Variations in the grain size, amount, and mineralogy of magnetic phases in layered volcanic rocks define a mapetic stratigraphy that can be identified by means of magnetic susceptibility measurements. In ash‐flow tuffs, grain size and other variables that control susceptibility are to a large extent a function of the cooling and alteration history. The phases responsible for the susceptibility of ashflow tuffs consist of post‐emplacement high‐temperature precipitates of Fe‐oxides in volcanic glass, and the phenocrystic Fe‐Ti oxides. At Yucca Mountain, Nevada, laterally‐continuous high‐susceptibility (∼ 10−2 SI) horizons exist in the Paintbrush Tuff due to the presence of either precipitates or phenocrystic Fe‐Ti oxides. The frequency dependence of magnetic susceptibility, x(ω), can be used to discriminate between horizons with supeiparamagnetic predpitates and horizons with multi‐domain phenocrystic material. The x(ω) exhibits a 26% decrease per decade of increasing frequency for precipitate grain sizes where the siperparamagnetic single‐domain state gives way to stable single‐domain behavior, and thus x(ω) offers an indirect method for rapid estimation of magnetic grain sizes. The interpretation of variations established by field and laboratory susceptibility data has been constrained by petrography and transmission electron microscopy (TEM).

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ConocoPhillips acquired a production 3D surface seismic survey in 2010. The survey size was about 410 square miles recorded by 10-Hz geophones with INOVA 364 vibrators that are capable of sweeping from 1 to 150 Hz. In addition, we carried out a field experiment recording a swath of 3D surface seismic data with colocated 2-Hz and 10-Hz geophones. The cost of acquiring the 2-Hz geophone data was negligible when compared with the cost of the entire 3D survey. Because the vibrators can produce energy down to 2 Hz, the use of the 2-Hz geophone is crucial in capturing this energy. To our knowledge, this was the first field experiment employing the 2-Hz geophones and vibrators that could transmit enough low-frequency signals into the ground. The questions we investigate in this field experiment are: (1) how much low-frequency signal can be recorded using 2-Hz geophones, (2) how much degradation of low-frequency signal results from the 10-Hz geophones when compared to the 2-Hz geophones, and (3) the possibility of using the colocated data sets to enhance the low-frequency signal of 10-Hz geophone data that include both experimental and production data. The analyses of the 10-Hz and 2-Hz geophone data in prestack and poststack domains concluded that 2-Hz geophone data clearly exhibited more low-frequency signal than the 10-Hz geophone data. The 2-Hz geophone stack had low-frequency signal down to 2 Hz, and a spiking deconvolution further extended the amplitude spectrum down to 1 Hz. In addition, we develop a novel technique to derive a deterministic match filter from the colocated data sets. The application of this filter on the 10-Hz geophone data recovers the low-frequency signal below 10 Hz.

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