Computational Seismology
Spectral Element Method - 1D Elastic Wave Equation

Seismo-Live: http://seismo-live.org

## Basic Equations

This notebook presents the numerical solution for the 1D elastic wave equation

$$\rho(x) \partial_t^2 u(x,t) = \partial_x (\mu(x) \partial_x u(x,t)) + f(x,t),$$

using the spectral element method. This is done after a series of steps summarized as follow:

1) The wave equation is written into its Weak form

2) Apply stress Free Boundary Condition after integration by parts

3) Approximate the wave field as a linear combination of some basis

$$u(x,t) \ \approx \ \overline{u}(x,t) \ = \ \sum_{i=1}^{n} u_i(t) \ \varphi_i(x)$$

4) Use the same basis functions in $u(x, t)$ as test functions in the weak form, the so call Galerkin principle.

6) The continuous weak form is written as a system of linear equations by considering the approximated displacement field.

$$\mathbf{M}^T\partial_t^2 \mathbf{u} + \mathbf{K}^T\mathbf{u} = \mathbf{f}$$

7) Time extrapolation with centered finite differences scheme

$$\mathbf{u}(t + dt) = dt^2 (\mathbf{M}^T)^{-1}[\mathbf{f} - \mathbf{K}^T\mathbf{u}] + 2\mathbf{u} - \mathbf{u}(t-dt).$$

where $\mathbf{M}$ is known as the mass matrix, and $\mathbf{K}$ the stiffness matrix.

The above solution is exactly the same presented for the classic finite-element method. Now we introduce appropriated basis functions and integration scheme to efficiently solve the system of matrices.

#### Interpolation with Lagrange Polynomials

At the elemental level (see section 7.4), we introduce as interpolating functions the Lagrange polynomials and use $\xi$ as the space variable representing our elemental domain:

$$\varphi_i \ \rightarrow \ \ell_i^{(N)} (\xi) \ := \ \prod_{j \neq i}^{N+1} \frac{\xi - \xi_j}{\xi_i-\xi_j}, \qquad i,j = 1, 2, \dotsc , N + 1$$

#### Numerical Integration

The integral of a continuous function $f(x)$ can be calculated after replacing $f(x)$ by a polynomial approximation that can be integrated analytically. As interpolating functions we use again the Lagrange polynomials and obtain Gauss-Lobatto-Legendre quadrature. Here, the GLL points are used to perform the integral.

$$\int_{-1}^1 f(x) \ dx \approx \int _{-1}^1 P_N(x) dx = \sum_{i=1}^{N+1} w_i f(x_i)$$


In [ ]:

# Import all necessary libraries, this is a configuration step for the exercise.
# Please run it before the simulation code!
import numpy as np
import matplotlib.pyplot as plt

from gll import gll
from lagrange1st import lagrange1st
from ricker import ricker

# Show the plots in the Notebook.
plt.switch_backend("nbagg")



### 1. Initialization of setup



In [ ]:

# Initialization of setup
# ---------------------------------------------------------------
nt    = 10000         # number of time steps
xmax  = 10000.        # Length of domain [m]
vs    = 2500.         # S velocity [m/s]
rho   = 2000          # Density [kg/m^3]
mu    = rho * vs**2   # Shear modulus mu
N     = 3             # Order of Lagrange polynomials
ne    = 250           # Number of elements
Tdom  = .2            # Dominant period of Ricker source wavelet
iplot = 20            # Plotting each iplot snapshot

# variables for elemental matrices
Me = np.zeros(N+1, dtype =  float)
Ke = np.zeros((N+1, N+1), dtype =  float)
# ----------------------------------------------------------------

# Initialization of GLL points integration weights
[xi, w] = gll(N)    # xi, N+1 coordinates [-1 1] of GLL points
# w Integration weights at GLL locations
# Space domain
le = xmax/ne        # Length of elements
# Vector with GLL points
k = 0
xg = np.zeros((N*ne)+1)
xg[k] = 0
for i in range(1,ne+1):
for j in range(0,N):
k = k+1
xg[k] = (i-1)*le + .5*(xi[j+1]+1)*le

# ---------------------------------------------------------------
dxmin = min(np.diff(xg))
eps = 0.1           # Courant value
dt = eps*dxmin/vs   # Global time step

# Mapping - Jacobian
J = le/2
Ji = 1/J    # Inverse Jacobian

# 1st derivative of Lagrange polynomials
l1d = lagrange1st(N)   # Array with GLL as columns for each N+1 polynomial



### 2. The Mass Matrix

Now we initialize the mass and stiffness matrices. In general, the mass matrix at the elemental level is given

$$M_{ji}^e \ = \ w_j \ \rho (\xi) \ \frac{\mathrm{d}x}{\mathrm{d}\xi} \delta_{ij} \vert_ {\xi = \xi_j}$$

#### Exercise 1

Implements the mass matrix using the integration weights at GLL locations $w$, the jacobian $J$, and density $\rho$. Then, perform the global assembly of the mass matrix, compute its inverse, and display the inverse mass matrix to visually inspect how it looks like.



In [ ]:

# Elemental Mass matrix
# ---------------------------------------------------------------
for i in range(0, N+1):
Me[i] = rho * w[i] * J    #stored as a vector since it's diagonal

# Global Mass matrix
# ---------------------------------------------------------------
k = -1
ng = (ne-1)*N + N + 1
M = np.zeros(2*ng)

for i in range(1, ne+1):
for j in range(0, N+1):
k = k + 1
if i>1:
if j==0:
k = k - 1
M[k] = M[k] + Me[j]

# Inverse matrix of M
# ---------------------------------------------------------------
Minv = np.identity(ng)
for i in range(0,ng):
Minv[i,i] = 1./M[i]

# ---------------------------------------------------------------
# Display inverse mass matrix inv(M)
# ---------------------------------------------------------------
plt.imshow(Minv)
plt.title('Mass Matrix $\mathbf{M}$')
plt.axis("off")
plt.tight_layout()
plt.show()



### 3. The Stiffness matrix

On the other hand, the general form of the stiffness matrix at the elemtal level is

$$K_{ji}^e \ = \ \sum_{k = 1}^{N+1} w_k \mu (\xi) \partial_\xi \ell_j (\xi) \partial_\xi \ell_i (\xi) \left(\frac{\mathrm{d}\xi}{\mathrm{d}x} \right)^2 \frac{\mathrm{d}x}{\mathrm{d}\xi} \vert_{\xi = \xi_k}$$

#### Exercise 2

Implements the stiffness matrix using the integration weights at GLL locations $w$, the jacobian $J$, and shear stress $\mu$. Then, perform the global assembly of the mass matrix and display the matrix to visually inspect how it looks like.



In [ ]:

# Elemental Stiffness Matrix
# ---------------------------------------------------------------
for i in range(0, N+1):
for j in range(0, N+1):
for k in range(0, N+1):
Ke[i,j] = Ke[i,j] + mu*w[k]*Ji**2 *J*l1d[i,k]*l1d[j,k]

# Global Stiffness Matrix
# ---------------------------------------------------------------
K = np.zeros([ng, ng])
# Values except at element boundaries
for k in range(1,ne+1):
i0 = (k-1)*N + 1
j0 = i0
for i in range(-1,N):
for j in range(-1,N):
K[i0+i,j0+j] = Ke[i+1,j+1]

# Values at element boundaries
for k in range(2,ne+1):
i0 = (k - 1)*N
j0 = i0
K[i0,j0] = Ke[0,0] + Ke[N,N]

# ---------------------------------------------------------------
# Display stiffness matrix K
# ---------------------------------------------------------------
plt.figure()
plt.imshow(K)
plt.title('Stiffness Matrix $\mathbf{K}$')
plt.axis("off")

plt.tight_layout()
plt.show()



### 4. Finite element solution

Finally we implement the spectral element solution using the computed mass $M$ and stiffness $K$ matrices together with a finite differences extrapolation scheme

$$\mathbf{u}(t + dt) = dt^2 (\mathbf{M}^T)^{-1}[\mathbf{f} - \mathbf{K}^T\mathbf{u}] + 2\mathbf{u} - \mathbf{u}(t-dt).$$


In [ ]:

# SE Solution, Time extrapolation
# ---------------------------------------------------------------

# initialize source time function and force vector f
src  = ricker(dt,Tdom)
isrc = int(np.floor(ng/2))   # Source location

# Initialization of solution vectors
u = np.zeros(ng)
uold = u
unew = u
f = u

# Initialize animated plot
# ---------------------------------------------------------------
plt.figure(figsize=(10,6))
lines = plt.plot(xg, u, lw=1.5)
plt.title('SEM 1D Animation', size=16)
plt.xlabel(' x (m)')
plt.ylabel(' Amplitude ')

plt.ion() # set interective mode
plt.show()

# ---------------------------------------------------------------
# Time extrapolation
# ---------------------------------------------------------------
for it in range(nt):
# Source initialization
f= np.zeros(ng)
if it < len(src):
f[isrc-1] = src[it-1]

# Time extrapolation
unew = dt**2 * Minv @ (f - K @ u) + 2 * u - uold
uold, u = u, unew

# --------------------------------------
# Animation plot. Display solution
if not it % iplot:
for l in lines:
l.remove()
del l
# --------------------------------------
# Display lines
lines = plt.plot(xg, u, color="black", lw = 1.5)
plt.gcf().canvas.draw()
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