particles was plotted together with the
in situ
distribution as bar diagrams. In this way it was possible to graphically determine the smallest equivalent floc size occurring in each sample as the intersect between the two distributions. Plotting this size against SSC, a linear relationship appeared: the greater the SSC, the smaller the smallest floc. It is suggested that this relates to high current velocities, at the same time causing both high values of SSC and high internal shear rates causing floc breakage. Plotting the settling velocities of the smallest floc against SSC yields an exponential relationship. Examples of the use of these relationships are presented. Flocs in the Dollard were larger than flocs sampled in the Danish Wadden Sea in earlier experiments. For comparative purposes, settling tube analyses were also carried out over the tidal flat.
Observations of the morphodynamic behaviour of an intertidal mudflat at different timescales
Authors: Whitehouse Richard J. S. , Mitchener Helen J. 
Published: April 2008

Abstract
This paper concerns the morphological (bed level) behaviour of the intertidal mudflat at Portishead on the Severn Estuary and discusses how the prevailing hydraulic and cohesive sediment processes contribute to observed changes in the bed level. The characteristics of the mudflat behaviour during a fortnightly cycle is presented, based upon a continuous high resolution time series of the bed level measured at one location during 31 tidal immersions. During spring tides the sediment supply to the mudflat is increased and the tidal-mean bed elevation is at least 10 mm higher following the peak of spring tides than on the previous or following neaps. On a tide by tide basis the bed level varies by 10–20 mm and the preservation of fresh mud deposits on the intertidal area is controlled by the hydraulic conditions prevailing during the shallow water phase of the tide (less than 0.5 m depth) and the processes operating during the dry part of the tide. Hence, the bed level is controlled by the phasing of the tidal range (ambient sediment supply) with local water depth and wave activity. In comparison, on the annual timescale the bed level variation on the middle and upper mudflat is of the order of 100 mm and seasonal changes in storminess, subaerial environment and biostabilization appear to play a significant role.
The erosion threshold of biotic sediments: a comparison of methods
Authors: Sutherland T. F. , Amos C. L. , Grant J. 
Published: April 2008

Abstract

Laboratory and field studies were carried out to determine the effect of biofilms on sediment erodibility. In the laboratory setting, a diatom culture of
Nitzschia curvilineata
was incubated on sediment and eroded in a recirculating flume at successive stages of growth. Several methods for determining erosion thresholds were employed: (1) the visual determination of particle and bed threshold, (2) an extrapolation method and (3) a digitiza tion method. A comparison of the derived erosion thresholds by these three methods is discussed. In the field setting, an
in situ
flume (Sea Carousel) was deployed at stations along a transect in a microtidal temperate estuary (Upper South Cove, Nova Scotia, Canada). The extrapolation method was used to determine the erosion thresholds of seabed constituents, such as chlorophyll, phaeopigment and sediment. The differences observed between the erosion threshold of chlorophyll, phaeopigment, and sediment lead to ambiguities in the definition of the erosion threshold of a seabed.