I understand this "This approach requires a grid (*grd format) with the Vs30 values (in m/s) at different

locations."

But, How is the best way to create the file?

I have created a file with QGIS, with a grid and modifying the value of z by Vs30.

I couldn't load this .grd

Is there some characteristic or format that this file must have, or only three columns. X value, Y value, Z value (Vs30)

I appreciate any contribution that allows me to interpret how to make this file