Abstract

The Jemez Tomography Experiment (JTEX) is a multidisciplinary study focused on the Valles Caldera and the Jemez Mountains, New Mexico. The objectives of the project are to create a high resolution crustal model of the subsurface structure of this silicic volcanic system and to develop an interpretation of its volcanic evolution. Use of Vibroseis sources in the acquisition of refraction/wide-angle reflection seismic data provided challenges beyond conventional explosive-source data. Processing of the JTEX Vibroseis data is an involved procedure consisting of sorting, cross-correlating, filtering, and stacking numerous individual seismograms in the production of final record sections. However, excellent results (high signal-to-noise seismograms at relatively small spacings) are obtainable with coherent arrivals at source-receiver distances of more than 60 km. The primary drawback in this approach lies in the massive volume of data that is necessary to produce record sections.

One benefit of the Vibroseis source used during JTEX was a method to decrease effective seismogram spacing. This technique, dubbed a “source-offset” technique, provides smaller overall seismograph station spacing by moving the Vibroseis sources during acquisition and leaving deployed seismographs stationary. After station corrections, this method effectively decreases station spacings and increases detail in resulting record sections. Various shallow crustal heterogeneities create travel-time advances and delays that affect the source-offset data differently than single-source data. Synthetic modeling demonstrates small travel-time discrepancies associated with the source-offset technique. However, the addition of traces with smaller station intervals clarifies secondary arrivals within record sections and aids in interpretation of these arrivals with a minimum amount of field effort required.

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