Last updated on August 12th 2016.

Note

The following links shows the example project as it should be just before step 14. You can use this to check your progress or restart the tutorial at this very point.

After Step 14: Misfit and Adjoint Sources

# 15. Model Update¶

LASIF per se does not aid you with actually updating the model. This is a concious design decision as a large part of the research or black art aspect in full seismic waveform inversions actually comes from the model update, potential preconditioning, and the actually employed numerical optimization scheme.

This parts illustrates a simple update. When running a real inversion, please consider spending a couple of days to scripts model updates and everything else required around a given LASIF project.

## 15.1. Actual model update¶

Because LASIF does not (yet) aid you with updating the model, you will have to do this manually. Keep in mind that the kernels SES3D produces initially are the raw gradients, and unsmoothed. You will probably need to smooth them before updating the model to get reasonable results without excessive focusing near sources and/or receivers.

## 15.2. Line Search/Model Update Check¶

At one point you will have to check the total misfit for a certain iteration and model - be it for a line search or to check a new test model. The recommended strategy to do this within LASIF is to use “side-iterations”, e.g. in the following we will test the misfit for a model update with a step length of 0.5:

$lasif create_successive_iteration 1 1_steplength_0.5$ lasif migrate_windows 1 1_steplength_0.5
# Copy synthetics for the test iteration - then calculate both misfits.
\$ mpirun -n 4 lasif compare_misfits 1 1_steplength_0.5


You will notice that the compare_misfits command always requires two iterations. This is necessary as any misfit must be calculated for exactly the same stations, windows, and weights for two iterations to be comparable. For steepest/gradient descent optimization methods it is fine to just take the current and next iteration, for others like conjugate gradient and L-BFGS make sure to choose the correct baseline iteration to get comparable misfit measurements!

## 15.3. Simple Update Script¶

The following is a short script that demonstrates the principles of how to perform a model update. It does a simple gradient descent update at various step lengths that then have to be tested. It requires the initial model as well as the kernels to be in the SES3D regular grid format and utilizes the Python tools shipping with SES3D.

#!/usr/bin/env python
# -*- coding: utf-8 -*-
import numpy as np
# This is from the SES3D Python tools.
import models as m

# Input. ----------------------------------------------------------------------
# Directory where the current model is located.
dir_current_model = "../MODELS/MODELS_3D/MODEL.66/"

# Directory where test models will be written.
dir_test_models = "../MODELS/MODELS_3D/MODEL.67/"

# Directory where current gradients are located.

# List of events for which kernels are available.
eventlist = [
"GCMT_event_NORTHERN_ITALY_Mag_4.9_2000-8-21-17",
"GCMT_event_NORTHWESTERN_BALKAN_REGION_Mag_5.9_1980-5-18-20"]

# Smoothing iterations.
sigma = 3.0

# Test step lengths.
gamma = [0.15, 0.5, 1.0]

print "read current model from directory " + dir_current_model

csv = m.ses3d_model()
csh = m.ses3d_model()
rho = m.ses3d_model()
cp = m.ses3d_model()

# Initialisation.

# Loop through events.
for k in eventlist:

print "read kernels for event " + str(k)

if k == eventlist[0]:

#  Clip the upper percentile to remove singularities.

else:

# Clip the upper percentile to remove singularities.

# Add to the previous kernels.

# Compute the new descent direction. ------------------------------------------

# Store new gradients and descent directions. ---------------------------------

h_csv.write(dir_kernels, 'h_csv_sum')
h_csh.write(dir_kernels, 'h_csh_sum')
h_rho.write(dir_kernels, 'h_rho_sum')
h_cp.write(dir_kernels, 'h_cp_sum')

# Apply smoothing. ------------------------------------------------------------
print "smoothing"

h_csv.smooth_horizontal(sigma, filter_type='neighbour')
h_csh.smooth_horizontal(sigma, filter_type='neighbour')
h_cp.smooth_horizontal(sigma, filter_type='neighbour')
h_rho.smooth_horizontal(sigma, filter_type='neighbour')

# Relative importance of the kernels. -----------------------------------------
sum_csv = 0.0
sum_csh = 0.0
sum_rho = 0.0
sum_cp = 0.0

sum_csv = sum_csv + np.sum(np.abs(h_csv.m[n].v))
sum_csh = sum_csh + np.sum(np.abs(h_csh.m[n].v))
sum_rho = sum_rho + np.sum(np.abs(h_rho.m[n].v))
sum_cp = sum_cp + np.sum(np.abs(h_cp.m[n].v))

max_sum = np.max([sum_csv, sum_csh, sum_cp, sum_rho])

scale_csv_total = sum_csv / max_sum
scale_csh_total = sum_csh / max_sum
scale_rho_total = sum_rho / max_sum
scale_cp_total = sum_cp / max_sum

print scale_csv_total, scale_csh_total, scale_rho_total, scale_cp_total

# Depth scaling.---------------------------------------------------------------
max_csv = []
max_csh = []
max_rho = []
max_cp = []

for n in range(h_csv.nsubvol):

max_csv.append(np.max(np.abs(h_csv.m[n].v[:, :, :])))
max_csh.append(np.max(np.abs(h_csh.m[n].v[:, :, :])))
max_rho.append(np.max(np.abs(h_rho.m[n].v[:, :, :])))
max_cp.append(np.max(np.abs(h_cp.m[n].v[:, :, :])))

max_csv = np.max(max_csv)
max_csh = np.max(max_csh)
max_rho = np.max(max_rho)
max_cp = np.max(max_cp)

damp = 0.3

for n in range(h_csv.nsubvol):

for k in range(len(h_csv.m[n].r) - 1):

h_csv.m[n].v[:, :, k] = h_csv.m[n].v[:, :, k] / (
damp * max_csv + np.max(np.abs(h_csv.m[n].v[:, :, k])))
h_csh.m[n].v[:, :, k] = h_csh.m[n].v[:, :, k] / (
damp * max_csh + np.max(np.abs(h_csh.m[n].v[:, :, k])))
h_rho.m[n].v[:, :, k] = h_rho.m[n].v[:, :, k] / (
damp * max_rho + np.max(np.abs(h_rho.m[n].v[:, :, k])))
h_cp.m[n].v[:, :, k] = h_cp.m[n].v[:, :, k] / (
damp * max_cp + np.max(np.abs(h_cp.m[n].v[:, :, k])))

h_csv = scale_csv_total * h_csv
h_csh = scale_csh_total * h_csh
h_rho = scale_rho_total * h_rho
h_cp = scale_cp_total * h_cp

# Store the smoothed descent directions. --------------------------------------
h_csv.write(dir_kernels, 'h_csv_sum_smoothed')
h_csh.write(dir_kernels, 'h_csh_sum_smoothed')
h_rho.write(dir_kernels, 'h_rho_sum_smoothed')
h_cp.write(dir_kernels, 'h_cp_sum_smoothed')

# Compute and store updates. --------------------------------------------------
for k in np.arange(len(gamma)):
csv_new = csv + gamma[k] * h_csv
csh_new = csh + gamma[k] * h_csh
rho_new = rho + gamma[k] * h_rho
cp_new = cp + gamma[k] * h_cp

csv_new.write(dir_test_models, 'dvsv.' + str(gamma[k]))
csh_new.write(dir_test_models, 'dvsh.' + str(gamma[k]))
rho_new.write(dir_test_models, 'drho.' + str(gamma[k]))
cp_new.write(dir_test_models, 'dvp.' + str(gamma[k]))