Other seismic techniques (reflection and/or refraction) We have focused on 2D vertical sections of seismic structure à sources and detectors in straight lines Other ways to collect seismic reflection data … Vertical Seismic Processing (VSP) à detectors located at different depths in a borehole, or sources in a borehole à  source at wellhead (zero-offset VSP) or at some distance away (offset VSP) à  Provides a better constraint on structures near Borehole, at depth à  Quieter (less noise) à  Direct arrival is more sensitive to depth (Kearey et al., 2002) Raypath geometry Main advantage of VSP: higher resolution Main drawback: asymmetric raypaths, difficulties in sorting/stacking and removal of multiples/reverberations Tabakov and Baranov, 2008 3D Seismic Surveys Problems with 2D (pseudo 3D) - features between adjacent 2D lines may not be imaged - some reflections generated by out-of-plane reflections Full 3D surveying à arrange detectors so that seismic rays are recorded from a 3D volume (not a line) On land: crossed-array method (dots are mid-points) (Kearey et al., 2002) Marine: multiple airgun-streamer surveys Petroleum Geo-Services Schematic illustration of a Ramform vessel towing a large GeoStreamer spread with deep towing depth. 3D seismic ray coverage: (Kearey et al., 2002) Processing is similar to 2D studies à note that migration is 3D (overcomes limitations of 2D assumption) 2D/3D + VSP: Similar to conventional surface seismics, use downhole/well receivers Tabakov and Baranov, 2008 Looking at 3D seismic data (data cube) (Kearey et al., 2002) Vertical slice = 2D seismic section Time slice / depth slice = horizontal plane through volume Seismic data cube over a salt dome in Gulf of Mexico Red = positive polarity Black = negative polarity Time slice at 3.76 seconds à shows that layers are dipping (Kearey et al., 2002) Three-component (3C) Surveys Most studies use vertical geophones à P-waves with nearly vertical incidence angles But the ground motion is three-dimensional à P-waves and S-waves Record using 3-component instruments à 3 mutually orthogonal geophones à 3 times as much data In marine surveys: Ocean bottom seismometers Ocean bottom cables Source of S-waves: - using special sources à horizontal or oblique impact (hammer strike) à horizontal vibrations of a Vibroseis truck - mode conversion of an oblique P-wave What do S-waves tell us? (1) Lithology – Vs to Vp ratio (or Poisson’s ratio σ) can be indicative of rock type - shales can overlap with other rock types (non-uniqueness) (Sheriff and Geldart, 1995) Poisson’s ratio: VP ⎡ 2(1 − σ)⎤ = ⎢ ⎥ VS ⎣ (1 − 2σ)⎦ 1 2 What do S-waves tell us? (2) Nature of pore fluids – P-waves travel through rock matrix and fluids, S-waves only travel through matrix à P-waves are sensitive to pore fluid composition (Sheriff and Geldart, 1995) Gas is more compressible than oil or water à P-wave velocities will decrease significantly if there is gas Gas is more compressible than oil or water à causes attenuation of P-waves à can enhance seismic images by using S-waves Gas escaping from a gas chimney in the North Sea P-waves S-waves (Caldwell, 1999) What do S-waves tell us? (3) Identification of a reservoir – some reflectors may only be detected with P-wave or S-wave data North Sea – sand reservoir overlain by shale à no P-wave reflection P-wave data from hydrophone streamer S-wave data from ocean bottom cable (Caldwell, 1999) Direct hydrocarbon indicators (DHI) Examine seismic reflections to determine if there is a hydrocarbon reservoir Attribute analysis (amplitude) à need true amplitudes à care must taken in data collection and processing à amplitude depends on distance travelled (spreading, attenuation, scattering) - deeper reflections have a lower amplitude - can correct for this Bright spots and dim spots •  P-waves are sensitive to pore fluid (gas vs. oil vs. water) •  gas results in a very low P-wave velocity •  large impedance contrast at top and bottom of gas-filled layer à large reflection amplitudes (negative / positive) (From earlier example) Seismic record: “BRIGHT SPOT” Non-unique: bright spots also caused by igneous sills and partial melt (Kearey et al., 2002) •  reflection amplitude also depends on the surrounding material •  if overlying rock has low velocity, there may not be a strong impedance contrast at top of gas reservoir •  low amplitude reflection = “dim spot” (Sheriff and Geldart, 1995) Flat spots Change in pore fluid composition = change in velocity = reflection/ refraction In a reservoir, pore fluids may be layered à may get a reflection from gas-oil or gas-water interface à horizontal (Kearey et al., 2002) LBR FlatSpotsBrightSpots01.odg 11/2011 What happens for a shale overlying a porous sandstone? σ= (Vp / Vs ) 2 − 2 2((Vp / Vs ) 2 − 1) (Sheriff and Geldart, 1995) (figures from Section 4.8) σ = 0.15-0.35 for shale Water-filled sandstone à σ is similar to that of shale Gas-filled sandstone à σ is less than that of shale, gas slows down P waves dramatically, not so much on S