Numerical Simulation Of The Acoustic And Elastic Wavefields Radiated By A Source In A Fluid-Filled Borehole Embedded In A Layered Medium

We present a method of calculation to simulate the propagation of acoustic and elastic waves generated by a borehole source embedded in a layered medium. The method is formulated as a boundary element technique where the Green's functions are calculated by the discrete wavenumber method. The restrictive assumptions are that the borehole is cylindrical and that its axis runs normal to the layer interfaces. The method is used to generate synthetic acoustic logs and to investigate the wavefield radiated into the formation. The simulations considered display the Stoneley wave reflections at the bed boundaries and show the importance of the diffraction which takes place where the borehole wall intersects the layer interfaces

Full Wave Logging In An Irregular Borehole

We present a boundary integral equation formulation for the problem of wave propagation in a borehole of irregular cross-section. The method consists in representing the wave field diffracted at the borehole-rock interface by the radiation from a distribution of surface sources applied along the borehole wall. The wave field in the borehole fluid and in the elastic rock are then expressed using the discrete wavenumber method. The application of the boundary conditions at discretized locations along the borehole wall leads to a linear system of equations. The inversion of this system yields the required source distribution. We have used the method to investigate the effect of changes in borehole diameter on the pressure wave field inside the borehole. The results show that when the change is smooth, the records obtained ahead of the discontinuity location are not affected by its presence. In the case of a steep variation, however, a significant amount of the Stoneley wave energy is reflected. When the borehole diameter is different at the source and at the receiver levels, the microseismograms obtained are somewhat of an average between the ones that would be recorded in boreholes of constant radius equal to the radius at the source and at the receiver. The presence of small-scale fluctuations in borehole diameter reduces the amplitude of the Stoneley wave and decreases its velocity and the pseudo-Rayleigh wave velocity.Massachusetts Institute of Technology. Full Waveform Acoustic Logging ConsortiumSociete Nationale Elf Aquitain

Borehole Seismic Source Radiation In Layered Isotropic And Anisotropic Media: Boundary Element Modeling

An algorithm based on the boundary element method is established for modeling seismic source radiation from open or cased boreholes in layered transversely isotropic (TI) media. The axis of symmetry of TI layers is assumed to be parallel to the borehole axis. Under this assumption, the problem is significantly simplified because the element discretization of the borehole remains one dimensional. For fluid-filled open boreholes, three equivalent sources on each element are required to represent the boundary effects on the inner fluid and the outer solid. The three boundary conditions for a f1uidsolid interface set up a system of equations for the equivalent sources on all elements. Once the sources are known, displacements in the solid and pressure in the fluid are obtained. For fluid-filled and cased boreholes, the method treats borehole fluid, casing, and cement as a cylindricaliy layered isotropic medium. In this case, the boundary conditions to be satisfied at the borehole wall are four (continuity of the normal and tangential displacements and stresses). The implementation of the method is illustrated through a few examples.United States. Dept. of Energy (Grant DE-FG02-89ERI4084

Seismic imaging of the 1999 Izmit (Turkey) Rupture inferred from the near‐fault recordings

International audienceWe use near-fault accelerograms to infer the space-time history of rupture on the fault during the Izmit earthquake. The records show that the ground displacement and velocity near the fault were surprisingly simple. Rupture propagated toward the west at a velocity of about 3 km/s, and toward the east at a remarkably high average velocity of 4.7 km/s over a distance of about 45 km before decelerating to about 3.1 km/s on the eastern segment. Slip on the fault is particularly large down to a depth of 20 km on the central portion of the fault where it reaches about 7 m. Slip is large also below 10 km on the eastern fault segment, and this may have contributed to the loading of shear stress on the Diizce fault. On the western fault segment, large slip seems confined to shallow depths. is located very close to ARC, and the ground velocities there display waveforms and amplitudes similar to those at ARC. The digital records at ARC are, however, of much better quality than the analog records at GBZ, and, for this reason, we shall use ARC as our modeling station. The other station that we did not use is YPT because records there are more complicated and have a longer duration than records at ARC and SKR, located further from the epicenter. This complexity suggests that at this station the records are more affected by the shallow crustal structure below the site or between the station and the fault

Observations d'interactions sismiques et d'une phase de nucléation de grands séismes

Dans cette thèse, nous présentons trois études concernant les interactions entre séismes et la phase de nucléation des forts tremblements de terre. La première partie de ce travail concerne le NO de la Turquie, où deux systèmes tectoniques sont présents : décrochement sur la Faille Nord Anatolienne et extension dans des essaims situés autour de la faille. Nous montrons que ces deux systèmes interagissent, mais répondent différemment à l'excitation du séisme d'Izmit (1999). Les calculs des changements de contraintes de Coulomb induits par le séisme d'Izmit indiquent que les évènements en décrochement répondent aux contraintes dynamiques, alors que les séismes en extension semblent contrôlés par la déformation statique de la croûte. Dans la seconde partie de cette thèse, nous analysons une importante séquence sismique qui s'est produite en 2008 en Grèce. Cette analyse a montré que les séismes en subduction ou en décrochement composant la séquence reflètent le mouvement du SO rigide de la Grèce vers le SSO accompagné du plongement de la plaque Africaine et du retrait vers le sud de la subduction. Cette première étape de déformation est suivie, quelques mois plus tard, de la déformation interne de la plaque Egée qui se manifeste par de l'extension N-S. Cette séquence sismique est le témoin de l'existence d'un couplage dynamique entre la subduction et le prolongement de la Faille Nord Anatolienne dans la région égéenne. Enfin, le but de la dernière partie de cette thèse a été d'apporter des éléments de réponse à la question : existe-t-il un phénomène observable par le biais de la sismicité qui précède de manière systématique les grands tremblements de terre? Pour cela nous avons étudié l'activité sismique précédant des séismes de M$geq 6.5$ dans des régions bien instrumentées (Japon, Cascades et NO des Etats-Unis). Nous montrons que la grande majorité des séismes interplaques est précédée d'une accélération de la sismicité, au contraire des séismes intraplaques. Nous mettons également en évidence une différence entre les séismes en décrochement et les séismes en subduction : les pré-chocs des séismes en subduction sont localisés dans une grande région, rendant un déclenchement en cascade des évènements et du choc principal impossible.We present three studies about earthquake interactions and the nucleation phase of big earthquakes. The first part of this work deals with the northwestern Turkey, where two tectonic regimes exist, with the transform North Anatolian Fault and extension clusters around it. We show that these two systems interact, but respond differently to Izmit earthquake (1999) stimulation. Computations of the Coulomb stress changes induced by the Izmit earthquake indicate that strike-slip events respond to dynamic stresses, whereas the extension ones seem to be controlled by the static deformation of the crust. In the second part, we analyze a seismic sequence that occurred in 2008 in Greece. This analysis shows that subduction or strike-slip earthquakes of the sequence depict the overall motion of the southwestern Greece to the SSW, accompanied by the plunge and the southward retreat of the slab. This first deformation stage is followed, few months later, by the internal deformation of the Aegean plate, which occurs as N-S extension. This seismic sequence depicts the existence of dynamic coupling between the Hellenic subduction and the prolongation of the North Anatolian Fault. The aim of the last part was to shed some lights on the question: is there a systematic seismic phenomena preceding the occurrence of big earthquakes? To do that, we studied seismic activity preceding M$geq$6.5 earthquakes in some well-instrumented areas of the North Pacific. We show that most of the interplate earthquakes are preceded by an increase of the activity, unlike the intraplate ones. We also highlight a different behavior between strike-slip and subduction events: foreshocks of subduction earthquakes are located in a large area, preventing a cascade triggering.SAVOIE-SCD - Bib.électronique (730659901) / SudocGRENOBLE1/INP-Bib.électronique (384210012) / SudocGRENOBLE2/3-Bib.électronique (384219901) / SudocSudocFranceF

How fast is rupture during an earthquake? New insights from the 1999 Turkey Earthquakes

We report that during the two devastating 1999 earthquakes in Turkey, rupture propagated over a large part of the nearly 200km long fault zone at supershear speed approaching 5km/s. We present observations and modeling which confirm the original inference of supershear rupture during the Izmit earthquake and we show that supershear rupture also occurred during the Düzce earthquake. We show that the rupture velocity measured—about √2 times the shear wave velocity—is the value predicted by theoretical studies in fracture dynamics. We look for clues to explain these observations

Observation of a Synchronicity between Shallow and Deep Seismic Activities during the Foreshock Crisis Preceding the Iquique Megathrust Earthquake

We analyze at a broad spatial scale the slab seismicity during one of the longest and best recorded foreshock sequence of a subduction earthquake to date: the M8.1 2014 Iquique earthquake in Chile.  We observe the synchronisation of this sequence with seismic events occurring in the deep slab (depth ~100km). This synchronisation supports the existence of long-range seismic bursts already observed in the Japan Trench subduction. It suggests that, like for the 2011 Tohoku earthquake, the deep slab was involved in the nucleation process of the Iquique earthquake.  We interpret these observations by the presence of pressure pulses propagating in transient fluid channels linking the deep slab where dehydration occurs to the shallow seismogenic zone before the earthquake. These observations may seem surprising but they are in line with the short-lived pulse-like channelized water escape from the dehydration zone predicted by recent studies in slab mineralogy and geochemistry

Intramolecular Cyclization of N-phenyl N'(2-chloroethyl)ureas leads to Active N-phenyl-4,5-dihydrooxazol-2-amines Alkylating β-Tubulin Glu198 and Prohibitin Asp40

International audienceThe cyclization of anticancer drugs into active intermediates has been reported mainly for DNA alkylating molecules including nitrosoureas. We previously defined the original cytotoxic mechanism of anticancerous phenyl '(2-chloroethyl)ureas (CEUs) that involves their reactivity towards cellular proteins and not against DNA; two CEUs subsets have been shown to alkylate β-tubulin and prohibitin leading to inhibition of cell proliferation by G/M or G/S cell cycle arrest. In this study, we demonstrated that cyclic derivatives of CEUs, -phenyl-4,5-dihydrooxazol-2-amines (Oxas) are two to threefold more active than CEUs and share the same cytotoxic properties in B16F0 melanoma cells. Moreover, the CEU original covalent binding by an ester linkage on β-tubulin Glu198 and prohibitin Asp40 was maintained with Oxas. Surprisingly, we observed that Oxas were spontaneously formed from CEUs in the cell culture medium and were also detected within the cells. Our results suggest that the intramolecular cyclization of CEUs leads to active Oxas that should then be considered as the key intermediates for protein alkylation. These results could be useful for the design of new prodrugs for cancer chemotherapy