The search for energy resources is primarily based on the use of seismic waves that allow geophysicists to image geological structures in the subsurface of the Earth. Seismic waves for these imaging experiments are usually created through detonation of small explosions or use of large truck-mounted vibrators that propagate waves into the Earth where they reflect and scatter from faults, geological strata, and hydrocarbon reservoirs. Research in this project is centered about the application of a new mathematical method to analyze seismic wave data collected from arrays of hundreds of seismic instruments that give a high-fidelity recording of the seismic wave over the Earth's surface. This method, called "wave gradiometry", is able to quantify how a seismic wave propagates in space and in time in such a way that the resulting inferred wave characteristics can be related more directly to the geological structure between the seismic source and recording array of instruments. This approach has not been tried before because appropriate datasets have not been available until now. Global Geophysical Services has deployed an experimental seismic array over a gas prospect in eastern Ohio that was designed to collect data that can be analyzed with wave gradiometry. We will be researching the use of this new technique on the field dataset to discover new seismic processing methods that will improve the imaging of Earth structure for resource development and scientific studies of the crust.
Data from a dense, 2D array consisting of 400 elements 30m apart with 3 vertical sensors at each element have been collected by Global Geophysical Services (Global) in conjunction with a 3D seismic reflection experiment in eastern Ohio. Approximately 7600 2-lb dynamite shots and auxiliary fill-in vibroseis signals were recorded by this array resulting in a ~1Tbyte dataset. These unique data were collected in consultation with investigators at the University of Memphis to test new seismic processing techniques centered on wave gradiometry. The dense, redundant array element spacing will allow computation of wave spatial gradients that, when combined with the original wave field, will yield wave attributes of slowness, azimuth, geometrical spreading change, and radiation pattern change for the wave field as a function of time over the area of the array. Small-scale gradiometers have been previously deployed at the University with off-the-shelf geophones to demonstrate feasibility of larger scale experiments such as this one. We will investigate the use of wave gradiometry in producing tomographic maps of wave attributes that will be used to estimate near-surface velocity structure to compute statics corrections from ground roll, investigate the nature and signature of shallow mined out voids under part of the array, and investigate wave characteristics of near-vertical reflections that could be used in stacking, migration, or AVO. Collecting data over spatial scales of a fraction of a seismic wavelength represents fertile new terrain in examining how stable the seismic wave field is with position. We will be developing processing techniques in computing wave spatial gradients and inverting tomographic maps of diverse wave attributes from particular seismic waves to determine seismic velocity structures or to examine particular aspects of the structure as in AVO analysis. This work represents an initial exploration of a unique dataset using a new paradigm of wave analysis that will very likely result in new insights and knowledge of seismic wave propagation.
