Why is the number of iterations different from the number of grids within the propagation distance in the x or y direction within the time window

[chi gao]

#title: No reflective layer
#domain: 3 1 0.003
#dx_dy_dz: 0.003 0.003 0.003
#time_window: 10e-9
#material: 1 0 1 0 air
#waveform: ricker 1 10e8 my_ricker
#hertzian_dipole: z 1.4 0.5 0 my_ricker
#rx: 1.6 0.5 0
#box: 0 0 0 3 1 0.003 air
#geometry_view: 0 0 0 3 1 0.003 0.003 0.003 0.003 gkone n

Why is the iteration count of my model file 1415 times instead of 1000 times? My model is relatively simple, and the propagation distance of waves should be 3m within 10 nanoseconds. My dx is 0.003, and there are 1000 grids in both the x and y directions. Why did it iterate more than 1000 times

[Antonis Giannopoulos]

The answer is in the documentation: http://docs.gprmax.com/en/latest/gprmodelling.html#guidance

The tine step in FDTD is determined by the CFL stability condition. So, the time step in 2D is sqrt(2) smaller than the one you thought it should be and in 3D ii will be sqrt(3) smaller. So, sqrt(2)*1000 is exactly what you got rounded to the nearest integer step.

Antonis

[chi gao]

Thank you, Professor. I understand this question and appreciate your reply. Additionally, I have a more basic question. In this post, I have attached an in file. The waveform obtained from the simulation is not an idealized Ricker wave, and the waveform is asymmetric on both sides. At the same time, its amplitude is not 1. Why did the received waveform change in this way and what is the reason for it

[Antonis Giannopoulos]

The answer to this question has to do with the radiation of the source which in the 2D case is a line source and in 3D will be a Hertzian Dipole. What you specify in a 2D model is the current waveform of the line source. This source will radiate an electromagnetic field that is not an exact replica of the current waveform. The amplitude you record at som other point is the Electric field at some distance and it is in V/m and not the same as the current on the source.

To understand these topics you need to study some basic electromagnetic theory and GPR fundamentals. The answers are there.

[chi gao]

Thank you, Professor. Thank you for taking the time to reply. I have a general understanding of what you mean