$\renewcommand{\div}{\nabla\cdot\,} \newcommand{\grad}{\vec \nabla} \newcommand{\curl}{{\vec \nabla}\times\,} \newcommand{\Acf}{{\mathbf A_c^f}} \newcommand{\Ace}{{\mathbf A_c^e}} \newcommand{\dcurl}{{\mathbf C}} \newcommand{\dgrad}{{\mathbf G}} \newcommand{\u}{\vec{u}} \renewcommand{\dudt}{\frac{\partial\u}{\partial t}} \renewcommand{\dphidt}{\frac{\partial\phi}{\partial t}} \renewcommand{\dsdt}{\frac{\partial s}{\partial t}} \newcommand{\M}{{\mathbf M}} \newcommand{\MfMui}{{\M^f_{\mu^{-1}}}}$

# Acoustic wave equation¶

## Backgrounds¶

Time domain acoustic wave equation in 1st order form:

\begin{align}\begin{aligned}\mu^{-1}\frac{\partial \vec{u}}{\partial t} = \nabla \phi\\\rho\frac{\partial \phi}{\partial t} = \nabla \cdot \vec{u}+\frac{\partial s}{\partial t}\delta(\vec{r}-\vec{r}_s)\end{aligned}\end{align}

where $$\rho$$ is the density, $$\mu$$ is the adiabatic compression modulus, $$\delta(\vec{r}-\vec{r}_s)$$ represents point source, and $$\phi$$ is the pressure.

By combining two equations we have 2nd order form:

$\rho\frac{\partial^2 \phi}{\partial t^2} - \div\mu\grad\phi = \frac{\partial^2 s}{\partial t^2}\delta(\vec{r}-\vec{r}_s)$

By applying Fourier transform to frequency domain we have:

$-\rho\omega^2\phi - \div\mu\grad\phi = -\omega ^2 s \delta(\vec{r}-\vec{r}_s)$

where $$\omega = 2\pi f$$ is angular frequency. Assuming contant $$\mu$$ and $$\rho$$ we have Helmholtz equation:

$(\grad^2 + k^2)\phi = \mu^{-1}\omega^2 s \delta(\vec{r}-\vec{r}_s)$

where $$k = \omega\sqrt{\frac{\rho}{\mu}}=\frac{\omega}{v}$$ is the wave propagation constant.

## Discretization of problem¶

To compute the solution of time or frequency dependent partial differential equation (PDE) as discussed above, we need to discretize those equations in both time and space. Our domain in real situation (i.e., earth) can be considered as infinite in space. However, in discrete space where we compute the solution of those PDE should be finite. Therefore, we need to implement artificial boundary conditions to get rid of out going wave, which propagates beyond our domain of interest. We first implement sponge boundary condition then perfectly matched layer (PML) boundary condition.

Once Loop Reflect

### Sponge boundary¶

A fundamental idea of sponge boundary condition is adding damping term to acoustic wave equation:

$\rho\frac{\partial^2 \phi}{\partial t^2} + c\frac{\partial \phi}{\partial t}- \div\mu\grad\phi = \frac{\partial^2 s}{\partial t^2}\delta(\vec{r}-\vec{r}_s)$

Similarly in frequency domain we have:

$-\rho\omega^2\phi +\imath\omega c\phi- \div\mu\grad\phi = -\omega ^2 s \delta(\vec{r}-\vec{r}_s)$

The 1st order form of time domain equation can be written as:

\begin{align}\begin{aligned}\mu^{-1}\frac{\partial \vec{u}}{\partial t} = \nabla \phi\\\rho\frac{\partial \phi}{\partial t} +c\phi- \nabla \cdot \vec{u}= +\frac{\partial s}{\partial t}\delta(\vec{r}-\vec{r}_s)\end{aligned}\end{align}

We discretize 2nd order acoustic wave equation in time (CITE) with central difference:

$\rho\triangle t^{-2}(\phi^{n+1}-2\phi^{n}-\phi^{n-1}) +\frac{c}{2}(\phi^{n+1}-\phi^{n-1})- \nabla \cdot \grad \phi^{n} = q$

where

$q = \triangle t^{-2}(s^{n+1}-2s^{n}-s^{n-1})\delta(\vec{r}-\vec{r}_s)$

By rearranging them, we have:

$\phi^{n+1} = (1+\frac{c\rho}{2}\triangle t)^{-1}[2\phi^{n}-\phi^{n-1}+\frac{c\rho}{2}\triangle t\phi^{n-1} +\triangle t^2\rho\div\mu\grad\phi^n + \triangle t^2\rho q]$

By letting $$f = (1+\frac{c \rho}{2}\triangle t )^{-1}$$, we can substitute $$c = \frac{1-f}{f}\triangle t^{-1}2 \rho$$. Range of $$f$$ for time and frequency domain are 0.98-1 and 0.9-1, respectively. And 35 cells are used for the sponge boundary.

Using staggered grid, we discretize acoustic wave equation in time (CITE):

\begin{align}\begin{aligned}\mu^{-1}\triangle t^{-1}(\u^{n+1}-\u^{n}) = \nabla \phi\\\rho\triangle t^{-1}(\phi^{n+1}-\phi^{n}) +\rho\sigma\phi^{n}- \nabla \cdot \vec{u}^{n+1/2}= +\triangle t^{-1}(s^{n+}-s^{n})\delta(\vec{r}-\vec{r}_s)\end{aligned}\end{align}

where $$\sigma = 2 \frac{1-f}{f} \triangle t^{-1}$$.

Using mimetic finite volume approach, we spatially discretize above equations:

\begin{align}\begin{aligned}\mathbf{diag}(\Acf^T\mu^{-1})\frac{\mathbf{u}^{n+1/2}-\mathbf{u}^{n-1/2}}{\triangle t} + \mathbf{diag}(\Acf^T\mu^{-1}\sigma)\mathbf{u}^{n+1/2}= \mathbf{Grad} \mathbf{\phi}^{n}\\\mathbf{diag}(\rho)\frac{\mathbf{\phi}^{n+1}-\mathbf{\phi}^{n}}{\triangle t} + \mathbf{diag}(\rho\sigma)\mathbf{\phi}^n -\mathbf{Div} \mathbf{u}^{n+1/2}= +\mathbf{diag}(\mathbf{vol})\frac{\mathbf{s}^{n+1}-\mathbf{s}^{n}}{\triangle t}\end{aligned}\end{align}

where $$\mathbf{Div}$$ and $$\mathbf{Grad}$$ and discrete differential operators and $$\Acf$$ is averaging operator from cell face to center. $$\mu$$, $$\rho$$ and $$\sigma$$ are defined on the cell center, and $$\mathbf{vol}$$ is volume of each cell. When we compute these, we first compute:

$\mathbf{u}^{n+1/2} = \mathbf{u}^{n-1/2} - \triangle t\mathbf{diag}(\sigma)\mathbf{u}^{n-1/2}+\triangle t\mathbf{diag}(\Acf^T\mu^{-1})^{-1}\mathbf{Grad} \mathbf{\phi}^{n}$

Then we compute:

$\mathbf{\phi}^{n+1} = \mathbf{\phi}^{n} - \triangle t\mathbf{diag}(\sigma)\mathbf{\phi}^n + \mathbf{diag}(\rho)^{-1}\triangle t[\mathbf{Div}\mathbf{u}^{n+1/2}+\mathbf{diag}(\mathbf{vol})^{-1}\frac{\mathbf{s}^{n+1}-\mathbf{s}^{n}}{\triangle t}]$

Note

Choice of $$\sigma$$ in sponge boundary condition case can be expanded to PML case.

$\sigma = 2 \frac{1-f}{f} \triangle t^{-1}$

And range of $$f$$ is 0.98-1, which can be useful reference property.

### PML boundary¶

PML has two fundamental factors: a. matching the impedance and b. damping. These purposes can be realized by considering solution of Helmholtz equation on complex plane:

\begin{align}\begin{aligned}\tilde{x} = x + \frac{1}{\imath \omega}\int^x_0 \sigma^x(\xi) d\xi\\\partial x = \frac{\imath\omega}{\sigma^x+\imath\omega} \partial\tilde{x}\end{aligned}\end{align}

By parameterizing the physical coordinate as

\begin{align}\begin{aligned}\tilde{x} = \tilde{x}(x)\\x<0 \ : \ \text{real axis}\\x>0 \ : \ \Im[\tilde{x}] < 0 \ (\text{decaying term})\end{aligned}\end{align}

Another core treatment of PML is decomposing $$\phi$$ as:

\begin{align}\begin{aligned}\begin{split}\phi_d = \begin{bmatrix} \phi^x \\[0.3em] \phi^y \\[0.3em] \phi^z \end{bmatrix}\end{split}\\\phi = [1,1,1]\phi_d = \phi^x + \phi^y +\phi^z\end{aligned}\end{align}

Substituting those yields:

\begin{align}\begin{aligned}\begin{split}\imath\omega \mu^{-1}\u = \begin{bmatrix} \frac{1}{\sigma^x+\imath\omega} \frac{\partial \phi}{\partial x} \\[0.3em] \frac{1}{\sigma^y+\imath\omega} \frac{\partial \phi}{\partial y} \\[0.3em] \frac{1}{\sigma^z+\imath\omega} \frac{\partial \phi}{\partial z} \end{bmatrix}\end{split}\\\begin{split}\imath\omega\rho \begin{bmatrix} \phi^x \\[0.3em] \phi^y \\[0.3em] \phi^z \end{bmatrix} -\rho\imath\omega \begin{bmatrix} \frac{1}{\sigma^x+\imath\omega} \frac{\partial u^x}{\partial x} \\[0.3em] \frac{1}{\sigma^y+\imath\omega} \frac{\partial u^y}{\partial y} \\[0.3em] \frac{1}{\sigma^z+\imath\omega} \frac{\partial u^z}{\partial z} \end{bmatrix} = \frac{1}{3}\imath\omega \delta(\vec{r}-\vec{r}_s) \begin{bmatrix} s \\[0.3em] s \\[0.3em] s \end{bmatrix}\end{split}\end{aligned}\end{align}

With some linear algebra:

\begin{align}\begin{aligned}\mu^{-1}(\imath\omega \u + \Sigma\u) = \grad\phi\\\begin{split}\imath\omega\rho \begin{bmatrix} \phi^x \\[0.3em] \phi^y \\[0.3em] \phi^z \end{bmatrix} -\rho \begin{bmatrix} \sigma^x\phi^x \\[0.3em] \sigma^y\phi^y \\[0.3em] \sigma^z\phi^z \end{bmatrix} + \begin{bmatrix} u^x \\[0.3em] u^y \\[0.3em] u^z \end{bmatrix} = \frac{1}{3}\imath\omega \delta(\vec{r}-\vec{r}_s) \begin{bmatrix} s \\[0.3em] s \\[0.3em] s \end{bmatrix}\end{split}\end{aligned}\end{align}

where

$\begin{split}\Sigma = \begin{bmatrix} \sigma^x & 0 & 0 \\[0.3em] 0 & \sigma^y & 0 \\[0.3em] 0 & 0 & \sigma^z \end{bmatrix}\end{split}$

In time domain we have:

\begin{align}\begin{aligned}\mu^{-1}(\frac{\partial \vec{u}}{\partial t} + \Sigma \u) = \nabla \phi\\\begin{split}\rho\frac{\partial \phi_d}{\partial t} +\rho\Sigma \phi_d - \begin{bmatrix} \frac{\partial u^x}{\partial x} \\[0.3em] \frac{\partial u^y}{\partial y} \\[0.3em] \frac{\partial u^z}{\partial z} \end{bmatrix} = \frac{1}{3}\imath\omega \delta(\vec{r}-\vec{r}_s) \begin{bmatrix} s \\[0.3em] s \\[0.3em] s \end{bmatrix}\end{split}\end{aligned}\end{align}

We discretize above equations in both space and time:

\begin{align}\begin{aligned}\MfMui \triangle t^{-1} (\mathbf{u}^{n+1/2}-\mathbf{u}^{n-1/2}) + \MfMui\mathbf{\Sigma}^{f}\mathbf{u}^{n-1/2} - \mathbf{Grad}\mathbf{I}_d\phi_d^{n} = 0\\\mathbf{\Omega}^{cc} \triangle t^{-1} (\phi_d^{n+1}-\phi_d^{n}) + \mathbf{\Omega}^{cc} \mathbf{\Sigma}^{cc} \phi_d - \mathbf{Div}_{vec} \mathbf{u} = \triangle t^{-1}\mathbf{diag}(\mathbf{vol})^{-1}(\mathbf{s}_d^{n+1}-\mathbf{s}_d^{n})\end{aligned}\end{align}

where

\begin{align}\begin{aligned}\begin{split}\MfMui = \mathbf{diag}(\Acf^T\mu^{-1}), \ \mathbf{\Sigma}^{f} = \mathbf{diag}(\Acf_{vec}^T \begin{bmatrix} \sigma^x \\[0.3em] \sigma^y \\[0.3em] \sigma^z \end{bmatrix} )\end{split}\\\begin{split}\mathbf{I}_d = [\mathbf{I}^{cc}, \mathbf{I}^{cc}, \mathbf{I}^{cc}], \ \mathbf{\Omega}^{cc} = \begin{bmatrix} \rho & 0 & 0 \\[0.3em] 0 & \rho & 0 \\[0.3em] 0 & 0 & \rho \end{bmatrix},\end{split}\\\begin{split}\mathbf{\Sigma}^{cc} = \begin{bmatrix} \sigma^x & 0 & 0 \\[0.3em] 0 & \sigma^y & 0 \\[0.3em] 0 & 0 & \sigma^z \end{bmatrix}\end{split}\\\begin{split}\mathbf{s}_d =\frac{1}{3} \begin{bmatrix} \mathbf{s} \\[0.3em] \mathbf{s} \\[0.3em] \mathbf{s} \end{bmatrix}\end{split}\end{aligned}\end{align}

Similarly we first compute:

$\mathbf{u}^{n+1/2} = \mathbf{u}^{n-1/2} - \triangle t \mathbf{\Sigma}^{f}\mathbf{u}^{n-1/2} + \triangle t \MfMui^{-1}\mathbf{Grad}\mathbf{I}_d\phi_d^{n}$

Then we compute:

$\phi_d^{n+1} = \phi_d^{n} - \triangle t \mathbf{\Sigma}^{cc} \phi_d + \mathbf{\Omega}^{cc \ -1} \triangle t\mathbf{Div}_{vec} \mathbf{u} +\mathbf{\Omega}^{cc \ -1}\mathbf{diag}(\mathbf{vol})^{-1}(\mathbf{s}_d^{n+1}-\mathbf{s}_d^{n})$

Once Loop Reflect