Rotation and strike

MT data are recorded in one coordinate frame, but a mesh or a 2D model often needs another. In this section we rotate data files with rotate_data, and see how the 2D workflows find and use the geoelectric strike.

Rotate a data file

Turning the axes by an angle θ (clockwise) changes the impedance tensor Z and the tipper T as

Z' = R Z Rᵀ,   T' = T Rᵀ,   R = [cos θ  sin θ; −sin θ  cos θ]

and the errors are carried along. This is the same for 1D, 2D and 3D data.

rotate_data("data.obs", 30.0)                                  # writes data-r.obs
rotate_data("data-r.obs", 9.5; kind = :declination)            # updates data-r.obs
rotate_data("data.obs", [9.4, 9.5, 9.6]; kind = :declination)  # one angle per site
d = rotate_data(load_data_modem("data.obs"), 30.0)             # in memory

The rotated file is written next to the input as <stem>-r<ext>, so data.obs becomes data-r.obs. A file that already ends in -r is updated in place.

Three kinds of rotation

kind	What it does	Use it when
:mesh (default)	turns the data and the station positions	your mesh is rotated
:strike	the same, for the 2D strike frame	a 2D workflow does this for you
:declination	turns the data by −D, keeps the station positions	the data were recorded with x on magnetic north

The rotation line of the ModEM header (> 30.00) always holds the total angle of the data's x axis from geographic north. Latitude and longitude never change.

Warning

A site and period without the full impedance tensor (or both tipper components) cannot be rotated. It is dropped with a warning.

Every rotation is recorded

Each call adds a step to the first # line of the data block, for example:

# Rotated by MTGeophysics.jl | rotated: mesh +30.00; declination [JK01 +9.40, JK02 +9.50, JK03 +9.60]

The package reads this history back with the data and carries it into every file it writes, so you can always see what was done to a data set.

The 2D strike

The 2D solver assumes that x runs along the geological strike and y along the profile. Every 2D workflow therefore first rotates the data to the strike given in fwd.ctrl:

Strike (deg)             : auto

• auto (the default) estimates the strike from the phase tensors of the data.

• A number, such as 32.5, sets it in degrees clockwise from north.

You can also estimate the strike or rotate to it yourself:

s = EstimateStrike2D(load_data2d("data.obs"))   # strike, with consistency and skew
RotateToStrike2D("data.obs")                    # writes data-r.obs

Inversions write the rotated data to their run folder and the strike to Summary.txt.

Note

The mesh depends on the station positions in the strike frame. If you change the strike, run the mesh tool again.