Automatic Bayesian polarity determination

The polarity of the first motion of a seismic signal from an earthquake is an important constraint in earthquake source inversion. Microseismic events often have low signal-to-noise ratios, which may lead to difficulties estimating the correct first-motion polarities of the arrivals. This paper describes a probabilistic approach to polarity picking that can be both automated and combined with manual picking. This approach includes a quantitative estimate of the uncertainty of the polarity, improving calculation of the polarity probability density function for source inversion. It is sufficiently fast to be incorporated into an automatic processing workflow. When used in source inversion, the results are consistent with those from manual observations. In some cases, they produce a clearer constraint on the range of high-probability source mechanisms, and are better constrained than source mechanisms determined using a uniform probability of an incorrect polarity pick.This work was funded under a Natural Environment Research Council (NERC) studentship as a CASE award with Schlumberger. Seismometers were borrowed from the NERC SEIS-UK (loan 842), who also archive the data.This is the author accepted manuscript. The final version is available from Wiley via http://dx.doi.org/10.1093/gji/ggw146

A Bayesian method for microseismic source inversion

Earthquake source inversion is highly dependent on location determination and velocity models. Uncertainties in both the model parameters and the observations need to be rigorously incorporated into an inversion approach. Here, we show a probabilistic Bayesian method that allows formal inclusion of the uncertainties in the moment tensor inversion. This method allows the combination of different sets of far-field observations, such as P-wave and S-wave polarities and amplitude ratios, into one inversion. Additional observations can be included by deriving a suitable likelihood function from the uncertainties. This inversion produces samples from the source posterior probability distribution, including a best-fitting solution for the source mechanism and associated probability. The inversion can be constrained to the double-couple space or allowed to explore the gamut of moment tensor solutions, allowing volumetric and other non-double-couple components. The posterior probability of the double-couple and full moment tensor source models can be evaluated from the Bayesian evidence, using samples from the likelihood distributions for the two source models, producing an estimate of whether or not a source is double-couple. Such an approach is ideally suited to microseismic studies where there are many sources of uncertainty and it is often difficult to produce reliability estimates of the source mechanism, although this can be true of many other cases. Using full-waveform synthetic seismograms, we also show the effects of noise, location, network distribution and velocity model uncertainty on the source probability density function. The noise has the largest effect on the results, especially as it can affect other parts of the event processing. This uncertainty can lead to erroneous non-double-couple source probability distributions, even when no other uncertainties exist. Although including amplitude ratios can improve the constraint on the source probability distribution, the measurements are often systematically affected by noise, leading to deviation from their noise-free true values and consequently adversely affecting the source probability distribution, especially for the full moment tensor model. As an example of the application of this method, four events from the Krafla volcano in Iceland are inverted, which show clear differentiation between non-double-couple and double-couple sources, reflected in the posterior probability distributions for the source models.NERCThis is the final version of the article. It first appeared from Oxford University Press via https://doi.org/10.1093/gji/ggw186

Effect of flood basalt stratigraphy on the phase of seismic waveforms recorded offshore Faroe Islands

The generation of short-period multiples between highly heterogeneous layers of basalt flows can strongly alter transmitted seismic wavefields. These layers filter and modify penetrating waves, producing apparent attenuation and phase changes in the observed waveforms. We investigated the waveform and apparent phase changes of the primary seismic signal using mainly the maximum kurtosis approach. We compared the seismic recordings from two short-offset vertical seismic profiles (VSPs) with synthetic seismograms, generated from sonic logs in the same wells, and we found that short-period multiples cause a rapid broadening of the primary arrivals and strong apparent phase changes within a short depth interval below the top of the basalt flows. Relatively large uncertainties were associated with estimating constant phase shifts of the seismic arrivals within the topmost 250 m of the basalt sequences, where complex scattering occurred. Within this interval of the Brugdan I well, a phase-only compensation of the first arrivals with a frequency-independent, combined scattering, and intrinsic attenuation operator was unfeasible. At a greater depth, we found that the phase shifts, predicted by a VSP-derived effective [Formula: see text] value, were similar to those estimated from the VSP signals using the kurtosis method. Thus, phase-only compensation with a combined scattering and intrinsic attenuation operator could work well depending on the seismic signal bandwidth and the distribution, depth, and magnitude of the impedance contrasts in the basalt sequence. We wish to thank Shell UK Ltd. and BP for providing the data sets and for the permission to publish them. The views expressed herein, however, are those of the authors, who are solely responsible for any errors. We thank J. Neep and two anonymous reviewers for critically reading the manuscript. Thanks go to A/S Norske Shell, Schlumberger Gould Research, and the Natural Environment Research Council (grant no. NE/H025006/1) for financial support. St. Edmund’s College and the Cambridge Philosophical Society further supported the first author during field work.This is the final published version. It first appeared at http://geophysics.geoscienceworld.org/content/80/3/D265.abstract