NOLET , Guust

SUMMARY The delay times reported for 427 17 476 mantle and core phases in the ISC-EHB Bulletins for the years 1964–2018 are averaged into 120 41 671 summary rays. For each summary ray, I estimate the standard variance of the mean delay and an upper bound for use in tomographic inversion. The trade-off between model norm and the fit to mantle...

International audience

International audience

International audience

International audience

We investigate finite-frequency effects in local tomography. For this purpose, we developed an inversion method including the sensitivity kernels in an 3D tomographic method. The influence of the frequency content is analyzed when considering variable velocity structure. Both travel-time and amplitude estimations are required to compute the...

In this paper we investigate finite-frequency effects in crustal tomography. We developed an inversion procedure based on an exact numerical computation of the sensitivity kernels. In this approach we compute the 3D travel-time sensitivity kernels by using (1) graph theory and an additional bending to estimate accurately both rays and...

We estimate the SH-wave velocity and attenuation structures of the western US upper mantle using the dense network of the USArray and new techniques: we observe a multiple-frequency data set of both traveltime and amplitude anomalies, and interpret these with full 3-D finite-frequency sensitivity kernels. Amplitudes show stronger frequency...

ABSTRACT We describe an algorithm to pick event onsets in noisy records, characterize their error distributions, and derive confidence intervals on their timing. Our method is based on an Akaike information criterion that identifies the partition of a time series into a noise and a signal segment that maximizes the signal-to-noise ratio. The...

We analyse the consistency of the delay time data in the most recent version of the ISC-EHB bulletin published by the International Seismological Centre covering the years 1964-2018. Considering that the delays are influenced by the lateral heterogeneity in the Earth's mantle, we construct a tomographic matrix. We use singular value...

We present a new approach to reduce a sparse, linear system of equations associated with tomographic inverse problems. We begin by making a modification to the commonly used compressed sparse-row format, whereby our format is tailored to the sparse structure of finite-frequency (volume) sensitivity kernels in seismic tomography. Next, we...

International audience

We study wavefield effects of direct P- and S -waves in elastic and isotropic 3-D seismicstructures derived from the temperature field of a high-resolution mantle circulation model.More specifically, we quantify the dispersion of traveltime residuals caused by diffraction instructures with dynamically constrained length scales and magnitudes of...

A network of moored hydrophones is an effective way of monitoring seismicity of oceanic ridges since it allows detection and localization of underwater events by recording generated T waves. The high cost of ship time necessitates long periods (normally a year) of autonomous functioning of the hydrophones, which results in very large data sets....

In a linear ill-posed inverse problem, the regularisation parameter (damping) controls the balance between minimising both the residual data misfit and the model norm. Poor knowledge of data uncertainties often makes the selection of damping rather arbitrary. To go beyond that subjectivity, an objective rationale for the choice of damping is...

Autonomous floats have been used for decades to monitor physical properties of the oceans. More recently, these instruments have been used to record seismic signals in the oceans in order to improve tomographic images resolution at the global scale. A hydrophone is used to monitor the acoustic landscape but all the data cannot be sent through...

In a linear ill-posed inverse problem, the regularisation parameter (damping) controls the balance between minimising both the residual data misfit and the model norm. Poor knowledge of data uncertainties often makes the selection of damping rather arbitrary. To go beyond that subjectivity, an objective rationale for the choice of damping is...

Monitoring of the oceans with autonomous floats is of great interest for many disciplines. Monitoring on a global scale needs a multidisciplinary approach to be affordable. For this purpose, we propose an approach that allows oceanographers from different specialities to develop applications for autonomous floats. However, developing such...

Il y a une dizaine d'années, l'un de nous (Gusst Nolet) travaillant avec Frederik Simons à l'Université de Princeton, avait observé une onde sismique P d'un séisme lointain de magnitude 6. Bien qu'il n'est pas rare de voir un séisme de cette ampleur, dont environ 200 se produisent chaque année, leur méthode d'observation était unique : aidé par...

Mermaid stands for Mobile Earthquake Recording in Marine Areas by Independent Divers. This autonomous, freely-drifting underwater robot records acoustic conversions of earthquake signals while floating at a water depth of 1.5 km, rising to the surface to transmit such seismograms by satellite. Mermaid is already in use for seismic tomography,...

We propose a new method to study the response of a hydrophone at very low frequencies. In our method, the hydrophone is placed in a calibration chamber filled with water and, by instantaneously changing the water height, an abrupt pressure increase of about 1000 Pa is produced. The pressure variation mathematically corresponds to an input...

At 2000 m depth in the oceans, one can hear biological, seismological, meteorological, and anthropogenic activity. Acoustic monitoring of the oceans at a global scale and over long periods of time could bring important information for various sciences. The Argo project monitors the physical properties of the oceans with autonomous floats, some...

Our understanding of the internal dynamics of the Earth is largely based on images of seismic velocity variations in the mantle obtained with global tomography. However, our ability to image the mantle is severely hampered by a lack of seismic data collected in marine areas. Here we report observations made under different noise conditions (in...

We present a realistic application of an inversion scheme for global seismic tomography that uses as prior information the sparsity of a solution, defined as having few nonzero coefficients under the action of a linear transformation. In this paper, the sparsifying transform is a wavelet transform. We use an accelerated iterative...