The 660-kilometer discontinuity, which separates Earth's upper and lower mantle, has been detected routinely on a global scale in underside reflections of precursors to SS shear waves. Here, we report observations of this discontinuity in many different regions, using precursors to compressional PP waves. The apparent absence of such precursors in previous studies had posed major problems for models of mantle composition. We find a complicated structure, showing single and double reflections ranging in depth from 640 to 720 kilometers, that requires the existence of multiple phase transitions at the base of the transition zone. The results are consistent with a pyrolite mantle composition.

A B S T R A C TRecently there has been much interest in the use of data from surface arrays in conjunction with migration-based processing methods for passive seismic monitoring. In this study we use an example of this kind of data recorded whilst 18 perforation shots, with a variety of positions and propellant amounts, were detonated in the subsurface. As the perforation shots provide signals with known source positions and origin times, the analysis of these data is an invaluable opportunity to test the accuracy and ability of surface arrays to detect and locate seismic sources in the subsurface. In all but one case the signals from the perforation shots are not visible in the raw or preprocessed data. However, clear source images are produced for 12 of the perforation shots showing that arrays of surface sensors are capable of imaging microseismic events, even when the signals are not visible in individual traces. We find that point source locations are within typically 45 m (laterally) of the true shot location, however the depths are less well constrained (∼150 m). We test the sensitivity of our imaging method to the signal-to-noise ratio in the data using signals embedded in realistic noise. We find that the position of the imaged shot location is quite insensitive to the level of added noise, the primary effect of increased noise being to defocus the source image. Given the migration approach, the array geometry and the nature of coherent noise during the experiment, signals embedded in noise with ratios ≥0.1 can be used to successfully image events. Furthermore, comparison of results from data and synthetic signals embedded in noise shows that, in this case, prestack corrections of traveltimes to account for near-surface structure will not enhance event detectability. Although, the perforation shots have a largely isotropic radiation pattern the results presented here show the potential for the use of surface sensors in microseismic monitoring as a viable alternative to classical downhole methods. *

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A B S T R A C TMicroseismic monitoring in petroleum settings provides insights into induced and naturally occurring stress changes. Such data are commonly acquired using an array of sensors in a borehole, providing measures of arrival times and polarizations. Events are located using 1D velocity models, P-and S-wave arrival times and the azimuths of P-wave particle motions. However in the case of all the sensors being deployed in a vertical or near-vertical borehole, such analysis leads to an inherent 180 • ambiguity in the source location. Here we present a location procedure that removes this ambiguity by using the dip of the particle motion as an a priori information to constrain the initial source location. The new procedure is demonstrated with a dataset acquired during hydraulic fracture stimulation, where we know which side of the monitoring well the events are located. Using a 5-step location procedure, we then reinvestigate a microseismic data set acquired in April 1997 at the Ekofisk oilfield in the North Sea. Traveltimes for 2683 candidate events are manually picked. A noise-weighted analytic-signal polarization analysis is used to estimate the dip and azimuth of P-wave particle motions. A modified t-test is used to statistically assess the reliability of event location. As a result, 1462 events are located but 627 are deemed to be statistically reliable. The application of a hierarchal cluster analysis highlights coherent structures that cluster around wells and inferred faults. Most events cluster at a depth of roughly 3km in the Ekofisk chalk formation but very little seismicity is observed from the underlying Tor chalk formation, which is separated from the Ekofisk formation by an impermeable layer. We see no evidence for seismicity in the overburden but such events may be too distant to detect. The resulting picture of microseismicity at Ekofisk is very different from those presented in previous studies.

We develop and apply an imaging procedure for simultaneous location and characterization of seismic source properties called Moment Tensor Migration Imaging. The procedure constructs images for moment tensor components using a weighted diffraction stack migration, and combines ray‐theoretical Green's functions with a reverse time moment tensor imaging methodology. By applying an approximation we term the ‘ray‐angles only approximation’, we form an expression for Moment Tensor Migration Imaging where the migration weights depend only on the take‐off and arrival angles for rays leaving receiver positions and incident upon the image points. Moment Tensor Migration Imaging retains the benefits of diffraction stack procedures for source location and characterization, namely speed, flexibility, and the potential for incorporating non‐linear stacking procedures, whilst also providing the benefits of moment tensor imaging such as: the inclusion of multiple phase and multiple component data; the collapsing of the source radiation pattern; estimation of the moment tensor. We examine variations of the imaging procedure through a synthetic test. We show that although the assumptions required for the imaging and ray‐angles only approximation may not be strictly valid for realistic survey geometries, a simple weight adjustment can be used to obtain more accurate and stable results in these situations. In our synthetic example we find that the use of a P‐wave only migration without this reweighting structure produces poor results, whereby the resulting images show activity upon incorrect moment tensor components. However, many of these effects are mitigated by use of the reweighting scheme and the results are further improved through the introduction of non‐linear stacking operators such as semblance weighted stacks. The highest quality moment tensor images (for the synthetic test examined here) are obtained through the use of both P‐wave and S‐wave wave fields. This highlights the importance of multicomponent data and multiphase modelling when characterizing seismic sources. We also find that the imaged moment tensor components vary proportionately when the input velocities are perturbed by a scale factor. This suggests, for the geometry investigated here, derived source properties such as fault‐plane solutions and shear‐tensile components will not be influenced by bulk changes in seismic velocities. Finally, we show the application to a real microseismic event observed using a surface array during hydraulic fracturing. We find that the procedure collapses the seismic radiation pattern into an anomaly with a maximum at the hypocentre and our derived mechanism is consistent with the observed radiation pattern from the source.

[1] A number of studies have confirmed the global existence of a transition zone discontinuity at 410 km depth by aligning large numbers of long-period seismograms on a surface reflection phase before stacking. In particular, SS and PP precursors from the 410-km discontinuity (termed P410P and S410S) have revealed long-wavelength topography of this discontinuity. Here we extend these techniques to examine the reflection coefficient of the 410-km discontinuity. Using our measurements of P410P and S410S amplitudes, we constrain the impedance contrasts across the 410-km discontinuity. We also show lateral variations in the P wave impedance contrast at 410 km, which is typically low under North America and China and higher beneath the North Pacific. The S wave impedance contrast shows less variability on the regional scale. However, analysis of P410P and S410S amplitudes over smaller areas (by binning traces into spherical caps) shows that the S wave reflection coefficient varies over much shorter scale lengths than that for P waves. The different patterns of variation for P410P and S410S reflection amplitudes could be due to the presence of melt, water, or other chemical heterogeneities in the transition zone. Other factors such as temperature or mantle olivine content variations could also influence precursor amplitudes, but they would be expected to lead to correlated variations, and so they cannot explain all the variation that we observe.Citation: Chambers, K., A. Deuss, and J. H. Woodhouse (2005), Reflectivity of the 410-km discontinuity from PP and SS precursors,

Although not as widespread as their use in other settings, there is a growing realization that distributed acoustic sensing (DAS) systems are suitable for traffic monitoring applications. One such application is demonstrated here using data from a surface DAS array recorded at Brady Hot Springs, Nevada, USA. Although this data set was acquired with the original intent of monitoring changes in a geothermal reservoir, it is shown that the data can also be used to identify and monitor vehicle movements on a nearby highway. Analysis of moveout patterns and recorded amplitudes confirm that this data set is dominated by signals generated by passing vehicles. During nighttime periods, the reduced traffic levels provide isolated signals that are more straightforward to analyze and interpret. During the day, however, increased traffic levels result in the signals from multiple vehicles overlapping to create a complex pattern of amplitudes recorded on the DAS array, making analysis and interpretation more challenging. Nonetheless, these signals can be separated and multiple vehicles identified along with their speeds and timings through the application of an automated workflow based on velocity stacking. The use of DAS for traffic monitoring purposes is an emerging technology, and despite challenges stemming from the nature of the measurement and the signals recorded, it can provide valuable information for the effective management of a transport network.

Noise is a persistent feature in seismic data and so poses challenges in extracting increased accuracy in seismic images and physical interpretation of the subsurface. In this paper, we analyse passive seismic data from the Aquistore carbon capture and storage pilot project permanent seismic array to characterise, classify and model seismic noise. We perform noise analysis for a three month subset of passive seismic data from the array and provide conclusive evidence that the noise field is not white, stationary, or Gaussian;characteristics commonly yet erroneously assumed in most conventional noise models. We introduce a novel noise modelling method that provides a significantly more accurate characterisation of real seismic noise compared to conventional methods, which is quantified using the Mann-Whitney-White statistical test. This method is based on a statistical covariance modelling approach created through the modelling of individual noise signals. The identification of individual noise signals, broadly classified as stationary, pseudo-stationary and non-stationary, provides a basis on which to build an appropriate spatial and temporal noise field model. Furthermore, we have developed a workflow to incorporate realistic noise models within synthetic seismic datasets providing an opportunity to test and analyse detection and imaging algorithms under realistic noise conditions.