Geotechnical and Geological Hazard (Geohazard) studies are an essential part of any major infrastructure project whether it is in the onshore, nearshore or offshore environment.
The challenge is to quantify and image the physical properties and geological stratigraphy such that the uncertainty about the natural environment can be reduced to an acceptable level. A higher understanding of soil parameters or natural hazards can enable one to confidently extrapolate away from sampling locations.
Consequently, quality soil/sediment or rock samples are highly valuable not just from a monetary perspective from the cost of acquisition, but also from a project perspective. Geotek’s non-destructive core logging (MSCL) and imaging (X-ray) are therefore the perfect techniques to maximise the data recovered from this precious resource. These can be used to identify good and poor quality samples and to determine, at a cm-scale resolution, the engineering geological stratigraphy directly from the core samples ahead of opening them, or on archived material.
Any geotechnical or geohazard study can benefit from non-destructive geophysical logging using Multi-Sensor Core Logger (MSCL) technology. MSCL data acquisition simultaneously provides a suite of continuous downcore physical and geochemical properties at cm-scale resolutions typically through plastic or metal core liners, or on the core surface. The data provide a quantified geological stratigraphy that can be unitised and lithology interpretations extracted guiding sub-sampling and visual logging. The data also help to improve descriptions and reduce the re-work of additional sub-sampling for future laboratory testing programmes. The high resolution of MSCL data and non-destructive nature enable an assessment of core disturbance, identifying areas of increased disturbance. More importantly, the data can identify areas of good quality sample for advanced laboratory testing thus preventing expensive testing programmes of poor quality samples being run.
Similar to cone penetration testing (CPT), the continuous data acquired by the MSCL gives more validity and meaning to geotechnical descriptions. This enables engineers or geologists to more adequately describe the structure or heterogenous nature of the material, such as the identification of low or high density laminae, their composition, their spacing and thickness, and whether the contacts of the laminae are sharp or gradational. Geochemical properties such a natural gamma, magnetic susceptibility or X-ray fluorescence (XRF) can then be used to correlate units between core locations. This enables positive identification of condensed or extended sequences for age dating, or improving confidence in the extrapolation of physical properties between core locations such as density or P-wave velocity, both of which are good indicators of sediment shear strength.
MSCL data are typically acquired either in the field or just after fieldwork in the laboratory. Field-based MSCL core logging provides a rapid assessment of sample quality and of the engineering geological stratigraphy that can be fed directly to the offshore engineers, geologists, operators and client representatives allowing them to make informed engineering, geological, or operational decisions in real time.
Laboratory-based studies are usually conducted just before the main geotechnical, or sedimentological laboratory programmes, equipment is sent to our client laboratory, or conducted at Geotek’s specialist core logging laboratory in the UK for analyses. Data are then fed to our clients in batches for analysis and to run geotechnical laboratory programmes in tandem.
The use of X-ray imaging techniques such as 2D radiography or Computed Tomography (CT) have long been recognised in the fields of soil mechanics and sedimentology (ASTM D4452-14). X-ray techniques image differences in density through sediment or rock samples often being able to identify features that are not visible with the human eye, or they can be discerned using other more conventional logging and testing techniques. There are two deliverables for X-ray imaging for geotechnical and geohazard projects: 1) Multi-angle 2D transmissions and 2) 3D Computed Tomography (CT). Multi-angle 2D X-ray transmissions, otherwise known as radiographs, are fast and efficient X-ray core slabs, which provide 2D information about the natural and artificial features within the core samples such as laminae or fractures. X-ray CT datasets provide axial slabs through the core samples, which can then be reconstructed into a 3D volume and processed into core slabs in any direction. X-ray CT is often acquired on smaller sub-samples such as waxed sub-samples or at areas of interest within core samples.
Geotek’s X-ray instruments are specifically developed for handling a range of core materials including unconsolidated sediment cores in plastic, aluminium or steel liners providing image resolutions of 20 μm to 150 μm per pixel.
The high resolution, accurate core motion and flexible geometry set-up provides our clients with an unparallelled ability to obtain highly detailed visualisations of the quality of core samples. These can include: disturbance, U-shaped pull down, remoulding, undercut core, swelling fractures, gas exsolution, drilling/sampling induced fractures, as well as natural features such as: fine-scale lamination, bedding contacts, cementation profiles, porosity, sand versus clay, fractures, faulting, gravels, authigenic precipitates, bioturbation, and much more.
Geotek’s X-ray radiography and CT equipment enables engineers and geologists to make informed decisions about sub-sampling or sample suitability for expensive geotechnical testing, sediment age dating, or mineralogical testing; and often provides valuable additional information about the natural structure of sediment cores that may have otherwise gone unknown, and hence the uncertainty about the sediment would remain.
The reduction in uncertainty from being able to adequately resolve the natural structure of geological samples is exceptionally important for geohazard studies. Often small scale (mm to cm) sedimentary structures can be missed by obliquely splitting core samples to dipping beds, or not being large enough or visually different enough to describe visually. Ahead of core splitting, X-ray imaging can be used to orientate cores to ensure that key sedimentological structures are not missed during logging. The identification of features such as cross-bedding or sharp erosive contacts might significantly change the interpretation of a geohazard such as a turbidity current, which will affect the geohazard risk inventory and ultimately the geohazard assessment.
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