The atmospheric wind and temperature can be estimated through the traveltimes of infrasound between pairs of receivers. The traveltimes can be obtained by infrasonic interferometry. In this study, the theory of infrasonic interferometry is verified and applied to modeled stratospherically refracted waves. Synthetic barograms are generated using a raytracing model and taking into account atmospheric attenuation, geometrical spreading, and phase shifts due to caustics. Two types of source wavelets are implemented for the experiments: blast waves and microbaroms. In both numerical experiments, the traveltimes between the receivers are accurately retrieved by applying interferometry to the synthetic barograms. It is shown that microbaroms can be used in practice to obtain the traveltimes of infrasound through the stratosphere, which forms the basis for retrieving the wind and temperature profiles.
Summary We propose a method for estimating the reflection coefficient of a subvertical boundary and the the quality factor of the medium between a receiver and the subvertical boundary. The method uses surface waves from transient deterministic sources and is inspired by the occurrence of non-physical arrivals in seismic-interferometry results due to intrinsic losses in the medium. The quality-factor estimation with our method can be used as an alternative to and confirmation of results from the spectral-ratio method. We demonstrate our method on data from ultrasonic laboratory measurements.
<p>We previously developed a physics-based model relating changes in pore pressure and vertical stress to seismic velocity variations and validated the model in a small area of Groningen gas field. Using the entire Groningen seismic network, near-surface velocity changes are estimated over a three-year period, using passive image interferometry. Using our developed model, we invert these observations of velocity change for pore pressure variations as a function of space and time, and thus we construct a 4D pore pressure model for the shallow subsurface of Groningen. Pressure-head recordings in the southeastern region of Groningen allow us to calibrate our inference tool.</p>
Seismic interferometry (SI) studies the interference phenomenon between pairs of signals in order to obtain information from the differences between them. SI is now regularly used in exploration and global seismology with active and/or passive sources, i.e., artificial sources (dynamite, vibroseis, sledge hammer, etc.) or natural sources (earthquakes, anthropogenic noise, ocean microseisms, etc.). SI allows one to extract subsurface information from complicated or random wavefields.This research aims to contribute to the knowledge of the subsurface structure at Planchon-Peteroa Volcano Complex (PPVC) by using SI technique. Inspired by the theory and applications in Wapenaar (2003) and Ruigrok and Wapenaar (2012), this work applies SI to fracture seismicity originated at PPVC or in active geologic faults located nearby this volcanic complex. Applying autocorrelation to a selected time window at each event, zero-offset reflection responses were obtained for each station. This response can be used to determine the location of shallow subsurface reflectors underneath each station.This application uses seismic data recorded by stations deployed in Argentina and Chile. The Argentine data was recorded by an array of six 2-Hz 3-component stations on the eastern flank of the volcano, deployed during the MalARRgue project in 2012. The Chile data is provided by OVDAS-SERNAGEOMIN (South Andes Volcanic Observatory, Chile). OVDAS has six 3-component 30-seconds stations located on the western flank of the volcano; these stations overlap in the same time period as the Argentine data.Events had been identified and located independently by the arrays deployed in each of the flanks (Casas, 2014; RAV SERNAGEOMIN, 2012). In order to obtain accurate locations of the detected events, the two datasets were used together to relocate them. This result constitutes a necessity for enhancing the resolution of subsurface images obtained by application of SI at PPVC.Preliminary results of this research will be presented.
Seismic interferometry is an effective tool to retrieve surface waves between two receiver stations by cross-correlating ambient background noise over sufficiently long recording times. This method assumes an azimuthally uniform distribution of noise sources. Unfortunately this assumption is not always fulfilled in practice. If noise sources are located on one side of a receiver array only, surface waves can also be retrieved by multi-dimensional deconvolution of passive records. We show how this method can effectively correct for azimuthal variations in the noise source distribution. We do not take backscattering of the surface waves into account, but this can be overcome if wavefield decomposition is incorporated.
Topography and near-surface heterogeneities lead to traveltime perturbations in surface land-seismic experiments. Usually, these perturbations are estimated and removed prior to further processing of the data. A common technique to estimate these perturbations is the delay-time method. We have developed the “modified delay-time method,” wherein we isolate the arrival times of the virtual refraction and estimate receiver-side delay times. The virtual refraction is a spurious arrival found in wavefields estimated by seismic interferometry. The new method removes the source term from the delay-time equation, is more robust in the presence of noise, and extends the lateral aperture compared to the conventional delay-time method. We tested this in an elastic 2D numerical example, where we estimated the receiver delay-times above a horizontal refractor. Taking advantage of reciprocity of the wave equation and rearranging the common shot gathers into common receiver gathers, isolated source delay times could also be obtained.
