19,639 research outputs found
Relative relocation of earthquakes without a predefined velocity model: an example from a peculiar seismic cluster on Katla volcano's south-flank (Iceland)
Relative relocation methods are commonly used to precisely relocate
earthquake clusters consisting of similar waveforms. Repeating waveforms are
often recorded at volcanoes, where, however, the crust structure is expected to
contain strong heterogeneities and therefore the 1D velocity model assumption
that is made in most location strategies is not likely to describe reality. A
peculiar cluster of repeating low-frequency seismic events was recorded on the
south flank of Katla volcano (Iceland) from 2011. As the hypocentres are
located at the rim of the glacier, the seismicity may be due to volcanic or
glacial processes. Information on the size and shape of the cluster may help
constraining the source process. The extreme similarity of waveforms points to
a very small spatial distribution of hypocentres. In order to extract
meaningful information about size and shape of the cluster, we minimize
uncertainty by optimizing the cross-correlation measurements and
relative-relocation process. With a synthetic test we determine the best
parameters for differential-time measurements and estimate their uncertainties,
specifically for each waveform. We design a relocation strategy to work without
a predefined velocity model, by formulating and inverting the problem to seek
changes in both location and slowness, thus accounting for azimuth, take-off
angles and velocity deviations from a 1D model. We solve the inversion
explicitly in order to propagate data errors through the calculation. With this
approach we are able to resolve a source volume few tens of meters wide on
horizontal directions and around 100 meters in depth. There is no suggestion
that the hypocentres lie on a single fault plane and the depth distribution
indicates that their source is unlikely to be related to glacial processes as
the ice thickness is not expected to exceed few tens of meters in the source
area
Advancements in seismic tomography with application to tunnel detection and volcano imaging
Thesis (Ph.D.) University of Alaska Fairbanks, 1998Practical geotomography is an inverse problem with no unique solution. A priori information must be imposed for a stable solution to exist. Commonly used types of a priori information smooth and attenuate anomalies, resulting in 'blurred' tomographic images. Small or discrete anomalies, such as tunnels, magma conduits, or buried channels are extremely difficult imaging objectives. Composite distribution inversion (CDI) is introduced as a theory seeking physically simple, rather than distributionally simple, solutions of non-unique problems. Parameters are assumed to be members of a composite population, including both well-known and anomalous components. Discrete and large amplitude anomalies are allowed, while a well-conditioned inverse is maintained. Tunnel detection is demonstrated using CDI tomography and data collected near the northern border of South Korea. Accurate source and receiver location information is necessary. Borehole deviation corrections are estimated by minimizing the difference between empirical distributions of apparent parameter values as a function of location correction. Improved images result. Traveltime computation and raytracing are the most computationally intensive components of seismic tomography when imaging structurally complex media. Efficient, accurate, and robust raytracing is possible by first recovering approximate raypaths from traveltime fields, and then refining the raypaths to a desired accuracy level. Dynamically binned queuing is introduced. The approach optimizes graph-theoretic traveltime computation costs. Pseudo-bending is modified to efficiently refine raypaths in general media. Hypocentral location density functions and relative phase arrival population analysis are used to investigate the Spring, 1996, earthquake swarm at Akutan Volcano, Alaska. The main swarm is postulated to have been associated with a 0.2 km\sp3 intrusion at a depth of less than four kilometers. Decay sequence seismicity is postulated to be a passive response to the stress transient caused by the intrusion. Tomograms are computed for Mt. Spurr, Augustine, and Redoubt Volcanoes, Alaska. Relatively large amplitude, shallow anomalies explain most of the traveltime residual. No large amplitude anomalies are found at depth, and no magma storage areas are imaged. A large amplitude low-velocity anomaly is coincident with a previously proposed geothermal region on the southeast flank of Mt. Spurr. Mt. St. Augustine is found to have a high velocity core
COSMOGRAIL: the COSmological MOnitoring of GRAvItational Lenses XIII: Time delays and 9-yr optical monitoring of the lensed quasar RX J1131-1231
We present the results from nine years of optically monitoring the
gravitationally lensed z=0.658 quasar RX J1131-1231. The R-band light curves of
the four individual images of the quasar were obtained using deconvolution
photometry for a total of 707 epochs. Several sharp quasar variability features
strongly constrain the time delays between the quasar images. Using three
different numerical techniques, we measure these delays for all possible pairs
of quasar images while always processing the four light curves simultaneously.
For all three methods, the delays between the three close images A, B, and C
are compatible with being 0, while we measure the delay of image D to be 91
days, with a fractional uncertainty of 1.5% (1 sigma), including systematic
errors. Our analysis of random and systematic errors accounts in a realistic
way for the observed quasar variability, fluctuating microlensing magnification
over a broad range of temporal scales, noise properties, and seasonal gaps.
Finally, we find that our time-delay measurement methods yield compatible
results when applied to subsets of the data.Comment: 11 pages, 9 figures, minor additions to the text only, techniques and
results remain unchanged, A&A in pres
Blind multi-user combining at the base station for asynchronous CDMA systems
This paper studies the potential benefits of antenna arrays in cellular CDMA communications and proposes a powerful scheme to undertake the array processing at the base station in CDMA mobile systems. The proposed technique exploits the temporal structure of CDMA signals. The necessary information is extracted directly from the received signals, thus no training signal orPeer ReviewedPostprint (published version
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