problem with the receiver's location

[ayaywzn]

Hi all,
I have a problem with the receiver's location and your help is appreciated. I am running the specfem3D_Cartesian. The SEM model considers the actual topography of the simulation region. I specify several receivers at the surface with depth of 0 m. However, the target receivers are inconsistent with my preconceptions original coordinates I specified, i.e.:

station # 2 N N3308N
original latitude: 3260730.
original longitude: 230386.0
original x: 230386.0
original y: 3260730.
original depth: 0.0000000E+00 m
horizontal distance: 43.3977160057072
target x, y, z: 230386.0 3260730. 1187.291
closest estimate found: 11.62000 m away
receiver located in slice 159
in element 11250
in elastic domain
at coordinates:
xi = 0.624910007199495
eta = 1.00000000000000
gamma = 1.00000000000000
rotation matrix:
nu1 = 1.00000000000000 0.000000000000000E+000
0.000000000000000E+000
nu2 = 0.000000000000000E+000 1.00000000000000
0.000000000000000E+000
nu3 = 0.000000000000000E+000 0.000000000000000E+000
1.00000000000000
x: 230386.000000000
y: 3260722.00000000
depth: 8.42759222328186 m
z: 1178.86330133141

I do not know why the receivers are moved. I would to know how to solve this problem in order to get the results in the accurate position.
Thank you in advance,
Zhang.

[danielpeter]

The finite-element mesh with topography depends obviously on the grid discretization as well as the topography surface resolution. The elements are taking the corner point positions to interpolate and fill the element with GLL points. This will lead to a difference between the actual elevation at a certain lat/lon or x/y position and the actual height of the element boundary, in particular for positions other than at the exact corners of an element.

In your case above, the nearest possible position found within a grid element is bound by the element boundaries. Those won't exactly match the elevation nor the lat/lon or x/y when the grid elements are distorted due to topography.