Purpose: Invert seismic data to full-bandwidth acoustic impedance either in a deterministic mode (the optimal AI volume is produced) or in a stochastic mode (N realizations are computed), so that uncertainties in the inversion process can be analyzed and probability output volumes can be generated. Theory: Stochastic inversion is complementary to deterministic inversion. Stochastic inversion is used to understand the uncertainty in seismic inversion, via the stochastic inversion utilities, and allows the user to explore the impact of this geophysical uncertainty on the lithology, porosity or reservoir volumes over the 3D seismic volume inverted. For thin intervals, a stochastic inversion is particularly appropriate for understanding uncertainty and reservoir volume and connectivity. It also allows the user to improve the estimation obtained from a deterministic inversion as the mean of many realizations (calculated using the utilities) will behave like a global convergence solution from computationally expensive algorithms. Software: OpendTect + MPSI (MPSI consists of five modules that are implemented as attributes in OpendTect's attribute engine: Model building, Error grid. and Deterministic Inversion are released as a bundle to perform deterministic inversion. Stochastic inversion and Utilities are add-ons needed for the stochastic inversion).
Workflow:
Open the attribute set window and select "EW3DModelBuilder" attribute. Define the model by selecting the wells, zones (horizons) and the smoothing parameter.
Select "EW2DErrorGrid" attribute and specify a variogram that captures the uncertainty in the model as a function of distance from the wells.
Select "EWDeterministicInversion" and specify the 3D model, the error grid, the seismic data, the wavelet and some parameters to perform the deterministic inversion. Press "Perform Preprocessing" to compute the required pre-processing step. Test in the usual way (see tips below) and if satisfied invert the entire volume by applying the attribute in batch mode under "Processing - Create Seismic output".
Select "EWStochasticInversion" and specify the 3D model, the error grid, the deterministic inversion result, the seismic data and some parameters to compute N stochastic realizations. Typically the number N is 100. Press "Perform pre-processing" or create a parameter file that can be executed in batch mode to perform the required pre-processing. Stochastic realizations can be inspected on the current element (say an inline) with the "Evaluate attributes" option. Press this icon and select the realizations that you wish to inspect in a movie-style manner using the slider in the Evaluate attributes window.
Select "EWUtilities" to perform post-processing on the N stochastic realizations. You can choose to create a Mean volume (which will be more or less similar to the volume created in the deterministic mode), a standard deviation volume (which gives you an estimate of the uncertainties in the inversion process), probability cubes and trends and joined probability cubes and trends. Probability cubes return the probability of finding a certain range of acoustic impedance values, e.g. corresponding to the sand distribution.
Tips:
The implementation as attributes allows on-the-fly calculation and inspection of each step in the workflow. For example use the "Redisplay" icon to inspect the 3D model, the error grid, the deterministic inversion result, or any of the post-processing results from the stochastic realizations on the current element (say an inline) in the scene.
Only the stochastic realizations cannot be inspected with the "Redisplay" icon. As explained above use the "Evaluate attributes" icon to movie-style inspect different realizations.
The version of MPSI in OpendTect v3.2 does not support wavelet estimation functionality. Use "Manage wavelets" to either import a wavelet, or to create a synthetic (Ricker, Sinc) one.
