Understanding Seismic Waves Generated by Train Traffic via Modeling: Implications for Seismic Imaging and Monitoring.

Trains are now recognized as powerful sources for seismic interferometry based on noise correlation, but the optimal use of these signals still requires a better understanding of their source mechanisms. Here, we present a simple approach for modeling train‐generated signals inspired by early work in the engineering community, assuming that seismic waves are emitted by sleepers regularly spaced along the railway and excited by passing train wheels. Our modeling reproduces well seismological observations of tremor‐like emergent signals and of their harmonic spectra. We illustrate how these spectra are modulated by wheel spacing, and how their high‐frequency content is controlled by the distribution of axle loads over the rail, which mainly depends on ground stiffness beneath the railway. This is summarized as a simple rule of thumb that predicts the frequency bands in which most of train‐radiated energy is expected, as a function of train speed and of axle distance within bogies. Furthermore, we identify two end‐member mechanisms—single stationary source versus single moving load—that explain two types of documented observations, characterized by different spectral signatures related to train speed and either wagon length or sleeper spacing. In view of using train‐generated signals for seismic applications, an important conclusion is that the frequency content of the signals is dominated by high‐frequency harmonics and not by fundamental modes of vibrations. Consequently, most train traffic worldwide is expected to generate signals with a significant high‐frequency content, in particular in the case of trains traveling at variable speeds that produce truly broadband signals. Proposing a framework for predicting train‐generated seismic wavefields over meters to kilometers distance from railways, this work paves the way for high‐resolution passive seismic imaging and monitoring at different scales with applications to near‐surface surveys (aquifers, civil engineering), natural resources exploration, and natural hazard studies (landslides, earthquakes, and volcanoes).

The accepted version and the supplementary material can be downloaded as a compressed file from this webpage.

The link to the published version is provided below as well.

Executive summary: Across the globe, the mineral industry is seeking new technologies to replace or complement existing geological, geochemical and geophysical methods to improve exploration efficiency at depth and to help design safer and more productive mines. These industries are increasingly using seismic methods for a wide range of commodities including base metals, uranium, diamonds, and precious metals. Seismic methods usually can be used for direct targeting of mineral deposits but particular care must be taken during acquisition and processing of the data. To achieve the best results, different processing sequences based on the target of the project are applied. Here we compare and discuss how such workflows are used when treating active seismic data in order to provide a basis for their use in the development of the passive seismic methods that form the basis of the PACIFIC project.

Executive summary: The physical properties of rocks and minerals, particularly their density and elasticity, control the velocitywith which they transmit seismic waves. The acoustic impedance, which is the product of density and seismic velocity, is a useful property to characterize different lithologies. Available data indicates that there are strong contrasts in acoustic impedance between common types of rock and, most importantly, between common rocks and ore minerals. These differences provide a basis for relating passive seismic tomographic models with models based on geological and previously acquired geophysical data.

Executive Summary: Seismic methods provide high-resolution images of geologic structures hosting mineral deposits and, in a few cases, can be used for direct targeting of deposits. Active reflection techniques have been successfully used in the minerals sphere, especially for structural control on deep targets. Although useful, a disadvantage of this methodology is that it is expensive and logistically difficult in locations without easy access for source generation. In contrast to active seismology, passive methods exploit ambient seismic noise and do not require specific seismic sources. In this report, we compare active and passive seismic methods in general and discuss different data processing sequences that have been used in previous passive seismic studies. The quality of the results in passive seismic methods strongly depends on (1) the spatial-temporal properties of the noise source distribution and (2) the number and disposition of seismic receiver pairs on which the noise correlation is performed. We then discuss how to apply these processing sequences to extract body-waves in the PACIFIC project, with a view to developing reflection seismic images analogous to active reflection seismic work.

Executive summary: Active seismic sources such as explosives, air guns and vibroseis generate energetic P-waves well suited for reflection seismic studies. However, they can have negative environmental impacts and are expensive, both of which have motivated the development of passive seismic methods. Passive seismic methods utilise ambient noise from meteorological and anthropogenic activity. They have been successful for surface wave recovery but extracting body waves for reflection imaging is still a challenge. A key goal of the PACIFIC project is to develop methodologies for extracting body waves from passive seismic data, and for using these body waves for subsurface imaging. This report describes the development of synthetic velocity models that characterise the geological structure and seismic reflectivity at the Marathon Cu-PGE prospect Ontario, Canada. Synthetic seismic signals generated in these models will then be used to develop and test processing procedures for body wave recovery and body wave imaging. A first velocity model consists of two vertical sections obtained by interpolation of lithological contacts identified in drillholes. One section is perpendicular to the dip of the main gabbro intrusion, the other is parallel. A second model is obtained by blind 3D interpolation between drillholes and uses velocities measured on hand samples and drill core. Work in progress uses dedicated geological modelling software to generate a 3D block model that honors geological structures and cross-cutting relationships. A recently acquired downhole acoustic log avoids negative velocity biases from microfractures that can be introduced during depressurisation (e.g. of drill core). This will be used to calibrate a new velocity forward model.

Download the PACIFIC flyer https://www.pacific-h2020.eu/wp-content/uploads/pacific-flyer.pdf 

 Charlie Beard's presentation can be viewed on the link https://www.youtube.com/watch?v=g_qdwQGHNhs 

Retrieving reflection arrivals from passive seismic data using Radon correlation

Since explosive and impulsive seismic sources such as dynamite, air guns, gas guns or even vibroseis can have a big impact on the environment, some companies have decided to record ambient seismic noise and use it to estimate the physical properties of the subsurface. Big challenges arise when the aim is extracting body waves from recorded passive signals, especially in the presence of strong surface waves. In passive seismic signals, such body waves are usually weak in comparison to surface waves that are much more prominent. To understand the characteristics of passive signals and the effect of natural source locations, three simple synthetic models were created. To extract body waves from simulated passive signals we propose and test a Radon-correlation method. This is a time-spatial correlation of amplitudes with a train of time-shifted Dirac delta functions through different hyperbolic paths. It is tested on a two-layer horizontal model, a three-layer model that includes a dipping layer (with and without lateral heterogeneity) and also on synthetic Marmousi model data sets. Synthetic tests show that the introduced method is able to reconstruct reflection events at the correct time-offset positions that are hidden in results obtained by the general cross-correlation method. Also, a depth migrated section shows a good match between imaged horizons and the true model. It is possible to generate off-end virtual gathers by applying the method to a linear array of receivers and to construct a velocity model by semblance velocity analysis of individually extracted gathers.

Executive summary: Permitting of the seismic survey and the acquisition of data are the first steps in WP3, the pilot test of the passive reflection seismic technique in the Marathon deposit. The processing and development stages of the Work Package rely directly on the successful acquisition of ambient seismic noise data from the Marathon test site.
Between September 17th and October 26th of 2018, at the Marathon test site, a 1025 sensor passive seismic survey was completed. The sensors equipment was rented from SAExploration. 1024 sensors were successfully deployed; however, only 1019 were recovered. The loss of sensors was due to animal activity or being buried by a rock slide.
The grid design was composed of two overlapping grids, a 416-sensor array and a 609-sensor profile line. The array had a grid spacing of 150m, while the profile line had a grid spacing of 50m. Both grids designs were configured along the main noise source of Lake Superior in the direction of 250deg to the west.
The sensors selected for the survey were ZL and C1, vertical direction sensors with a 10hz range. Once the sensors were retrieved, they were shipped back to SAExploration for download. The data was successfully downloaded and shipped to Sisprobe for analysis.

Executive summary: A key goal of the PACIFIC project is to develop methodologies for the extraction of body waves from passive seismic data, for use in the environmentally sustainable environments. Recovering body waves from ambient noise data has proved to be challenging as they are usually weak and ambient noise fields are rich in surface waves. Here we propose and test a method, based on the Radon Transformation, that helps suppress surface waves and enhance reflected body waves. The method exploits the ‘moveout’ differences between reflected body (hyperbolic) and surface waves (linear) and is tested on synthetic 2D & 3D model data prior to its application to ambient noise field data. We refer to it as Radon Correlation. Synthetic tests are very encouraging, showing clear body wave recovery that cannot be seen in raw cross-correlated data. Using these synthetics to have a choice of parameters, we then move to field passive data from the Marathon site within PACIFIC. We generate virtual shot gathers by applying Radon Correlation to single virtual sources into a linear array of receivers. Again, results are very encouraging with clear reflected body wave recovery from the ambient noise data and determined by clear hyperbolic arrivals on the virtual shot gathers. There is a hint that using time windows that contain active blast seismic coda possibly further enhances body wave recovery. Finally, velocity analysis on these virtual shot gathers leads to a P-wave velocity model that compares well with models derived from surface wave dispersion analysis of the same ambient noise data. However, these models are not currently publicly available and hence are not shown here, in this report.