Peter Styles

Felt seismicity associated with shale gas hydraulic fracturing: The first documented example in Europe

We describe the origin of felt seismicity during the hydraulic fracturing of the Carboniferous Bowland Shale at the Preese Hall 1 exploration well near Blackpool in the UK during 2011. The seismicity resulted from the interaction of hydraulic fracturing and a fault, the location of which was unknown at the time but has subsequently been located and does not intersect the well borehole. Waveform cross correlation is used to detect 50 events in the sequence. A representative hypocenter and strike-slip focal mechanism is calculated using the best recorded seismic event. The hypocenter is calculated to lie 300–400 m east, and 330–360 m below the injection point and shown to lie on a fault imaged using 3-D seismic at a depth of about 2930 m. The 3-D survey shows that not only the event hypocenter but also the focal mechanism correlates strongly with a subsequently identifiable transpressional fault formed during the Late Carboniferous (Variscan) basin inversion.

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.

The use of microgravity for cavity characterization in karstic terrains

Microgravity is the interpretation of changes in the subsurface density distribution from the measurement of minute variations in the gravitational attraction of the Earth. As a technique, it is particularly suited to the investigation of subsurface structures, mapping of geological boundaries and, most importantly in this case, the location and characterization of voids or cavities. Gravity variations due to the geological/petrophysical changes associated with fracturing and changes in pore composition are superimposed upon much larger variations due to elevation, latitude, topography, Earth tides and regional geological variations. However, these external changes can be modelled or monitored with sufficient accuracy to be removed from the data. With the recent development of high-resolution instruments, careful field acquisition techniques and sophisticated reduction, processing and analysis routines, anomalies as small as 10 microgal can be detected and interpreted effectively. This paper describes the ‘state-of-the-art’ application of the microgravity technique for the detection and characterization of karstic cavities in a variety of limestone terrains, including the Carboniferous Limestone of the United Kingdom and Eire and the coral limestones of the Bahamas. The case study examples show how the recorded gravity anomalies have revealed the location of density variations associated with underground cave systems and, ultimately, provided information on their depths, shapes and morphology from a combined analysis of their spectral content, characteristic gradient signatures and modelling responses. In addition, mass deficiencies have been estimated, directly from the anomaly map, by the use of Gausss theorem without any prior knowledge of the exact location, or nature, of the causative bodies.

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.

A new gravity map of southern Chile and its preliminary interpretation

Un nuevo mapade la gravedad en el sur de Chile y su interpretacion preliminar. Se han recolectado datos de gravedad en el sur de Chile en la region continental frente al Punto Triple Nazca-Sudamerica-Antartica. El objetivo de este proyecto fue investigar los efectos de la subduccion de una dorsal bajo el margen continental. El mapa de anomalias de Bouguer muestra una disminucion regional de ellas hacia el este, correspondiendo con un engrosamiento de la cuna mantelica, un gran bajo circular de ca. 60 mGal de la anomalia de Bouguer en la vecindad del lago General Carrera y una tendencia linear en el borde occidental de la cordillera Andina Principal a lo largo de la zona de Fallas Liquine-Ofqui. Se utilizo la desconvolucion de Euler para determinar la profundidad del cuerpo causativo principal, el que yace a profundidades subcorticales (ca. 65 km). La anomalia yace cerca de la posicion predicha para los segmentos subducidos de la Dorsal de Chile y los autores sugieren que la anomalia esta, por lo tanto, relacionada con el proceso de subduccion de dorsales

Thermal and dynamic modeling of deep subduction of a spreading center : Implications for the fate of the subducted Chile Rise, southern Chile

Our objective is to determine whether the Chile Rise spreading center, currently subducting south of the Chile Triple Junction in southern Chile, continues or ceases to separate after subduction. A two-dimensional kinematic thermal model is used to predict the temperature history of the subduction zone during spreading center subduction. Density anomalies, calculated from the temperature fields of the thermal model, are then used to define body forces that drive the motion of a dynamic viscous fluid model of spreading center subduction. Viscous models that include only local in-plane driving forces predict that the Chile Rise spreading center ceases to separate after ridge subduction. If global in-plane plate forces are also included in the viscous model, continuing separation of the subducted Chile Rise is predicted, but at a greatly diminished rate. Fully continuing separation of the subducted Chile Rise only occurs if the subducted Nazca and Antarctic plates remain rigid, allowing the lateral transmission of global plate forces from along strike; however, this situation is unlikely as young subducted plates are believed to fragment. Continuing separation but at a greatly diminished rate is believed to be the most plausible hypothesis for spreading center subduction in southern Chile. Dynamic viscous modeling of subduction in the central Andes suggests a viscosity of 2.5 x 10 20 Pa s for upper mantle above the 400 km phase transition.

A Case Study of Low Frequency Noise Assessed Using DIN 45680 Criteria

This paper describes a case study in which low frequency noise was suspected of causing disturbance in a semi-rural location close to an industrial estate. Previous attempts using conventional acoustic measurement techniques to resolve the case, or even prove the existence of a real acoustic problem, had proved unsuccessful. In the present study, the authors applied a novel integrated acoustic/microseismic measurement system, and assessed the resultant data using criteria from the German national standard DIN 45680. Using this approach, the authors successfully resolved the low frequency noise problem and, after a test involving a sequential shutdown at a suspect industrial site, established the precise cause of the disturbance. The paper thus supports the criteria in DIN 45680 as a predictor of annoyance due to low frequency noise and as an aid in resolving such problems. It also illustrates the flexibility of the combined acoustic/microseismic technique and the advantages of the method over conventional techniques.

Perceptions of the Public of Low Frequency Noise

This paper describes the results of a series of measurements using a variety of systems comprising vibration sensors and sound-level measuring devices. These were carried out at the homes of people experiencing the effects of low frequency noise and in other environments where low-frequency noise was likely to be generated. Suggestions are made as to the possible reasons for the perception of low-frequency noise by sufferers.

Simultaneous estimation of shape factor and depth of subsurface cavities from residual gravity anomalies using feed-forward back-propagation neural networks

We develop a new method of using feed-forward back-propagation (FFBP) neural networks to simultaneously estimate shape factor and depth of gravity anomalies. The advantages compared to neural network methods are the following: no pre-assumptions are made on source shape, the FFBP neural network estimates both depth and shape factor of source bodies and, once trained, works well for any new data in the training space, without repeating the initial calculations.The optimum number of neurons in the hidden layer was achieved with a novel multi-start algorithm. The FFBP model after training with suitable data sets and testing with different levels of noisy data is more robust than non-linear least squares minimization methods, especially for data with higher noise levels.The FFBP was tested for two sets of gravity field data over a major container terminal at Freeport, Grand Bahama, and a cavity anomaly at the Medford site, Florida, USA. The estimated parameters of the cavities agree well with the actual values.

Depth Estimation of Cavities from Microgravity Data through Multi Adaptive Neuro Fuzzy Interference System

We aim to estimate the depth of subsurface cavities from gravity data by a new method through a Multiple Adaptive Neuro Fuzzy Interference System (MANFIS); this method is an intelligent way to interpret microgravity data and gain an estimation of depth and shape of the most probable cavities. The MANFIS model was trained for two main models of cavities: sphere and cylinder in the related domains of radius and depth. We tested different MANFIS’s with different number of rules and obtained the optimum value for number of in the hidden layer. Then it was tested in the presence of 20% Gaussian noise and showed good robusnesst to noise. The method was also tested for real microgravity data from Bahamas Free Port. The results are in good agreement with ground-thruthed drilled values for the depth of subsurface cavities.