Full waveform inversion of 2-D acoustic wave

[chenzhaoxingxin]

Hi Daniel,
First of all, thank you very much for sharing the program for everyone to use and learn; also thank you very much for your achievements in the professional field.
Recently, I was learning the code you shared. Your code is very practical. I encountered a problem when using the program to retrieve the full waveform of two-dimensional sound waves. The error report screenshot is as follows:

I used the P component of velocity as the observation data and set the density to a constant value: 1000kg / m3. When the inversion is carried out in four frequency ranges, the inversion results can be obtained in the first frequency range, and the above errors appear when the program continues to run. I looked at the current inversion results, the velocity value and density value are - Nan.
I want to ask you, have you ever encountered such a problem in the test, and how did you solve it?
Thank you again for your work and help. We look forward to your reply.