R. C. Flint

Geophysical imaging inside masonry structures

Abstract Geophysical techniques offer the capability to non-invasively investigate engineering structures, both in terms of their spatial variability, and the prediction of properties of interest to the engineer from the ‘pixel’ values of the geophysical tomograms. The combined use of different geophysical methods provides a means of identifying the signature of salient engineering conditions. The response of seismic, radar and electrical resistivity tomography to simple targets within masonry structures is demonstrated. The benefits of combining rapid standard radar surveys with more detailed tomography is discussed and strategies for investigating typical structures developed. The response of the three methods, acoustic, electrical and radar to changes in the condition of masonry, such as reduction in strength and ingress of water, are discussed, with particular reference to a case study utilising the Ribblehead viaduct as an example. The corresponding responses to changes in ground conditions are also presented and shown to be important in understanding possible further applications of these methods, such as investigating behind retaining walls.

Two- and Three-Dimensional Heterogeneity in Carbonate Sediments Using Resistivity Imaging

Abstract Volume heterogeneity was investigated in carbonate sediments using fine-scale electrical resistivity data, which are sensitive to porosity and to sediment macrostructure and microstructure. Variability in sediment density and porosity was assessed by X-radiographic and two-dimensional (2D) electrical resistivity measurements in 3-cm-thick slabs collected from the Dry Tortugas in the lower Florida Keys. A comparison of the 2D correlation lengths calculated from each assessment indicates that, using methods that differ in resolution, these two methods measure quite different horizontal and vertical fluctuations in sediment density and porosity. Other electrical resistivity experiments were conducted using a 2D network of resistivity electrodes on the surface of freshly collected box cores. The resulting three-dimensional (3D) resistivity data provide a unique insight into the spatial variability of sediment porosity and structure at the cm scale. Complementary X-radiograph cores provide 2D datasets of resistivity and porosity at higher resolution. These data are used to establish the porosity–resistivity–microstructure relationships for these carbonate sediments, with microstructure being described in terms of tortuosity. These relationships are used to extend our interpretation in terms of porosity and tortuosity to the corresponding 3D box core resistivity datasets. Sediment tortuosity is investigated by numerically modeling the flow of electrical currents through a range of pore morphologies in three dimensions. The results show particular sensitivity to the intra-particle porosity, particle shape and the relative sizes of pores and throats. The 3D methodology shows promise as a non-invasive measurement of buried inhomogeneities that could lead to improving models for predicting acoustic backscattering from the sediment volume.

Two-dimensional variability in porosity, density, and electrical resistivity of Eckernförde Bay sediment

Image-based analysis techniques are employed to compare sediment density heterogeneity as interpreted from X-radiography and electrical resistivity measurements. Assessments of sediment heterogeneity in two dimensions through the use of the two methods, with some exceptions, yield similar results. Autocorrelation functions estimated from density fluctuations reveal an anisotropy between vertical and horizontal sediment structure indicated by horizontal correlation lengths being greater than vertical correlation lengths. In addition to the anisotropy, discrepancies between values of correlation lengths calculated from two statistical methods are indicative of the scale-dependent nature of the sediment structure. This scale dependence is also exemplified by the differences between the images of X-radiography and electrical resistivity. The unequal volume of sediment over which the respective methods are integrated produces results differing by the amount and type of information included in each image. Electrical microresistivity is more sensitive than X-radiography to many smaller-scale variations in sediment density. Combining the results from X-radiography and microresistivity imaging allows the calculation of Archie’s m parameter, an extremely sensitive indicator of sediment microstructure. In addition, electrical microresistivity gives a measurement of sediment tortuosity coincident with sediment porosity and density.

Cross-hole seismic measurements for detection of disturbed ground beneath existing structures

Cross-hole seismic surveys are well established for non-invasive investigations of geological structure and are well suited for detecting hazards beneath buildings. Inversion of the seismic data confirms the value of tomographic imaging as a means of expressing the results in a form that is of practical value to the engineer. Physical property research has identified petrophysical factors, which control the attenuation and velocity of compressional and shear waves in reservoir-type rocks. This indicates that there is value in assessing changes in amplitude of seismic waves in addition to their velocity. The practical value of such surveys is demonstrated using case histories. While minimising the losses between the borehole casing and the formation (e.g. using bentonite grout) is required for amplitude studies, it is noted that diffraction causes a bias towards higher values in velocity studies. Two surveys are described where cross-hole compressional waves have been used to investigate the foundation conditions under domestic dwellings in response to subsidence. The results show that a wide range of velocities and signal loss is possible above the water table and that consideration of amplitudes enables areas of disturbed ground to be identified. Automatic data acquisition and processing has enabled a 3-D dataset to be generated using 16 pairs of boreholes and over 5000 individual seismic rays.

Evaluation of sediment heterogeneity using microresistivity imaging and X-radiography

Microresistivity imaging and X-radiography of 3-cm-thick sediment core slabs collected by divers from Eckernförde Bay, Germany, indicate subtle delineations and features created by hydrodynamic and biological processes within the top 20 cm of the sediment. Variations in the images of both techniques are controlled by the physical properties of the sediment. The X-radiographs record attenuation by solids throughout the 3-cm thickness of the sediment slab, while microresistivity images depend on pore water in terms of amount, salinity, and distribution. The microresistivity method is shown to detect layered structures and the presence of shells within the highporosity sediment.

Rapid non-contacting resistivity logging of core

Abstract We demonstrate a non-contact approach to whole-core and split-core resistivity measurements, imaging a 15 mm-thick, dipping, conductive layer, producing a continuous log of the whole core and enabling the development of a framework to allow representative plugs to be taken, for example. Applications include mapping subtle changes in grain fabric (e.g. grain shape) caused by variable sedimentation rates, for example, as well as the well-known dependencies on porosity and water saturation. The method operates at relatively low frequencies (i.e. low induction numbers), needing highly sensitive coil pairs to provide resistivity measurements at the desired resolution. A four-coil arrangement of two pairs of transmitter and receiver coils is used to stabilize the measurement. One ‘coil pair’ acts as a control, enabling the effects of local environmental variations, which can be considerable, to be removed from the measurement at source. Comparing our non-contact approach and independent traditional ‘galvanic’ resistivity measurements indicates that the non-contact measurements are directly proportional to the reciprocal of the sample resistivity (i.e. conductivity). The depth of investigation is discussed in terms of both theory and practical measurements, and the response of the technique to a variety of synthetic ‘structures’ is presented. We demonstrate the potential of the technique for rapid electrical imaging of core and present a whole-core image of a dipping layer with azimuthal discrimination at a resolution of the order of 10 mm. Consequently, the technique could be used to investigate different depths within the core, in agreement with theoretical predictions.

High-resolution petrophysical characterization of samples from an aeolian sandstone: the Permian Penrith sandstone of NW England

Abstract The Penrith Sandstone is an orange/red, mainly homogeneous, friable rock made up of well-rounded, highly spherical quartz grains, often showing euhedral overgrowths of quartz. Sandstone samples from Stoneraise Quarry, NW England, exhibit a remarkable degree of rounding and very high sphericity, along with frosted textures typical of aeolian deposits. Chemically, the rock is predominantly SiO2 (>95%), with no evidence of carbonate cements. Quartz predominates with a small proportion (10%) of feldspar. The grain size across heterogeneous zones varies from very fine (100 µm) to coarse sand (700 µm). There is no evidence of the presence of clay minerals. Petrophysically, based on the measurements made in this study, the Penrith Sandstone is a typical clean sandstone characterized by moderate porosity (12%) and core-plug permeability (10−14−10−12 m2), and Archie ‘m’ exponents between 1.90 and 1.91, suggesting a reasonably clean ‘Archie’ rock with no excess conductivity associated with clays or bound water. Capillary pressure curves for four samples demonstrate unimodal pore-size distributions with a single modal range that varies between 25–50 and 70–80 µm. Because of the relative simplicity of its petrophysics, the sandstone is thus potentially very useful in fundamental studies, and also in the trialling of new techniques. We use imaging techniques to investigate the degree of heterogeneity and the fabric of the Penrith Sandstone. Conventional optical images are complemented by electrical resistivity, porosity and mini-permeametry images. These two-dimensional maps of resolution of approximately 5 mm show a spatial similarity determined by the rock fabric. The detailed images show a wider degree of variation and heterogeneity than the plug-averaged values. The success of the resistivity imaging method suggests that the technique could be used in deriving correlations that could be used to interpret borehole resistivity imaging logs. However, in the present study, correlations of property values derived from the imaging do show considerable scatter: this suggests that heterogeneity even below the scale of the imaging is also important, a conclusion supported by thin-section and electronmicroscope data.

Fracture mapping with electrical core images

Abstract Naturally fractured reservoirs often contain a range of different fracture types and networks; fractures that are relatively permeable and relatively impermeable, unconnected and connected to the part of the fracture network that carries fluid flow, and naturally occurring or drilling induced. Consequently, in terms of their fluid connectivity, fractures may be open or closed, while individual fractures may be isolated or well connected. We have adapted our approach to imaging sedimentary fabric in the laboratory, where we related electrical core images to properties such as porosity, permeability, grain size and cementation, to enable electrical imaging of fractures in core. Our approach uses similar principles to those employed in down-hole electrical imaging. The results demonstrate an ability to image conductive fractures in fully saturated low-porosity water-bearing core: these fractures being electrically connected from the flat measurement surface through to the outer surface of the core. Published results for numerical modelling of down-hole electrical imaging tools show the electrical response is related to fracture depth and fracture aperture. Our experimental results on fractured core in the laboratory support these numerical observations, increased current flowing into the fracture as the aperture increases. The finite size of the electrode, however, means that this technique cannot distinguish between a single fracture and smaller groups of fractures intersecting the electrode.

Measurement scale and formation heterogeneity: effects on the integration of resistivity data

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.

A non-contacting resistivity imaging method for characterizing whole round core while in its liner

Abstract Recent laboratory experimentation has shown that non-contacting whole-core resistivity imaging, with azimuthal discrimination, is feasible. It has shown the need for very sensitive coil pairs in order to provide resistivity measurements at the desired resolution. Independent high-resolution ‘galvanic’ resistivity estimations show the ‘non-contacting’ measurements to be directly proportional to the resistivity of core samples. The response of the technique to a variety of synthetic ‘structures’ is presented. A whole-core image of a dipping layer is used to demonstrate the three dimensional response of the technique and to show that the resolution of the measurements is of the order of 10 mm. Experiments are described which show that the technique is capable of investigating to different depths within the whole round core. The results agree with theoretical predictions and indicate that the technique has the potential to assess invasion near the surface of the core. The technique is intrinsically safe and has the potential to be packaged in a form that would be suitable for whole-core imaging at the well site, or laboratory, without taking core from their liners. Thus it is possible to acquire information crucial for core selection, in addition to acquiring resistivity data at a resolution not too far removed from that of the downhole imaging tools.