S.M. Toon

Seismic emissions from a surging glacier: Bakaninbreen, Svalbard

Abstract Bakaninbreen is a polythermal glacier in southern Spitsbergen, Svalbard, which surged between 1985 and 1995. For 9 days in spring 1987, when the surge front was travelling at ∼2.5-3.0 m d–1, three single-component geophones and two accelerometers were deployed in a T-shaped array immediately downstream of the surge front to record seismic emissions. The events were characterized by their waveforms and spectral content. At least three different categories have been identified: impulsive P- and S-waveforms, surface P- and S-wave trains, and harmonic (75-130 Hz) events. We interpret the impulsive events to originate at the base of the glacier, at or downstream of the surge front; the surface P- and S-wave trains from near-surface brittle fracture associated with the surface expression of the surge front itself; and the harmonic events from deep sources that involve resonance in a water-filled fracture, associated with the base of the surge front. We believe the basal events are related to the activation of stagnant ice downstream of the surge front, which allows water to access the bed and provides the mechanism for its propagation.

Microgravity as a tool for the detection, characterization and prediction of geohazard posed by abandoned mining cavities

The presence of mining-related cavities or karstic features in the rock mass and their actual or potential collapse pose a severe geohazard and a range of subsidence-related problems for both current and future users of that land. Cavities constitute a hazard to both development and redevelopment as their migration to the surface, as sinkholes or fractured and disturbed ground, may seriously damage property and services, and in severe and catastrophic failure, cause potential significant loss of life. The most common natural targets in karst environments are solution-related features such as voids, extended cavern systems, and the collapse/drainage features associated with swallow holes (or sinkholes). Manmade cavities, including mine workings, shafts and tunnels, are just as hazardous and can be even more prevalent than natural features, particularly in industrialized environments. Prior to the development (or redevelopment) of a site, the most common method of site investigation has been to drill an extensive pattern of boreholes over the target area in an attempt to locate and then define the spatial extent of any cavities. Indirect techniques such as geophysics can give a cost-effective, non-invasive method of cavity delineation with targeted drilling used as a verification tool rather than a primary search technique. The existence of a cavity alters the physical state of the strata and results in a contrast between the cavity and the host stratum that can be detected using suitable geophysical methods if the contrasts are large enough and the features are of a sufficient size (McDowell, 2002). Microgravity involves measuring minute changes in the gravitational pull of the Earth and interpreting the presence of subsurface density variations, such as those produced by voids and cavities, from an analysis of these readings. A cavity usually has a lower density than the surrounding material and may be filled with water, sediment, collapse material, or a mixture of all of these. A void therefore represents a mass deficiency in the subsurface and a very a small reduction in the pull of the Earth’s gravity is observed, which is called a negative gravity anomaly. Although the method is simple in principle, measurement of the minute variations in the gravity field of the Earth to a few parts per billion requires the use of highly sensitive instruments, strict data acquisition procedures, stringent quality controls, careful data reduction, and sophisticated digital data analysis techniques in order to evaluate and interpret the data. These gravity anomalies are superimposed onto much larger variations produced by elevation, topography, latitude earth tides, and regional geological variations and are, usually, almost undetectable by conventional gravity investigations. Microgravity surveying has developed considerably over the last 10 years with the development of modern, high resolution instruments, careful field acquisition procedures, sophisticated data reduction methods, and advanced analysis techniques. Qianshen (1996) presents a thorough review of the fundamentals of the microgravity technique although interpretation in particular has developed significantly since then. It is now possible to detect and interpret anomalies as small as 10 microgal with a repeatability of a few microgals. Not only can the isolated anomalies reveal the location of mines, caverns and voids, either natural or man-made, but they also provide information on their depths, shapes and morphology. Through the use of Euler deconvolution and Gauss’s theorem, the topology and the ‘missing mass’ associated with the void can be calculated in order to provide vital information for the development of remediation strategies and, ultimately, the costs associated with cavity filling. Through the targeted use of repeated post-remediation microgravity surveys, assessments can be made on the success, or not, of the remediation process and help verify the location and distribution of materials used to fill the void space. These attributes have led to the method becoming widely used in hydrogeological, engineering and geotechnical investigations with the significant advantage of leaving the ground completely undisturbed. Conventional site investigation techniques, nowadays sometimes guided by laser cavity scanning, are then employed as directed by the microgravity results to verify the areas deficient in mass. Emsley et al. (1992) and Bishop et al. (1997) describe the application of the microgravity method in the detection of both karstic and man-made cavities and also describe how the resulting data can be enhanced by image processing to better define the anomalies associated with the targets. This paper describes two detailed applications of the microgravity technique for the delineation of mining-related geohazards, the first in a currently operational open-cut gold mine at Kalgoorlie in Western Australia, and the second for the detection of historic chalk mining in the United Kingdom which caused the collapse of the main A2 trunk road into central London in 2002. Both required detailed terrain corrections to be made, in the first case for the effects of the main open cut workings and, in the second, for the influence of surrounding buildings as well as topography. The methods by which these are calculated are very different for the two different environments but are essential if interpretation of small-amplitude subtle anomalies is to be made.

Training the next generation of near-surface geophysicists: team-based, student-led, problem-solving field exercises, Cumbria, UK

Discussions with employers of graduate applied geophysicists (reinforced by recent literature) indicate a progressive reduction in the numeracy and literacy of graduating students. In particular, there is a perception that problem-solving and quantitative analysis skills are not being gained during university studies, which could be partly attributed to an emphasis on classroom lectures and timetable constraints rather than research-informed and active learning in the field. This paper provides a pedagogic overview of a Masters level, student-led residential field exercise in the Lake District, Cumbria, UK that has run for eight years. The valley has complex glaciated bedrock buried by recent sediments, which poses a challenge for students to recognize, understand and quantify in three dimensions. Participating student ‘companies’ are set a competitive task to win a contract for a full geophysical valley survey to determine the route of a gas pipeline. Students initially complete a desk study, collating available multi-disciplinary (geology, remote sensing and geotechnical) data sets. The student-led field exercise then acts as a geophysical reconnaissance mission, with teams mapping depth to bedrock and estimating extents of any coastal salinity incursions. Full costings are produced to simulate a real work contract and the successful company is awarded the ‘contract’, based on ‘client’ presentations on the final day of the exercise. Comments on the student learning outcomes are provided, including employability skills in team working, problem-solving, quantitative data analysis, project and budget management and client presentation skills. Recent student evaluations are discussed with very positive comments from graduate geophysicists who have entered related employment emphasizing how the exercise has prepared them for the workplace.

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.

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.

Microseismic Investigations of Low Frequency Vibrations and Their Possible Effects on Populations

Environmental low frequency noise and vibration (or “hum”) is a growing annoyance and potential health hazard to many people, according to surveys conducted by the UKs Low Frequency Noise Sufferers Association. This paper describes attempts to measure the “hum” using microseismic methods. Sensitive three-component seismometers and high resolution digital dataloggers were employed in the homes of “hum” sufferers, and three-dimensional vibrational data were recorded overnight. Frequency analyses were then performed on the time series data using Fast Fourier Transform methods. In one case study, a clear signal was observed corresponding to a fundamental vibrational frequency peak of 10 Hz. At least one harmonic frequency peak (30 Hz) was also apparent. It is suggested that local industry of some kind (with machinery running overnight at an operating frequency of 10 Hz) may be responsible for this ground-borne disturbance. In another house occupied by “hum” sufferers, no notable ground velocities were recorded overnight with the seismometer mounted on ground floor concrete foundations. Significant levels of vibration were only observed in an upstairs bedroom, by placing the instrument on floorboards which appear to be excited by acoustic waves in the room. It is inferred that in this case the “hum” is predominantly air-borne; again, local industry is a likely source. Low frequency “hum”, both acoustic and ground-borne, is a very real problem which causes sufferers great distress. It is possible to measure the “hum” and determine its frequency characteristics using microseismic techniques. Urgent revision of current legislation is needed to limit industrial output levels of low frequency noise/vibration, especially at night time.

Comparing Shear Wave Refraction and Continuous Surface Wave Surveys in Sand and Gravel

The aims of this study were to evaluate some surface wave based methods and their limitations with regard to aggregate variability and thickness determinations. We compared the results of field assessments of sand and gravel sequences using two different surface wave survey approaches. The first, followed a seismic refraction approach, and the second, a CSW survey methodology. Further probing using an ultra-lightweight cone penetrometer provided verification of results, and also, an active extraction programme at the field site provided the opportunity to directly observe the subsurface geology post-survey.

Surface and borehole microseismic monitoring of mining-induced seismicity

Abstract To determine whether 130 felt earth tremors around Edwinstowe, Nottinghamshire, UK, which also experienced severe surface fissuring, were caused by coal extraction, a surface seismometer array was established around Thoresby Colliery. Over the next year, 785 microseismic events were detected. The spatial and temporal variations in seismicity are clearly associated with the commencement, continuing extraction and closure of faces. Of particular note are events which locate at the surface and appear to be related to the active fissuring. Events occur within days of commencement of production and cease when production finishes, with good correlation between face advance and hypocentral position. Naturally occurring microseismic events have also been detected up to 1 km ahead of active longwall faces in the Midlands using triaxial geophone packages grouted into the seam together with a surface seismometer in the top of the borehole. The quality of these data was very high and guided waves can clearly be seen with the dispersive characteristics associated with seam waves. In one experiment, more than 2000 events were detected in only two days of monitoring even in a relatively noisy surface environment. This paper demonstrates how useful locations can be generated using three-component digital data from only one borehole. The event distributions give a dynamic, three-dimensional image of the developing patterns of fracturing above, below and ahead of the longwall face with important implications for roof control, subsidence prediction and gas migration.