sebastien Chastin | Glasgow Caledonian University (original) (raw)

Papers by sebastien Chastin

Journal of the Chinese Institute of …, Jan 1, 2002

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Oil companies are currently making multi-million dollar investments in time-lapse (4D) monitoring... more Oil companies are currently making multi-million dollar investments in time-lapse (4D) monitoring of producing reservoirs. These are interpreted as if moving OWCs and associated fluid and pressure variations are the only cause of changes in oil-field seismograms. We report an example of significant changes in seismic travel times for low-level seismicity at substantial distances. This implies that oil field time-lapse seismics are probably contaminated by remote seismic and volcanic disturbances.

… General Assembly, Nice, 21-26 April …, Jan 1, 2002

ABSTRACT We present clear observations of seismic velocity variations following comparatively dis... more ABSTRACT We present clear observations of seismic velocity variations following comparatively distant seismic activity and a consequently change in tectonic stress. The Stress- Monitoring Site SMSITES at Húsavík in Northern Iceland, described in paper SMS2 at this meeting, was used to monitor short-term variations in absolute seismic velocity. The survey took place between the 10th and 23rd of August 2001. The signal from the controlled source, a DOV (Downhole Orbital Vibrator), was transmitted repeatedly between two boreholes at 315m-offset along a single sub- horizontal direct ray path ray path at ~500m depth. The azimuthal direction between the wells is almost parallel to the strike of the Húsavík-Flatey Transform Fault (HFF) of the Mid-Atlantic Ridge, and is about 200m from the surface break. Source and receivers are in a 200m-thick layer of sandstone sandwiched between fractured layers. The DOV source was swept every 12s and the three-component recordings summed every 100 sweeps. This routine was repeated 24hours a day for 13days. Statistical analysis of the source signal stability indicates the source is coherent at 20µs and that velocity variations can be resolved at close to 1 part in 10-5. Variations in relative velocities of 2% to 5%, are observed peak to peak for Vp, Vsh, and Vsv. The behaviour of P-wave and S-wave velocities is strikingly different and correlates with a swarm of ~110

Geophysical …, Jan 1, 2003

Geophys. J. Int, Jan 1, 2004

Measurements of time-delays in seismic shear-wave splitting above small earthquakes typically dis... more Measurements of time-delays in seismic shear-wave splitting above small earthquakes typically display a scatter of often as much as ±80 per cent about the mean. Changes in the average timedelay appear to be related to changes of stress, but applications of this potentially powerful tool have been handicapped by the previously inexplicable scatter in time-delays above earthquakes. In contrast, measurements of shear-wave time-delays in controlled-source exploration seismics are typically well controlled and display little scatter. Previous estimates of possible causes of scatter cannot produce sufficient variation specifically above earthquakes. Here we show that 90 • -flips in shear-wave polarizations due to fluctuating high pore-fluid pressures on seismicallyactive fault planes are the most likely cause of the scatter.

Age and …, Jan 1, 2011

Background: recent evidence suggests that the interaction between periods of sedentary and activi... more Background: recent evidence suggests that the interaction between periods of sedentary and activity behaviour is important for health; providing distinctive information to assessment of activity alone. This study quantified activity and sedentary behaviour in older, community-dwelling adults. Methods: fifty-six community-dwelling older adults with an average age 79 (SD) years wore an ActivPAL accelerometer for 7 days and were assessed for a range of motor, cognitive and affective characteristics. Seven variables derived from accelerometry considered to represent four characteristics of habitual behaviour (volume, frequency, intensity and variability) were submitted to principal components factor analysis (PCA). Factor scores were retained and used as dependent variables in regression analysis. Results: three significant orthogonal factors emerged from the PCA, accounting for 80% of the variance in test scores: 'walking behaviour' which accounted for 39% of variance in the model; 'sedentary behaviour' explaining 24.3% of total variance; and 'postural transitions' which accounted for 16.7% of total variance. For the regression analysis, younger age and lower body mass index (BMI) emerged as significant predictors of physical behaviour, explaining 36% of the total variance. For postural transitions, lower BMI was the unique contributor, explaining 15% of total variance. Significant predictors of sedentary behaviours were not identified. Conclusions: walking, sedentary and transitory behaviours are distinct from each other, and together explain daily function. Further research on a larger sample is indicated to explore the characteristics that explain these behaviours, in particular the interplay between sedentary behaviour and periods of physical activity.

European journal of …, Jan 1, 2009

Chastin, Sebastien F.; Dall, Philippa M.; Tigbe, William W.; Grant, P. Margaret; Ryan, Cormac G.;... more Chastin, Sebastien F.; Dall, Philippa M.; Tigbe, William W.; Grant, P. Margaret; Ryan, Cormac G.; Rafferty, Danny; and Granat, Malcolm H., "Compliance with physical activity guidelines in a group of UK based postal workers using an objective monitoring technique" (2009). School of Health. Paper 112.

Seismological …, Jan 1, 2003

Geophys. Res. Lett, Jan 1, 2003

1] We analyse time series of daily seismic event rate for the Kilauea, Hawaii, volcano between 19... more 1] We analyse time series of daily seismic event rate for the Kilauea, Hawaii, volcano between 1959 and 2000. Individual eruptions are not always preceded by an increase in event rate, and many increases in event rate do not lead to eruption. However, a mean field accelerating behaviour does emerge 10-15 days before eruption in data stacked in phase with the eruption time. In phase space the preeruptive dynamics is well defined by Voight's [1988] equation, but so is that of the seismicity in the period between eruptions. We conclude that the underlying dynamics of the 'background' seismicity is similar to that of magma eruption. We use Bayesian methods to compare different time-to-failure models that have been suggested for precursors. Only a short-term forecast can be achieved, using a linear fit to inverse rate. INDEX TERMS: 7223 Seismology: Seismic hazard assessment and prediction; 7280 Seismology: Volcano seismology (8419); 7299 Seismology: General or miscellaneous; 8419 Volcanology: Eruption monitoring (7280). Citation: Chastin, S. F. M., and I. G. Main, Statistical analysis of daily seismic event rate as a precursor to

Journal of applied geophysics, Jan 1, 2003

This is a preliminary report on two sets of recent observations from a region of active tectonics... more This is a preliminary report on two sets of recent observations from a region of active tectonics that provide comparatively direct evidence for the critical state of the fluid-saturated microcracked crust. The first data set from crosshole seismics in a controlled source stress-monitoring site (SMS) shows that the crust of the Earth is highly compliant and responds to low-level changes of tectonic stress at substantial distances. The second set of data from earthquake seismograms shows that the seismically active Húsavík -Flatey Fault plane is pervaded by critically high pore-fluid pressures, which cause 90j flips in the polarisations of seismic shear waves. We suggest that both sets of observations confirm previous hypotheses for a compliant crack-critical (CCC) crust. This is a new understanding of low-level pre-fracturing deformation that has fundamental implications for a range of applications in solid earth geophysics. These applications range from monitoring hydrocarbon production with time-lapse seismics to monitoring tectonic stress in in situ rock and stress-forecasting the times and magnitudes of impending large earthquakes. D

Geophysical Journal …, Jan 1, 2004

Geophysical Journal International, Jan 1, 2003

Over the last 15 years, it has become established that crack-induced stress-aligned shear wave sp... more Over the last 15 years, it has become established that crack-induced stress-aligned shear wave splitting, with azimuthal anisotropy, is an inherent characteristic of almost all rocks in the crust. This means that most in situ rocks are pervaded by fluid-saturated microcracks and consequently are highly compliant. The evolution of such stress-aligned fluid-saturated grainboundary cracks and pore throats in response to changing conditions can be calculated, in some cases with great accuracy, using anisotropic poro-elasticity (APE). APE is tightly constrained with no free parameters, yet dynamic modelling with APE currently matches a wide range of phenomena concerning anisotropy, stress, shear waves and cracks. In particular, APE has allowed the anisotropic response of a reservoir to injection to be calculated (predicted with hindsight), and the time and magnitude of an earthquake to be correctly stress-forecast. The reason for this calculability and predictability is that the microcracks in the crust are so closely spaced that they form critical systems. This crack-critical crust leads to a new style of geophysics that has profound implications for almost all aspects of pre-fracturing deformation of the crust and for solid-earth geophysics and geology.