LECTURE 18&19

LECTURE 18&19

1. SEISMIC REFLECTION SURVEY

   • minimises subsurface mapping risk away from hard data locations (wells)
   • acoustic imaging of the subsurface in 2way time
   • Acoustic pulse is partially transmitted+ reflected at boundaries betw. rocks with different acoustic impedence
   • complicated extension of echo (water depth) sounder
     • source emit sound down through earth- interfaces where impedance changes partially reflect sound up-record- reflection strength determined by difference acoustic impedence (Z) across interface.
     • reflection coefficient determine amplitude of reflected wave
     • move device in a line produce 2D mapping, in a grid produce 3D mapping. difficult to move device-3D is provide by many closely-space receivers in grid.
   • Depth resolution
     • poor relative to log+other well data (spatial/geographic resolution). typical range: quarter to half wavelenth
     • size of smallest distinguishable features in the data
   • Steps
     • 1. Acquisition
       • survey is designed+shot
       • compositions
         • source of sound
           • air guns (marine)
           • vibroseis truck (land)
             • frequency range: 8-80Hz
         • receiver detect returning echo
           • geophones (land)
           • hydrophones (marine)
         • amplify, digitise, record data instruments
       • Cheaper in sea (equipment loaded on boat, operate day+night) than land (manual load equipments, electric cables)
     • 2. Processing
       • intensive computer processing-image from seismic data
       • data pass through separate processes, different purposes
         • Random noise suppression
           • improve signal:sound
           • Common Midpoint Shooting (CMP)
             • data shot so many reflections record from same point-reflections add together- make stack section- better signal-noise ratio than original data
             • CMP gathers: raw, recorded shot records. not easy integret
           • combine CMPs -transform to geologically meaningful display that can be integret
           • Velocity Analysis for correcting Normal Moveout (NMO)
           • Stacking (summing) NMO-corrected CMP gathers - attenuate noise- improve S:N ratio
           • Range of possible S:N improvement
         • Coherent noice (multiples)
         • Resolution enhancement
           • better resolution -more detail image
         • Migration of received acoustic energy to correct reflector location
           • migration moves reflection to accurate location
           • when beds are not horizontal-reflections different from vertical receiver-distorted view of structure on seismic section
             • distortion increase with deep, dip, velocity
     • 3. Intepretation
       • image is interpreted from seismic data- maps, geological models of subsurface
       • to define explotation + development well locations
       • Good integretation: internally consistent with available well data+ basic geological concepts
       • Basic procedures
         • picks significant events+ faults in each seismic section
           • time+ amplitude of each event is recorded
         • mapping stratigraphic features
           • predict environments of deposition +rock types from reflection pattern
         • mapping reservoir properties
           • porosity map (well data)
           • seismic event amplitude at reservoir level correlation with porosity
           • transform seismic amplitude - more detailed porosity map
         • mapping pore fluids
         • dept conversion
           • calculate depth from seismic times. requires velocity from well/seismic data
       • Key intepretable features
         • seismic profile section
           • seismic horizons (coherent reflection events)
           • faults (displaced coherent events
           • 3D better than 2D
             • provide a volume of data
             • view in any orientation
             • able to see channels from origin