Wave Modeling: 1994 BP migration from topography dataset
Before getting started one should download velocity model from SEG page.
Reading data
Velocity model is represented by SEGY file. One should read it as Geo Volume: right click on Geo Volume treeview -> Import -> SEGY STACK. Read parameters:
Domain: TVD
Length units: decimeter (use this service to check whether units can be converted to any other)
Data units: m/s
Samp rate: -100
Byte start: IL: 17
Byte start: XL: 9
Byte start: X: 73
Byte start: Y: 77
Warning
As some trace headers are incorrect in this SEGY (see few last traces) most likely volume parameters (origin, spacings) should be manually fixed then. For that after SEGY is read go to the Geo Volumes module and select volume read. Make sure that origin is [0,0,-99900] and spacings are [150, 1, 100] decimeters. After setting this and clicking Update button the velocity model is ready.
Generating geometry
From data description geometry should contain 277 shots with 90m interval. 480 traces per shot spread from -3600m to 3600m with 15m interval. Receivers are moved along with source. Sampling rate is 4 ms, number of samples 1000.
In Colada geometry is Seis object. Thus go to the Seismic module and create new seismic with parameters:
Data type: PRESTACK
Survey type: TWO_D
Domain: TWT
Number of traces: let it be 480 (will be changed anyway)
Number of samples: 1000
Sampling rate: 4
Trace chunking: 480 (see HDF5 chunking)
length units: m
Temporal units: ms
Click Create button.
Then in the Geometry section set:
Source geometry x0,dx,nx: 0,90,277
Receiver geometry x0,dx,nx: -3600,15,480
Check on move receivers with sources
Click Generate Geometry button.
Forward modeling
Go to the Wave Modeling module. Set velocity model as input. Generated geometry as Field Records (Geometry)
Modeling settings:
Computing type: Forward modeling
Ricker frequency: 0.02 kHz
Select Save file prefix and Output dir
Click Start Computing.
Note
It is recommended to always compute one or small number of shots before starting computation on all shots. This allows to check if the solution is accurate enough or not and make changes to settings. For that in Additional settings set Compute each Nth shot to some big number.
After computations are done one can see the result, for example this is 129 shot:
Reverse Time Migration (RTM)
RTM works with field data thus geometry input file must be replaced by the computed synthetic records.
Then in Common computing settings:
Computing type: RTM
Limit modeling domain: 0 (if set to 3000 then about 35 Gb RAM is required)
Change Save file prefix and Output dir (the result will be Geo Volume)
and in Additional settings*:
Mute model: On
Mute model (m): 170
Mute model taper (m): 10
Mute data: mute turning (upper triangle mute)
Mute data start time (t0,ms): 140 (approximate length of a 20 Hz Ricker signal)
Mute data velocity (km/s): 4
Click Start Computing
The result may be similar to this:
and this is velocity model overlayed by RTM:
Hope you liked it.