Seismic Forward Modelling
The seismic forward modelling is based on a method developed by the Norwegian Computing Centre, sponsored by Equinor. Full documentation can found here: Seismic Forward - User manual
seismic-forward runs are controlled by XML files. Multiple versions of synthetic seismic can be generated, such as different angle stacks (full, near, mid, far), or angle gathers, both in time and depth domain. The most common use in sim2seis is to produce one or more angle stacks, commonly full, near and/or far.
The resulting angle stacks are compared to observed/acquired seismic data, and for this reason the parameters in the XML files should be set to match the seismic acquisition and processing.
Yaml File Section
Default settings are usually sufficient for seismic forward modelling. Figure 1 shows the seismic-forward-part of the sim2seis configuration file.
## Section for seismic forward 4D
#______________________________________________________________________________________________________________________#
seismic_fwd:
# Comment out lines with full/near/far models if you don't want those stacks modelled
# At least one line must be present
stack_models:
full: model_file.xml
# near: model_file_near.xml
# far: model_file_far.xml
# stack_model_path: ../../sim2seis/model
# attribute: &seismic_attribute amplitude
# pem_output_dir: ../../sim2seis/output/pem
# seismic_output_dir: ../../share/results/cubes
# twt_model: ../../sim2seis/model/model_file_twt.xml
# segy_depth: syntseis_temp_seismic_depth_stack.segy
# segy_time: syntseis_temp_seismic_timeshift_stack.segy # <= not used anymore
# pickle_file_prefix: seis_4dFigure 1: Parameters in the sim2seis configuration file related to seismic forward.
XML Model Files
An example XML file is shown in Figure 2 below. The following paragraphs discuss, some of the most common XML tags, which are likely to be edited to match the case. Sections in the XML file that are commented out show alternatives that are normally not altered.
<seismic-forward>
<elastic-param>
#grid with exported PEM parameters:
<eclipse-file>../../sim2seis/output/pem/pem.grdecl</eclipse-file>
# parameters in overburden (top), underburden (bot) and inactive cells (mid):
<default-values>
<vp-top>2000</vp-top>
<vp-mid>2240</vp-mid>
<vp-bot>3000</vp-bot>
<vs-top>1100</vs-top>
<vs-mid>890</vs-mid>
<vs-bot>890</vs-bot>
<rho-top>2300</rho-top>
<rho-mid>2240</rho-mid>
<rho-bot>2240</rho-bot>
</default-values>
# names of parameters to use from .grdecl file:
<parameter-names>
<vp>VP</vp>
<vs>VS</vs>
<rho>DENS</rho>
</parameter-names>
# type of interpolation to use ++:
<cornerpt-interpolation-in-depth> no </cornerpt-interpolation-in-depth>
<zero-thickness-limit> 0.1 </zero-thickness-limit>
<remove-negative-delta-z> yes </remove-negative-delta-z>
</elastic-param>
# incident angle that are modelled. Here modelled 0 offset:
#<angle>
# <theta-0>0</theta-0>
# <dtheta>0</dtheta>
# <theta-max>0</theta-max>
#</angle>
# modeling gather with nmo stretch:
<nmo-stretch>
<seafloor-depth>150</seafloor-depth>
<velocity-water>1500</velocity-water>
<offset>
<offset-0> 0 </offset-0>
<doffset> 400 </doffset>
<offset-max> 2000 </offset-max>
</offset>
</nmo-stretch>
# Wavelet used for modelling (here a Ricker is used)
# A wavelet ca be used, see user manual
<wavelet>
<ricker>
<peak-frequency>30</peak-frequency>
</ricker>
<scale>1</scale>
</wavelet>
# Add white noise.
# Note that the noise is added to the inputgrid, and not to the seismic grid
# This means that the noise will vary depending on the grid. This is not ideal
# <white-noise>
# <standard-deviation>0.001</standard-deviation>
# </white-noise>
# Definition of output grid.
# A timesurface for top of the grid is needed to create seismic in time. Else, use a constant
# A segy cube can be used as areal layout.
<output-grid>
<top-time>
#<top-time-constant>0</top-time-constant>
<top-time-surface>../../sim2seis/input/seismic_forward/TopVolantis.txt</top-time-surface>
</top-time>
<area-from-segy>
<filename>../../sim2seis/input/seismic_forward/seismiclayout.segy</filename>
</area-from-segy>
<cell-size>
#<dx>50</dx>
#<dy>50</dy>
<dz>4</dz>
<dt>4</dt>
</cell-size>
<depth-window>
<top>1500</top>
<bot>2000</bot>
</depth-window>
<time-window>
<top>1500</top>
<bot>2000</bot>
</time-window>
<utm-precision>0.01</utm-precision>
</output-grid>
# Specification of output data
<output-parameters>
<prefix>../../share/results/cubes/syntseis_temp</prefix>
<seismic-time-segy>no</seismic-time-segy> # gathers
<seismic-depth-segy>no</seismic-depth-segy> # gathers
<twt>no</twt>
<seismic-timeshift-segy>no</seismic-timeshift-segy> # timeshifted gathers
<seismic-stack>
<timeshift-segy>yes</timeshift-segy> # timeshifted stacked seismic to timeshift-twt
<depth-segy>yes</depth-segy> # stacked seismic
</seismic-stack>
<elastic-parameters-depth-segy>no</elastic-parameters-depth-segy>
<elastic-parameters-time-segy>no</elastic-parameters-time-segy>
</output-parameters>
# This will import twt from base survey, and use this as twt also for monitor
<timeshift-twt>../../share/results/cubes/syntseis_base_twt.storm</timeshift-twt>
<project-settings>
# number of threads/cpu. Set to 1 when running in batch
<max-threads>1</max-threads>
</project-settings>
</seismic-forward>Figure 2: Example XML file for full stack synthetic seismic in depth domain.
Default Values
As the geomodel and reservoir simulator model are likely to cover the reservoir section only, background values for Vp, Vs and density (&rho) need to be defined. Top refers to the interval above the reservoir, mid within the reservoir and bot below the reservoir. To estimate the seismic response, it is required to know the contrast in elastic properties. It is not correct to assume that these values are zero outside the reservoir, hence the need for background/default values.
To estimate the correct set of default values, Vp, Vs and &rho logs from available wells should be studied, and average values to the top, mid and bottom intervals calculated.
Angle / NMO Stretch
Normal moveout (nmo) stretch compensates for the increase in seismic travel time with offset between seismic source and receiver. This compensation is applied both in angle gather displays and for stacked seismic cubes. To generate cubes without estimation and correction of nmo-stretch, angle option is used instead. The nmo-stretch option takes precedence over the angle option.
In the Drogon case, nmo-stretch is used for the full stack and angle for near and far partial stacks.
Wavelet
The wavelet can either be a Ricker wavelet with a specified peak frequency, or an estimated wavelet read from a file. Only Landmark ASCII wavelet format is supported for file input. <scale> should be used to match the dynamic range of the observed seismic data. If a Ricker wavelet is used, the peak frequency should be set to match the frequency content of the observed seismic data.
Noise
There are two options for modelling noise in the synthetic seismic data: white noise added to the seismic data or white noise added to the reflection coefficients. In both cases, the standard deviation of the noise distribution must be specified. The <white-noise> option adds noise to the seismic data, while <add-noise-to-refl-coef> adds the noise to the reflection coefficients. It is recommended to use the <white-noise> option.
Output Grid
Most settings for the output grid can be read from the simulation grid and a template segy file. Other settings should be fairly self-explanatory.
Output Parameters
Most parameters in this section should remain at their default values, as these are expected by the sim2seis workflow.
Timeshift TWT
A separate run of seismic-forward estimates the TWT of the base grid as a function of depth. This run is controlled by the XML file model_file_twt.xml. The parameter setting in this file should be harmonised with those used to generate the angle stacks.