Monitoring and Modelling the Vibrational Effects of Small (<50 kW) Wind Turbines on the Eskdalemuir IMS Station

It is known (Styles et al., 2005) that windfarms generate low frequency vibrations which propagate through the ground and have the potential to adversely affect sensitive installations, most notably seismometer arrays set up to monitor for nuclear tests. Significant work on the effects of large wind turbines has been carried out by Keele University as well as by Schofield (2002) and Fiori et al (2009). For the Eskdalemuir International Monitoring System station in Scotland, a vibration threshold was set, for wind farms within 50 km of Eskdalemuir, at frequencies around the 4 to 5 Hz region. However, with increased development, the threshold is being approached and small wind turbines (less than 50kW), even of the order of 15kW have also been restricted despite the differences in scale and modes of vibration. In order to protect Eskdalemuir a threshold limit was set for any turbine as a holding measure and a programme to try to establish whether they were really problematic has been carried out. Models for two wind turbine types from the manufacturers Proven and Gaia-Wind have been calculated and measurement programmes carried out. It has been possible to demonstrate that in most cases these small turbines do not generate significant energy in the band of concern and that the levels are low enough to be negligible. Small turbines once evaluated and monitored by Keele University and given approval by the UK Ministry of Defence, will receive clearance for deployment around the Eskdalemuir site at distances greater than 10km

Long term time-lapse microgravity and geotechnical monitoring of relict salt-mines, Marston, Cheshire, UK.

The area around the town of Northwich in Cheshire, U. K., has a long history of catastrophic ground subsidence caused by a combination of natural dissolution and collapsing abandoned mine workings within the underlying Triassic halite bedrock geology. In the village of Marston, the Trent and Mersey Canal crosses several abandoned salt mine workings and previously subsiding areas, the canal being breached by a catastrophic subsidence event in 1953. This canal section is the focus of a long-term monitoring study by conventional geotechnical topographic and microgravity surveys. Results of 20 years of topographic time-lapse surveys indicate specific areas of local subsidence that could not be predicted by available site and mine abandonment plan and shaft data. Subsidence has subsequently necessitated four phases of temporary canal bank remediation. Ten years of microgravity time-lapse data have recorded major deepening negative anomalies in specific sections that correlate with topographic data. Gravity 2D modeling using available site data found upwardly propagating voids, and associated collapse material produced a good match with observed microgravity data. Intrusive investigations have confirmed a void at the major anomaly. The advantages of undertaking such long-term studies for near-surface geophysicists, geotechnical engineers, and researchers working in other application areas are discussed

Seismic monitoring and vibrational characterization of small wind turbines: A case study of the potential effects on the Eskdalemuir International Monitoring System Station in Scotland

This paper presents a case study of the ground-borne vibrations generated by small wind turbines. This is of particular importance for assessing the possible impact on the detection capabilities of the International Monitoring System seismic array at Eskdalemuir in Scotland. Measurements were gathered from a selection of small wind turbines (≤50 kW), from three different manufacturers, varying in hub height, power and construction using a range of accelerometers and seismometers fixed to the tower and buried in the adjacent ground at increasing distances. Previous studies have shown that medium and large turbines generate harmonic vibrations. Planning guidelines exist but have inadvertently led to a ban on the deployment of individual small-turbines within 50 km of the array. This study investigates whether their inclusion in these guidelines is necessary. Vibration levels on the tower are in the region of 10 -3 m/sqrt(Hz), are transferred into the ground and decay rapidly away from the turbine (10 -5 m/sqrt(Hz) in the ground at 200 m). The spectral content of the vibrations shows variations between the turbines, but each have peaks in the band of interest for Eskdalemuir. Further, sources of the peaks in the spectra are identified as either originating from the blades’ rotation or the resonant modes of the turbines. Analysis of the relationship between wind speed and seismic amplitude on the tower shows an exponential trend, individual to each wind turbine and the frequency band of interest. Similarly, analysis of the seismic amplitude between the tower and foundation shows that this can be described by a power law, but this is individual to each turbine. It is our opinion that there is little requirement to include turbines of 50 kW or less in planning guidelines outside the statutory exclusion zone of 10 km, but each turbine should be considered on an individual basis

Geophysical characterization of derelict coalmine workings and mineshaft detection: a case study from Shrewsbury, United Kingdom

A study site of derelict coalmine workings near Shrewsbury, United Kingdom was the focus for multi‐phase, near‐surface geophysical investigations. Investigation objectives were: 1) site characterization for remaining relict infrastructure foundations, 2) locate an abandoned coalmine shaft, 3) determine if the shaft was open, filled or partially filled and 4) determine if the shaft was capped (and if possible characterize the capping material). Phase one included a desktop study and 3D microgravity modelling of the relict coalmine shaft thought to be on site. In phase two, electrical and electromagnetic surveys to determine site resistivity and conductivity were acquired together with fluxgate gradiometry and an initial microgravity survey. Phase three targeted the phase two geophysical anomalies and acquired high‐resolution self potential and ground penetrating radar datasets. The phased‐survey approach minimised site activity and survey costs. Geophysical results were compared and interpreted to characterize the site, the microgravity models were used to validate interpretations. Relict buildings, railway track remains with associated gravel and a partially filled coalmine shaft were located. Microgravity proved optimal to locate the mineshaft with radar profiles showing ‘side‐swipe’ effects from the mineshaft that did not directly underlie survey lines. Geophysical interpretations were then verified with subsequent geotechnical intrusive investigations. Comparisons of historical map records with intrusive geotechnical site investigations show care must be taken using map data alone, as the latter mineshaft locations was found to be inaccurate

Multispin correlations and pseudo-thermalization of the transient density matrix in solid-state NMR: free induction decay and magic echo

Quantum unitary evolution typically leads to thermalization of generic interacting many-body systems. There are very few known general methods for reversing this process, and we focus on the magic echo, a radio-frequency pulse sequence known to approximately "rewind" the time evolution of dipolar coupled homonuclear spin systems in a large magnetic field. By combining analytic, numerical, and experimental results we systematically investigate factors leading to the degradation of magic echoes, as observed in reduced revival of mean transverse magnetization. Going beyond the conventional analysis based on mean magnetization we use a phase encoding technique to measure the growth of spin correlations in the density matrix at different points in time following magic echoes of varied durations and compare the results to those obtained during a free induction decay (FID). While considerable differences are documented at short times, the long-time behavior of the density matrix appears to be remarkably universal among the types of initial states considered - simple low order multispin correlations are observed to decay exponentially at the same rate, seeding the onset of increasingly complex high order correlations. This manifestly athermal process is constrained by conservation of the second moment of the spectrum of the density matrix and proceeds indefinitely, assuming unitary dynamics.Comment: 12 Pages, 9 figure

Incentives facing UK-listed companies to comply with the risk reporting provisions of the UK Corporate Governance Code

Recent changes made to the UK Corporate Governance Code require UK firms to report new or enhanced narrative information concerning their principal risks, their risk management processes and their future viability. This paper analyses whether the level and nature of voluntary compliance with these new requirements is consistent with alternative economic and political visibility incentives. We analyse relevant sections of financial reports produced by industry matched samples of large-, mid-and small-cap UK listed firms during the transitional 2013-14 financial reporting years. Both specific and generic readability attributes of the reports are measured. We find that virtually no firm in our sample has provided any viability statement. Empirical analysis of disclosures concerning principal risk assessment and review processes appear to be primarily motivated by political visibility reasons. Examples of particularly good and cases of poor corporate risk reporting practices are also discussed. Possible implications for the actuarial profession are discussed

Studies of Vibrations from Wind Turbines in the Vicinity of the Eskdalemuir (AS104) IMS Station

Styles et al (2005) describe an extensive microseismic and infrasound monitoring programme to characterise the low-frequency vibration spectra produced by wind turbines of various types, both fixed and variable speed. They demonstrated that small but significant harmonic vibrations (modal eigentones) of the towers, excited by blade passing, tower braking and wind loading while parked, can propagate many kilometres and be detected on broadband seismometers. This meant that protective measures were required to protect the IMS seismic monitoring station (EKA), located at Eskdalemuir in the Scottish Borders, UK. Over 2 GW capacity of wind turbines were planned for this region and planning restriction were imposed to control development because of potential effects on the IMS station. This work established that vibrations of concern in the 2 to 6 Hz band, while small, were critical for this monitoring. Propagation laws were derived and an aggregate vibration budget established which would not prejudice the operation of Eskdalemuir, to aid planning and permit appropriate wind farm development. Subsequently, further work has been carried out to determine if small wind turbines (<50kW) should be covered by the same restrictions as large turbines. The UK Ministry of Defence has now issued new guidelines that should allow small wind projects to be developed in the vicinity of Eskdalemuir after modeling and measurement confirms that they do not generate significant vibrations within the band of concern. The work may have relevance to other IMS sites where new windfarm developments are planned or already exist