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CATEGORIES:Bullard Laboratories Wednesday Seminars
SUMMARY:Imaging metallic deposits using ambient seismic no
 ise - Yihe Xu\, Bullard Laboratories
DTSTART;TZID=Europe/London:20220202T160000
DTEND;TZID=Europe/London:20220202T170000
UID:TALK169544AThttp://talks.cam.ac.uk
URL:http://talks.cam.ac.uk/talk/index/169544
DESCRIPTION:High-frequency seismic surface waves sample the to
 p few tens of meters to the top few kilometres of 
 the subsurface. They can be used to determine 3-D 
 distributions of shear-wave velocities and to map 
 the depths of discontinuities (interfaces) within 
 the crust. Passive seismic imaging\, using ambient
  noise as the source of signal\, can thus be an ef
 fective tool of exploration for mineral\, geotherm
 al and other resources\, provided that sufficient 
 high-frequency signal is available in the ambient 
 noise wavefield and that accurate\, high-frequency
  measurements can be performed on this signal. Amb
 ient noise imaging using the ocean-generated noise
  at 5–30 s periods is now a standard method\, but 
 less signal is available at frequencies high enoug
 h for deposit-scale imaging (0.2–30 Hz)\, and few 
 studies have reported successful measurements in b
 road frequency bands. Here\, we develop a workflow
  for the measurement of high-frequency\, surface w
 ave phase velocities in very broad frequency range
 s. Our workflow comprises (1) a new noise cross-co
 rrelation procedure that accounts for the non-stat
 ionary properties of the high-frequency noise sour
 ces\, removes bandpass filtering\, replaces tempor
 al normalization with short time window stacking\,
  and drops the explicit spectral normalization by 
 adopting cross-coherence\; (2) a new phase-velocit
 y measurement method that extends the bandwidth of
  reliable measurements by exploiting the (resolved
 ) 2π ambiguity of phase- velocity measurements and
  (3) interstation-distance-dependent quality contr
 ol that uses the similarity of subgroups of disper
 sion curves to reject outliers and identify the fr
 equency ranges with accurate measurements. The wor
 kflow is highly automated and applicable to large 
 arrays. Applying our method to data from a large-N
  array that operated for one month near Marathon\,
  Ontario\, Canada\, we use rectangular subarrays w
 ith 150-m station spacing and\, typically\, 1 hr o
 f data and obtain Rayleigh-wave phase-velocity mea
 surements in a 0.5–30 Hz frequency range\, spannin
 g over 5.9 octaves\, twice the typical frequency r
 ange of 1.5–3 octaves in previous studies. Phase-v
 elocity maps and the subregion-average 1-D velocit
 y models they constrain show a high-velocity anoma
 ly consistent with the known\, west-dipping gabbro
  intrusions beneath the area. The new structural i
 nformation can improve our understanding of the ge
 ometry of the gabbro intrusions\, hosting the Cu-P
 GE Marathon deposit.
LOCATION:ONLINE - Details to be sent by email
CONTACT:Florian Millet
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