Since its reintroduction by Pratt (1999), full-waveform inversion (FWI) has gained a lot of attention in geophysical exploration because of its ability to build high-resolution velocity models more or less automatically in areas of complex geology. While there is an extensive and growing literature on the topic, publications focus mostly on technical aspects, making this topic inaccessible for a broader audience due to the lack of simple introductory resources for newcomers to computational geophysics. We will accomplish this by providing a hands-on walkthrough of FWI using Devito (Lange et al., 2016), a system based on domain-specific languages that automatically generates code for time-domain finite differences.
As usual, this tutorial is accompanied by all the code you need to reproduce the figures. Go to github.com/seg/tutorials-2017 and follow the links. In the Notebook, we describe how to simulate synthetic data for a specified source and receiver setup and how to save the corresponding wavefields and shot records. In Part 2 of this series, we will address how to calculate model updates — i.e., gradients of the FWI objective function — via adjoint modeling. Finally, in Part 3 we will demonstrate how to use this gradient as part of an optimization framework for inverting an unknown velocity model.
FWI tries to iteratively minimize the difference between data that were acquired in a seismic survey and synthetic data that are generated from a wave simulator with an estimated (velocity) model of the subsurface. As such, an FWI framework essentially consists of a wave simulator for forward modeling the predicted data and an adjoint simulator for calculating a model update from the data misfit. This first part of the tutorial is dedicated to the forward modeling part and demonstrates how to discretize and implement the acoustic wave equation using Devito (http://www.opesci.org/devito/).
Devito provides a concise, straightforward computational framework for discretizing wave equations, which underlie most FWI frameworks. We will show that it generates verifiable executable code at run time for wave propagators associated with forward and (in Part 2) adjoint wave equations. Devito frees the user from the recurrent and time-consuming development of performant time-stepping codes and allows the user to concentrate on the geophysics of the problem rather than on low-level implementation details of wave-equation simulators. This tutorial covers the conventional adjoint-state formulation of full-waveform tomography (Tarantola, 1984) that underlies most of the current methods referred to as FWI (Virieux and Operto, 2009). While other formulations have been developed to improve the performance of FWI for poor starting models, in these tutorials we will concentrate on the standard formulation that relies on the combination of a forward/adjoint pair of propagators and a correlation-based gradient. We discuss how to set up wave simulations for inversion, including how to express the wave equation in Devito symbolically and how to deal with the acquisition geometry.
Wave simulations for inversion
In the Model instantiation, vp is the velocity in km/s, origin is the origin of the physical model in meters, spacing is the discrete grid spacing in meters, shape is the number of grid points in each dimension, and nbpml is the number of grid points in the absorbing boundary layer. It is important to note that shape is the size of the physical domain only, while the total number of grid points, including the absorbing boundary layer, will be automatically derived from shape and nbpml.
Symbolic definition of the wave propagator
To model seismic data by solving the acoustic wave equation, the first necessary step is to discretize this partial differential equation (PDE), which includes discrete representations of the velocity model and wavefields, as well as approximations of the spatial and temporal derivatives, using finite differences. Unfortunately, implementing these finite-difference schemes in low-level code by hand is error prone, especially when we want performant and reliable code.
The primary design objective of Devito is to allow users to define complex matrix-free finite-difference approximations from high-level symbolic definitions, while employing automated code generation to create highly optimized low-level C code. Using the symbolic algebra package SymPy (Meurer et al. 2017) to facilitate the automatic creation of derivative expressions, Devito generates computationally efficient wave propagators.
At the core of Devito's symbolic application program interface are symbolic types that behave like SymPy function objects, while also managing data:
Function objects represent a spatially varying function discretized on a regular Cartesian grid. For example, a function symbol f = Function(name=“f”, grid=model.grid, space_order=2) is denoted symbolically as f(x, y). The objects provide auto-generated symbolic expressions for finite-difference derivatives through shorthand expressions like f.dx and f.dx2 for the first and second derivative in x.
TimeFunction objects represent a time-dependent function that has time as the leading dimension, for example g(time, x, y). In addition to spatial derivatives, TimeFunction symbols also provide time derivatives g.dt and g.dt2.
SparseFunction objects represent sparse components, such as sources and receivers, which are usually distributed sparsely and often located off the computational grid. These objects also therefore handle interpolation onto the model grid.
Setting up the acquisition geometry
The expression for time stepping we derived in the previous section does not contain a seismic source function yet, so the update for the wavefield at a new time step is solely defined by the two previous wavefields. However as indicated in equation 1, wavefields for seismic experiments are often excited by an active (impulsive) source q(x, y, t; xs), which is a function of space and time (just like the wavefield u). To include such a source term in our modeling scheme, we simply add the source wavefield as an additional term to equation 3:
Because the source appears on the right-hand side in the original equation (equation 1), the term also needs to be multiplied with dt2/m (this follows from rearranging equation 2, with the source on the right-hand side in place of 0). Unlike the discrete wavefield u however, the source q is typically localized in space and only a function of time, which means the time-dependent source wavelet is injected into the propagating wavefield at a specified source location. The same applies when we sample the wavefield at receiver locations to simulate a shot record — i.e., the simulated wavefield needs to be sampled at specified receiver locations only. Source and receiver both do not necessarily coincide with the modeling grid.
The symbolic expressions used to create Operator contain sufficient meta-information for Devito to create a fully functional computational kernel. The dimension symbols contained in the symbolic function object (time, x, y) define the loop structure of the created code while allowing Devito to automatically optimize the underlying loop structure to increase execution speed.
When this has finished running, the resulting wavefield is stored in u.data and the shot record is in rec.data. We can easily plot this 2D array as an image, as shown in Figure 2.
As demonstrated in the notebook, a movie of snapshots of the forward wavefield can also be generated by capturing the wavefield at discrete time steps. Figure 3 shows three time steps from the movie.
In this first part of the tutorial, we have demonstrated how to set up the discretized forward acoustic wave equations and associated wave propagator with run-time code generation. While we limited our discussion to the constant density acoustic wave equation, Devito is capable of handling more general wave equations, but this is a topic beyond this tutorial on simulating waves for inversion. In Part 2 of our tutorial, we will show how to calculate a valid gradient of the FWI objective using the adjoint-state method. In Part 3, we will demonstrate how to set up a complete matrix-free and scalable optimization framework for acoustic FWI.
This research was carried out as part of the SINBAD II project with the support of the member organizations of the SINBAD Consortium. This work was financially supported in part by EPSRC grant EP/L000407/1 and the Imperial College London Intel Parallel Computing Centre.