Jonathan P. Busby

The measured shallow temperature field in Britain

Abstract The move towards a lower carbon society is likely to lead to a greater utilization of geothermal heat as the UK meets the challenge of its EU renewable obligation to source 15% of its energy from renewables by 2020. The shallow temperature field can be estimated, but measured temperatures are the most accurate approach for defining the shallow heat resource. Available measured temperature data have been used to compile maps of temperatures at depths below ground level of 100, 200, 500 and 1000 m. From these data regional trends and anomalies have been defined. From a consideration of all of the data a geothermal gradient of 28 °C km−1 has been calculated for the upper 1 km of the sedimentary crust, which is slightly above the previously quoted value of 26 °C km−1. Elevated temperatures have been mainly observed in eastern and southern England and have been attributed to convection within some of the thicker Permo-Triassic sandstones and the thermal blanketing effect of Triassic and Jurassic argillaceous rocks. Some of the depressed temperatures are associated with thick sequences of Carboniferous arenaceous rocks.

Geothermal exploration in the Fell Sandstone Formation (Mississippian) beneath the city centre of Newcastle upon Tyne, UK: the Newcastle Science Central Deep Geothermal Borehole

The postulate that geothermal energy might be recoverable from strata laterally equivalent to the Fell Sandstone Formation (Carboniferous: Mississippian) beneath Newcastle upon Tyne has been examined by the drilling and testing of the 1821 m deep Newcastle Science Central Deep Geothermal Borehole. This proved 376.5 m of Fell Sandstone Formation below 1400 m, much of which resembled braided river deposits found at outcrop, although some lower portions were reddened and yielded grains of aeolian affinity. Downhole logging after attainment of thermal equilibrium proved a temperature of 73°C at 1740 m, and allowed estimation of heat flow at about 88 mW m−2. This relatively high value probably reflects deep convective transfer of heat over a distance of >8 km from the North Pennine Batholith, along the Ninety Fathom Fault. The Fell Sandstone traversed by the borehole proved to be of low hydraulic conductivity (c. 7 × 10−5 m d−1). The water that entered the well was highly saline, with a Na–(Ca)–Cl signature similar to other warm waters encountered in the region. It remains for future directional drilling to establish whether sufficient natural fracture permeability can be encountered, or wells stimulated, to support commercial heat production.

A new high-resolution aeromagnetic dataset over central Ayrshire: insights into the concealed geology

Synopsis High-resolution aeromagnetic data have been acquired over central Ayrshire as part of a multi-component environmental and resource survey. The data were recorded along easterly orientated flight lines at 200 m separation and show a very marked improvement in resolution over the pre-existing aeromagnetic data for the region. The distribution of the extensive volcanic and intrusive igneous rocks in the near sub-surface is defined more accurately, many new features are recognized and features distinguished in the pre-existing aeromagnetic data are now seen to be laterally continuous. Northwesterly trending lineaments are clearly defined and most can be attributed to Palaeogene dykes, the majority of which are reversely magnetized. Depth to source solutions and modelling suggest that even though the dykes have a limited width at outcrop, their widths increase with depth. Permo-Carboniferous easterly trending dykes are not well resolved in the new aeromagnetic data. This is most likely due to a low amplitude resultant magnetic vector that, combined with the easterly-orientated flight lines, results in few large anomalies. One significant northwesterly trending lineament is interpreted as the trace of the Cleveland Dyke across the Midland Valley. These new data should be of value to the extractive and water industries and assist in the planning and sustainable management of these resources.

An assessment of the ability to derive regional resistivity maps from geological mapping data

Abstract There is a requirement to understand the electrical resistivity structure of the near subsurface (i.e. the upper 10 m). This is the zone into which infrastructure is buried and electrical systems are earthed. Detailed resistivity surveys are carried out for site-specific purposes, but there is a lack of regional data. A synthetic resistivity map has been generated by assigning average intrinsic resistivity values to the superficial and bedrock geology and producing an average resistivity for the top 10 m using the superficial thickness as the weight. To test this approach the synthetic map has been compared with the measured resistivity arising from a high-frequency airborne electromagnetic survey over the Isle of Wight. Many general features of the synthetic and measured maps are in agreement, but some of the resistivity assignments are oversimplified. A revised synthetic map that takes into account the position in the landscape of the geological units and with revised resistivity ranges informed from the airborne survey has been generated that represents a good first approximation of the near-surface resistivity structure. A scheme for generating synthetic maps in the absence of measured airborne data is indicated.

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.

Keeping warm: a review of deep geothermal potential of the UK:

In 2015, the primary energy demand in the UK was 202.5 million tonnes of oil equivalent (mtoe = 848 EJ). Of this, about 58 mtoe (2.43 EJ) was used for space heating. Almost all of this heat was from burning fossil fuels either directly (50% of all gas used is for domestic purposes) or indirectly for power generation. Burning fossil fuels for heat released about 160 million tonnes of carbon dioxide in 2015. The UK must decarbonise heating for it to meet its commitments on emissions reduction. UK heat demand can be met from ultra-low-carbon, low enthalpy geothermal energy. Here we review the geothermal potential of the UK, comprising a combination of deep sedimentary basins, ancient warm granites and shallower flooded mines. A conservative calculation of the contained accessible heat in these resources is 200 EJ, about 100 years supply. Presently only one geothermal system is exploited in the UK. It has been supplying about 1.7MWT (heat) to Southampton by extracting water at a temperature of 76 ℃ from a depth of 1.7 km in the Wessex Basin. Like Southampton, most of the major population centres in the UK lie above or adjacent to major geothermal heat sources. The opportunity for using such heat within district heating schemes is considerable. The consequences of developing a substantial part of the UK’s geothermal resource are profound. The baseload heating that could be supplied from low enthalpy geothermal energy would cause a dramatic fall in the UK’s emissions of greenhouse gases, reduce the need for separate energy storage required by the intermittent renewables (wind and solar) and underpin a significant position of the nation’s energy security for the foreseeable future, so lessening the UK’s dependence on imported oil and gas. Investment in indigenous energy supplies would also mean retention of wealth in the UK.