Stephen Pearson

An evaluation of combined geophysical and geotechnical methods to characterize beach thickness

Beaches provide sediment stores and have an important role in the development of the coastline in response to climate change. Quantification of beach thickness and volume is required to assess coastal sediment transport budgets. Therefore, portable, rapid, non-invasive techniques are required to evaluate thickness where environmental sensitivities exclude invasive methods. Site methods and data are described for a toolbox of electrical, electromagnetic, seismic and mechanical based techniques that were evaluated at a coastal site at Easington, Yorkshire. Geophysical and geotechnical properties are shown to be dependent upon moisture content, porosity and lithology of the beach and the morphology of the beach–platform interface. Thickness interpretation, using an inexpensive geographic information system to integrate data, allowed these controls and relationships to be understood. Guidelines for efficient site practices, based upon this case history including procedures and techniques, are presented using a systematic approach. Field results indicated that a mixed sand and gravel beach is highly variable and cannot be represented in models as a homogeneous layer of variable thickness overlying a bedrock half-space.

Monitoring coastal change using terrestrial LiDAR

Abstract The paper describes recent applications by the British Geological Survey (BGS) of the technique of mobile terrestrial Light Detection And Ranging (LiDAR) surveying to monitor various geomorphological changes on English coasts and estuaries. These include cliff recession, landslides and flood defences, and are usually sited at remote locations undergoing dynamic processes with no fixed reference points. Advantages, disadvantages and some practical problems are discussed. The role of GPS in laser scanning is described.

Switching flow patterns within the last ice sheet in northern Scotland

Synopsis A confluence zone existed within the northern sector of the last British Ice Sheet (BIS) where ice flowing from the Northern Highlands met ice streaming out of the Moray Firth and across the plain of Caithness. At Wester Clett, a complex 35m thick sequence of glacigenic sediments provides important evidence of dynamic interactions between these ice lobes that are consistent with recent computer simulations of the behaviour the last BIS. Sedimentological evidence suggests that early advance of Northern Highlands ice, probably between 33.5 and 30.5 ka, was followed by a first incursion of Moray Firth ice. Ice flowing out of Strath Halladale then deposited sands and gravels along its margin. Later, around the time of the Last Glacial Maximum, a powerful flow of Moray Firth ice diverted Northern Highlands ice to flow to the NW across the present coastline, depositing tills of mixed provenance. Cosmogenic exposure dates of c. 18 ka for summits in southern Caithness indicate that the ice sheet thinned subsequently, but flow to the NW continued. The youngest till and its associated moraine systems record a final movement of Northern Highlands ice. Subsequent ice retreat was accompanied by uncoupling of ice lobes at the Caithness–Sutherland border soon after 16 ka.

A GIS TOOL FOR ANALYSIS AND INTERPRETATION OF COASTAL EROSION MODEL OUTPUTS (SCAPEGIS)

The SCAPE (Soft Cliff and Platform Erosion) model of cliff toe retreat, and a cliff-top recession model, have been linked with a new flexible GIS tool (SCAPEGIS) to provide visualisation and analytical capability for the model results. 45 model runs exploring different sealevel rise and wave climate scenarios and protection choices are available. Outputs are available in the form of maps, dynamic visualisation, and descriptive statistics of key parameters such as cliff toe and cliff top position. It also allows analysis with other datasets such as land use and building location for impact evaluation, and hence supports shoreline management and cliff-top land use planning. Some preliminary results and ideas for further development are presented.

Comparison of surface wave techniques to estimate shear wave velocity in a sand and gravel sequence: Holme Pierrepont, Nottingham, UK

This study evaluated the application of surface wave methods to aggregate variability and thickness determinations. We compared the results of field assessments of sand and gravel sequences using three surface wave survey approaches. The first was a seismic refraction approach, the second, a continuous surface wave (CSW) survey approach, and the third adopted a multi-channel analysis of surface waves (MASW) technique to the original refraction field set-up and records. The sand and gravel sequences were highly heterogeneous and the shear wave profiles were not normally dispersive (i.e. did not exhibit a monotonic increase in velocity with depth), which had a significant effect upon the performance of the three field approaches. Both CSW and MASW approaches provided information over a broad spectrum from which velocity–depth profiles were produced, but the upper frequency of operation was limited in both methods because of poorer signal quality at higher frequencies. Shear wave velocity profiles obtained using vertically vibrating sources during CSW surveys were different from profiles obtained using a horizontally polarized source in the refraction survey. This was attributed to different propagation paths and modes of propagation, which were illustrated via additional tomographic inversion of the refraction travel times but could also be attributed to data inversion methods. Probing using an ultra-lightweight cone penetrometer, continuous reflection profiling using ground-penetrating radar, and also an active extraction programme at the field site provided the opportunity to directly observe the subsurface geology and verify field results. Within the sand and gravel sequence, high-velocity layers were associated with matrix-supported coarse gravel lenses, some of which were weakly cemented. Localized high- and low-velocity zones within the underlying bedrock were interpreted as being related to lithostratigraphic heterogeneity and the development of an upper, weathered zone.

Geoelectrical Monitoring of Seasonal Moisture Content Changes in an Earth Embankment

Earth structures, such as embankments, require ongoing monitoring and maintenance to identify potential failure zones and to compensate for the effects of settlement. Extreme weather events leading to prolonged periods of desiccation or saturation are becoming more frequent and threaten embankment stability. In this paper we develop electrical resistivity tomography (ERT) as a non-invasive tool for characterising and monitoring earth embankments. A study is described in which ERT was applied alongside conventional intrusive techniques to investigate and monitor a section of Victorian Great Central Railway embankment, near Nottingham, UK. A number of modes of deployment were considered including linear 2D ERT arrays both parallel and perpendicular to the long-axis of the embankment and 3D imaging arrays. The resulting ERT images, when calibrated using intrusive geotechnical testing and core samples, revealed the spatial variability of the embankment soils. Parallel ERT sections were used to identify major discontinuities between material types at locations associated with poor track geometry. Perpendicular ERT sections also revealed significant internal heterogeneity, and were used to monitor seasonal changes in the moisture content within the embankment.

Bedrock Interface Detection for Sand and Gravel Mineral Reserve Assessment Using 3D ERT

A study describing the use of 3D ERT to determine depth to bedrock, and hence mineral thickness, at a sand and gravel extraction site within river terrace deposits is presented. Two approaches are considered which automatically extract interface depths from 3D ERT models. The first assumes that the interface is located at the maximum slope of the resistivity-depth curve, and is therefore referred to as the ‘steepest gradient method’ (SGM). The second uses an intrusive sample point to calibrate the model by identifying the resistivity iso-surface associated with the interface, and is referred to here as the ‘known interface method’ (KIM). The results of these two approaches have been tested against direct GPS observations of the interface position that were made after the bedrock had been exposed by quarrying of the river terrace deposits. In this case, the use of intrusive data for model calibration (i.e. the KIM) was essential for recovering accurate depth information from the 3D ERT model. Reliance upon using the steepest resistivity gradient as an indicator of a geological boundary (i.e. the SGM) produced a severe overestimate of interface elevation.