# Data geometry: rotation angles for receivers

Receiver rotations are described by three angles:   θ, α and β, as shown in the image below. These rotate from the model coordinates $x,y,z$ to the fully rotated measurement coordinates $x'',y'',z''$. For the rotation matrices associated with these angles, see Key and Lockwood (2010).   The  θ angle describes the horizontal rotation of the receiver's $x$  axis from the 2D model strike direction $x$ For a typical CSEM data set ,  the data should be rotated so that  θ = 0. Therefore, for an array of receivers inline to a horizontal electric dipole source (with transmitter azimuth of 90°), the inline electric field will be the $E_y$ component.  There angles α and β describe tilts of the receiver.  α is the vertical tilt of the $E_{x''}$ component. If  α =0, then β describes the vertical tilt of the $E_{y''}$, otherwise β  describes a slanted angle to $E_{y''}$.

##### References

Key, K., & Lockwood, A. (2010). Determining the orientation of marine CSEM receivers using orthogonal Procrustes rotation analysis. Geophysics, 75(3), F63–F70. doi:10.1190/1.3378765

# Data geometry: horizontal positions of transmitters and receivers

For MT receivers, the x coordinate of the receivers is ignored by MARE2DEM since the 2D MT fields are strike invariant. For controlled-source electromagnetic receivers, there are a few considerations you need to mind about the accuracy of MARE2DEM, which uses a total field implementation for the source and a wavenumber domain transformation to compute the spatially varying EM fields:

• The receiver  component should be close the to  transmitter position. Receivers don't need to be perfectly inline, but  if they are more than 1-2 km down-strike from the transmitter (gray regions below), the default settings for the wavenumber domain transforms in MARE2DEM may break down due to highly oscillatory kernel functions.
• Due to the source singularity in the 2.5D wavenumber domain, receivers located down strike from the transmitter (pink region below) may be inaccurate unless they are positioned much deeper than the source.
• If modeling point dipoles, remember that receivers closer than a few real dipole lengths from the transmitter will be inaccurate due to finite length dipole effects (red region below). You can instead specify a finite length dipole in the data file, but this comes at an increased numerical effort for MARE2DEM (more on that in another tutorial).