LECTURE 18&19

Hoa Truong
Mind Map by Hoa Truong, updated more than 1 year ago
Hoa Truong
Created by Hoa Truong over 6 years ago
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Description

Seismic reflection survey

Resource summary

LECTURE 18&19
  1. SEISMIC REFLECTION SURVEY
    1. minimises subsurface mapping risk away from hard data locations (wells)
      1. acoustic imaging of the subsurface in 2way time
        1. Acoustic pulse is partially transmitted+ reflected at boundaries betw. rocks with different acoustic impedence
          1. complicated extension of echo (water depth) sounder
            1. 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.
              1. reflection coefficient determine amplitude of reflected wave
                1. 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.
                2. Depth resolution
                  1. poor relative to log+other well data (spatial/geographic resolution). typical range: quarter to half wavelenth
                    1. size of smallest distinguishable features in the data
                    2. Steps
                      1. 1. Acquisition
                        1. survey is designed+shot
                          1. compositions
                            1. source of sound
                              1. air guns (marine)
                                1. vibroseis truck (land)
                                  1. frequency range: 8-80Hz
                                2. receiver detect returning echo
                                  1. geophones (land)
                                    1. hydrophones (marine)
                                    2. amplify, digitise, record data instruments
                                    3. Cheaper in sea (equipment loaded on boat, operate day+night) than land (manual load equipments, electric cables)
                                    4. 2. Processing
                                      1. intensive computer processing-image from seismic data
                                        1. data pass through separate processes, different purposes
                                          1. Random noise suppression
                                            1. improve signal:sound
                                              1. Common Midpoint Shooting (CMP)
                                                1. data shot so many reflections record from same point-reflections add together- make stack section- better signal-noise ratio than original data
                                                  1. CMP gathers: raw, recorded shot records. not easy integret
                                                  2. combine CMPs -transform to geologically meaningful display that can be integret
                                                    1. Velocity Analysis for correcting Normal Moveout (NMO)
                                                      1. Stacking (summing) NMO-corrected CMP gathers - attenuate noise- improve S:N ratio
                                                        1. Range of possible S:N improvement
                                                        2. Coherent noice (multiples)
                                                          1. Resolution enhancement
                                                            1. better resolution -more detail image
                                                            2. Migration of received acoustic energy to correct reflector location
                                                              1. migration moves reflection to accurate location
                                                                1. when beds are not horizontal-reflections different from vertical receiver-distorted view of structure on seismic section
                                                                  1. distortion increase with deep, dip, velocity
                                                            3. 3. Intepretation
                                                              1. image is interpreted from seismic data- maps, geological models of subsurface
                                                                1. to define explotation + development well locations
                                                                  1. Good integretation: internally consistent with available well data+ basic geological concepts
                                                                    1. Basic procedures
                                                                      1. picks significant events+ faults in each seismic section
                                                                        1. time+ amplitude of each event is recorded
                                                                        2. mapping stratigraphic features
                                                                          1. predict environments of deposition +rock types from reflection pattern
                                                                          2. mapping reservoir properties
                                                                            1. porosity map (well data)
                                                                              1. seismic event amplitude at reservoir level correlation with porosity
                                                                                1. transform seismic amplitude - more detailed porosity map
                                                                                2. mapping pore fluids
                                                                                  1. dept conversion
                                                                                    1. calculate depth from seismic times. requires velocity from well/seismic data
                                                                                  2. Key intepretable features
                                                                                    1. seismic profile section
                                                                                      1. seismic horizons (coherent reflection events)
                                                                                        1. faults (displaced coherent events
                                                                                          1. 3D better than 2D
                                                                                            1. provide a volume of data
                                                                                              1. view in any orientation
                                                                                                1. able to see channels from origin
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