Influence of Mantle Structures on Measurements of Anisotropy in the Inner Core

<p>The seismological exploration of the Earth’s inner core has revealed some structural complexities such as seismic anisotropy and hemispherical separation. Investigating the travel times of PKP waves from at least two different ray paths, a polar and an equatorial one, is one of the commonly used methods to probe the inner core’s anisotropy. Since the waves are traversing anomalous structures in the lowermost mantle before entering the core, these heterogeneities have to be taken into account when investigating anisotropy in the inner core.</p><p>In this study we use data from an equatorial path with events from Indonesia recorded in Morocco and a nearly polar one with earthquakes in New Zealand recorded in England. The two waves used in our study, PKPdf and PKPab, both propagate through mantle and outer core and PKPab additionally traverses the inner core. Within this work, we do not only analyse the travel times of the waves but rather investigate their deviations from the originally assumed path along with their incidence angle. This is done with the methods of array seismology, mainly its two parameters slowness and backazimuth.</p><p>The results of this study reveal opposite deviations of slowness and backazimuth of the polar in contrast to the equatorial path. While the polar waves travel shallower and closer to North, the equatorial waves propagate deeper and farther from North than predicted by ak135. Additionally we observe hemispherical differences between waves that sample the eastern and the ones that sample the western hemisphere for both ray paths, PKPdf and PKPab, which leads us to the assumption that the deviations are not caused by the inner core but are rather due to mantle structures.</p>

Mapping lithological boundaries in mines with array seismology and in situ acoustic emission monitoring

SUMMARY Knowledge of the position of lithological boundaries is key information for a realistic interpretation of geological settings. Especially in the mining environment, the exact knowledge of geometrical boundaries and characteristics of rock structures has a great impact for both economic decisions and safety awareness. For this purpose, we investigate the P-coda of high frequency acoustic emission (AE) events (picoseismicity) and test the application of array seismology techniques, usually used to study the Earth's deep interior, on a much smaller scale in a mining environment. In total 52 events were used, all of them recorded in the Asse II salt mine in Lower Saxony (Germany) using a network of 16 piezoelectric sensors. Many of these events show a pulse-like arrival in the late P-coda, suggesting the presence of a well-defined structure which scatters seismic energy. To explore the directional information of the signals in the seismograms we use the sliding-window slowness-backazimuth analysis, performed on the waveform envelope of the entire recording. Strong direct P-wave arrivals are clearly visible with observed slowness and backazimuth as expected for a homogenous medium. This implies straight ray paths from event to sensors indicating that the medium between the events and the sensors is homogeneous for wavelengths larger than about 60 cm. In the late P-coda we observe out-of-plane arrivals from southeast and, assuming single P-to-P scattering, we find that the scatterers responsible for these observations are clustered in space defining a sharp reflector corresponding to a known lithological boundary located at the southern flank of the salt dome. In agreement with the established geological model we observe no other dominant reflections in the analysed waveforms that would indicate previously unknown lithological boundaries. This study shows that array seismology can be applied to AEs in mines to gain more information on structures and heterogeneities located in the vicinity of the monitored rock volume. In micro-acoustically monitored mines, this technique could be a valuable addition to increase hazard awareness and mining efficiency at little or no extra costs.

Optimum suppression of coherent signals with linear moveout in seismic data

This paper presents a new technique for suppression of coherent signals with linear moveout in seismic data. This technique is implemented in two schemes which take an array of traces as input and produce either one single trace or another array of traces as output. Two seismic applications are discussed in detail. The first is in the area of separation of upward and downward travelling signals in vertical array seismology, and the second is concerned with the attenuation of ground roll in conventional land data and cable noise in marine data, respectively. In each case, an example of real seismic data is presented to illustrate the effectiveness of this technique.