154 Seismic Imaging The field data examples illustrate the potential of gathering two sections, extracted from different parts of the seismic wave field, for providing a means to resolve reflectivity and images from the ultra-shallow surface down to deeper structures. This is achieved using a simple seismic data processing method, with data acquired in a rapid and inexpensive manner, for refraction seismic surveying, without increasing the acquisition costs. 6.2 Refraction-surface waves imaging The main objective of this example is to encourage the processing of P-wave refraction and surface-wave data obtained from a single seismic survey. The geophysical survey was carried out in Yellowstone National Park (USA), in the Obsidian Pool Thermal Area. The goal of the seismic survey carried out at this site was to study shallow hydrothermal systems, characterize fluid pathways and improve understanding of the depths at which steam separates from liquid water. The area is characterized by extensive CO2 diffuse degassing and isolated thermal features with water temperatures between 21.9 °C and 84.0 °C. Seismic data were collected in July 2016 along a south-southwest−north-northeast transect, crossing a heat-flow anomaly between 50 and 120 m and a degassing feature between 86 and 96 m. The equipment and parameters used in the seismic survey were: • a 5.4 kg sledgehammer source swung onto a metal plate. The plate was hit five times at each position to increase the S/N, • 10 Geometrics Geode seismographs, with 24-channels in each one, • 4.5 Hz vertical component geophones spaced every 1 m, obtaining a 239 m long profile, • 25 shot gathers recorded every 10 m, • a sampling rate of 0.125 ms and a recording time of 0.75 s, to include the full surface wavefield. In addition, a GPS survey and airborne LiDAR data collection were carried out to extract the topography. The following results are extracted from Pasquet and Bodet (2017), who have developed an open-source MATLAB-based package that performs surface wave inversion and profiling (SWIP) to obtain 1D to 2D variations of S-wave velocity. The first step of the proposed velocity-estimation procedure concerns the P-wave velocity model. The construction of the P-wave velocity model from the first arrival times was accomplished iteratively by tomographic inversion. A large range of 100 initial models were tested to estimate the sensitivity and depth of investigation. The final P-wave velocity model produced is an average of all models satisfying the picked
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