Hydrogeological and geotechnical rock property characterization from geophysics

Boreholes drilled at Sellafield penetrated a variety of rock units. Core samples and wireline logs indicate that rock mass properties and in situ fluid properties vary from one unit to another, but that properties are relatively uniform for each unit within a particular borehole. Variability is superimposed upon the bulk rock mass properties, however, where faults or zones of fractured rock intersect a borehole. Furthermore, lateral variability within a particular unit may be expected between boreholes and throughout the rock volume. The primary objectives of the work were: to establish a means of determining the spatial heterogeneity and distribution of rock mass and fluid properties using measurements made on core samples and derived from wireline logs; to determine the spatial heterogeneity and distribution of rock mass properties away from boreholes and in three-dimensions, by extrapolating the detailed understanding gained from boreholes into the seismic survey volume; and to develop a visualization model of rock mass properties in three-dimensions. It has been demonstrated that acoustic impedance is the principal property in common between surface seismic and borehole measurements, and that it provides a link between the various scales of measurement. Dynamic rock quality and hydrogeological indices have been developed which allow qualitative comparisons to be made between the boreholes. Empirical relationships have also been established between acoustic impedance and rock quality, and between acoustic impedance and hydrogeological properties. These relationships enable continuous profile quantitative estimations of Rock Mass Rating and hydraulic conductivity to be made from wireline logs, and have allowed these properties to be extrapolated into the seismic acoustic impedance volume, thereby providing three-dimensional visualizations of the spatial heterogeneity of rock mass properties

The comparison of core and geophysical log measurements obtained in the Nirex investigation of the Sellafield region

The Sellafield region, west Cumbria, is the focus of one of the most thorough geological investigations in the United Kingdom. The Sellafield Site is defined as an area immediately around the potential repository, extending 6.5 km north-south by 8 km eastwest. Twenty six deep boreholes were drilled within the area up to the end of 1995, with a total depth of approximately 28 km. Most of these boreholes have been continuously cored, a total of over 17 kilometres of core, with average core recovery well in excess of 90%. All boreholes were logged with a comprehensive suite of geophysical logs, including many state of the art tools. Laboratory physical property analysis of hundreds of sample cores has been carried out. Pilot studies were carried out to compare and contrast datasets and to investigate the relationships between the different data scales. Various techniques, including fractal analysis and Artificial Neural Networks, were tried in order to explore the relationships of these data at a variety of measurement scales. The pilot study was conducted in two stages: (1) evaluation of the primary controlling factors of the physical properties; (2) testing the validity of ‘Up-scaling’. The rocks of the Borrowdale Volcanic Group provided the most challenging problems due to the physical properties being dominated by fracturing and associated alteration zones. Relationships between data types at different scales were established suggesting that the extrapolation of properties derived from core and wireline logs across three-dimensional seismic grids would allow an understanding of the properties throughout a three-dimensional volum

Physical properties of sediments and basalts from the Argo and Gascoyne Abyssal Plains in the Indian Ocean

The Wyllie time average equation has, for many years, been universally applied to predict porosities from compressional wave velocities, or visa-versa. However, it has long been recognized that the Wyllie equation does not adequately describe the actual relationship between these two parameters, and there have been many attempts to improve upon it. These have included the use of a simplified Wood equation, the concept of acoustic formation factor, and a wide range of empirical relationships. In many cases these models have been derived by testing them against a set of data representing a relatively narrow range of porosity values and, similarly, the use of the Wyllie equation has often been justified by virtue of a pseudo-linear relationship over a narrow range of porosity values. During the Ocean Drilling Program - Leg 123 two sites were drilled in the deep Indian Ocean. Continuous coring at Site 765 recovered over 930m succession of soft Quaternary through Lower Cretaceous sediments and a further 271m of oceanic basement with relatively fresh, glass-bearing pillow lava and massive basalt. Soon after core recovery, measurements were made of: saturated bulk density, grain density, water content, porosity, and compressional wave velocity. The porosity ranged from 89%, close to the sea floor, to 1.6% for the dense basalts. This self consistent set of measurements made on fresh samples, with a wide range of values, has enabled some of the descriptive models to be tested more rigorously. Some of' the limitations of the time average equation were also recognised by Wyllie and his co-workers who amended the Wood emulsion equation to partially take account of the rigidity of the materials. Further modifications to this Wyllie-Wood equation have been shown here to not only describe the relationship between porosity and velocity more closely than the time average equation, but also more closely than some of the alternative proposals suggested by contemporaries of Wyllie and since. Indeed, bearing in mind the Wyl1ie-Wood equation was discussed in the sam~ paper the time average equation was first proposed, it is somewhat curious that the time average equation has been adhered to for so long. A semi-empirical acoustic impedance relationship has been developed which is shown to provide a more accurate porosity-velocity transform than has hitherto been possible using realistic material parameters

Wireline geophysical logging of the Nirex deep boreholes in the Sellafield area : comparisons between BVG core and wireline derived formation factors

The formation (resistivity) factor of a rock unit is a parameter based on electrical measurements that can be related directly to the porosity and, to a lesser degree, to the permeability. A reasonable correlation had previously been found between core sample and wireline derived formation factors for the Borrowdale Volcanic Group (BVG) in Nirex Borehole RCF3 in the Sellafield area (Brereton et al., 1996). It was concluded that wireline logs are able to provide an effective means of estimating the broad characteristics of formation factor variability with depth in a borehole and furthermore that they may be suitable for use in estimating the rock matrix diffusion properties of the in situ rock. The objective of the work reported here has been to further test this approach using core sample and wireline derived formation factors from other Nirex deep boreholes in the Sellafield area. For the purposes of calculating the wireline derived formation factors, the method used in Brereton et al., 1996 has been adopted in which the pore water resistivity in the BVG in the vicinity of any particular borehole has been taken to be constant. Values of pore water resistivity have been estimated from measurements on ground water produced from the Nirex boreholes and on pore water extracted from core samples. The wireline derived formation factors have then been compared with formation factor measurements made by the BGS on core samples during the Nirex Core Characterisation Programme

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The assessment of borehole cement sealing characteristics by acoustic waveform analysis

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