Sviluppo di metodologie di localizzazione nel contesto del Comprehensive Nuclear Test-Ban Treaty
Creators: name: Giuntini, Alessandra <1976>
Publisher: name: Alma Mater Studiorum - Università di Bologna
Year: 2008
Subjects: GEO/10 Geofisica della terra solida

description: Since the first underground nuclear explosion, carried out in 1958, the analysis of seismic
signals generated by these sources has allowed seismologists to refine the travel times of seismic
waves through the Earth and to verify the accuracy of the location algorithms (the ground truth for
these sources was often known). Long international negotiates have been devoted to limit the
proliferation and testing of nuclear weapons. In particular the Treaty for the comprehensive nuclear
test ban (CTBT), was opened to signatures in 1996, though, even if it has been signed by 178
States, has not yet entered into force, The Treaty underlines the fundamental role of the
seismological observations to verify its compliance, by detecting and locating seismic events, and
identifying the nature of their sources. A precise definition of the hypocentral parameters represents
the first step to discriminate whether a given seismic event is natural or not. In case that a specific
event is retained suspicious by the majority of the State Parties, the Treaty contains provisions for
conducting an on-site inspection (OSI) in the area surrounding the epicenter of the event, located
through the International Monitoring System (IMS) of the CTBT Organization. An OSI is supposed
to include the use of passive seismic techniques in the area of the suspected clandestine
underground nuclear test. In fact, high quality seismological systems are thought to be capable to
detect and locate very weak aftershocks triggered by underground nuclear explosions in the first
days or weeks following the test.
This PhD thesis deals with the development of two different seismic location techniques: the
first one, known as the double difference joint hypocenter determination (DDJHD) technique, is
aimed at locating closely spaced events at a global scale. The locations obtained by this method are
characterized by a high relative accuracy, although the absolute location of the whole cluster
remains uncertain. We eliminate this problem introducing a priori information: the known location
of a selected event. The second technique concerns the reliable estimates of back azimuth and
apparent velocity of seismic waves from local events of very low magnitude recorded by a trypartite
array at a very local scale. For the two above-mentioned techniques, we have used the crosscorrelation
technique among digital waveforms in order to minimize the errors linked with incorrect
phase picking. The cross-correlation method relies on the similarity between waveforms of a pair of
events at the same station, at the global scale, and on the similarity between waveforms of the same
event at two different sensors of the try-partite array, at the local scale.
After preliminary tests on the reliability of our location techniques based on simulations, we
have applied both methodologies to real seismic events. The DDJHD technique has been applied to
a seismic sequence occurred in the Turkey-Iran border region, using the data recorded by the IMS.
At the beginning, the algorithm was applied to the differences among the original arrival times of
the P phases, so the cross-correlation was not used. We have obtained that the relevant geometrical
spreading, noticeable in the standard locations (namely the locations produced by the analysts of the
International Data Center (IDC) of the CTBT Organization, assumed as our reference), has been
considerably reduced by the application of our technique. This is what we expected, since the
methodology has been applied to a sequence of events for which we can suppose a real closeness
among the hypocenters, belonging to the same seismic structure. Our results point out the main
advantage of this methodology: the systematic errors affecting the arrival times have been removed
or at least reduced. The introduction of the cross-correlation has not brought evident improvements
to our results: the two sets of locations (without and with the application of the cross-correlation
technique) are very similar to each other. This can be commented saying that the use of the crosscorrelation
has not substantially improved the precision of the manual pickings. Probably the
pickings reported by the IDC are good enough to make the random picking error less important than
the systematic error on travel times. As a further justification for the scarce quality of the results
given by the cross-correlation, it should be remarked that the events included in our data set don’t
have generally a good signal to noise ratio (SNR): the selected sequence is composed of weak
events ( magnitude 4 or smaller) and the signals are strongly attenuated because of the large
distance between the stations and the hypocentral area.
In the local scale, in addition to the cross-correlation, we have performed a signal
interpolation in order to improve the time resolution. The algorithm so developed has been applied
to the data collected during an experiment carried out in Israel between 1998 and 1999. The results
pointed out the following relevant conclusions: a) it is necessary to correlate waveform segments
corresponding to the same seismic phases; b) it is not essential to select the exact first arrivals; and
c) relevant information can be also obtained from the maximum amplitude wavelet of the
waveforms (particularly in bad SNR conditions).
Another remarkable point of our procedure is that its application doesn’t demand a long time
to process the data, and therefore the user can immediately check the results. During a field survey,
such feature will make possible a quasi real-time check allowing the immediate optimization of the
array geometry, if so suggested by the results at an early stage.