Tag Archives: data

Data geometry: rotation angles for transmitters

MARE2DEM allows for arbitrary orientations of  transmitter dipoles. The orientation of  electric and magnetic dipole transmitters are described by the two angles of the dipole axis: the horizontal azimuth and the vertical dip angle.

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Example sources including the horizontal electric dipole (HED), vertical electric dipole (VED), horizontal magnetic dipole (HMD) and vertical magnetic dipole (VMD).  Since MARE2DEM allows for arbitrary source transmitter orientations, you only need to specify the source type as electric or magnetic in the data file and list its location and two orientation angles.
The transmitter azimuth is the angle of the dipole measured clockwise from the 2D strike direction x.
The transmitter azimuth is the angle of the dipole measured clockwise from the 2D strike direction x.  Thus, typical inline electric dipole CSEM data have a transmitter azimuth of about 90° (or 270°), while broadside data have a transmitter azimuth of  0° (or 180°).
The transmitter dip angle
The transmitter dip angle is defined as the angle down, clockwise from the azimuth of the transmitter.  A normal horizontal  electric dipole transmitter  has a dip of 0°.

 

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).

 

EM (not MT) receivers should be positioned with an x coordinate in the green region. Receivers too close the transmitter will suffer from inaccuracy due to the source singularity (in the pink region). If a point dipole is being modeled, receivers should be located at least a few dipole lengths away (red region). Finally, while technically receivers can be given any x coordinate, the default setting used by MARE2DEM for the 2.5D wavenumber domain transforms may break down if the receivers are located too far down strike from the transmitter (gray regions); typically this may be 1 -2 km or more offline.
EM (not MT) receivers should be positioned with an x coordinate in the green region. Receivers too close to the transmitter will suffer from inaccuracy due to the source singularity (in the pink region). If a point dipole is being modeled, receivers should be located at least a few dipole lengths away (red region). Finally, while technically receivers can be given any x coordinate, the default setting used by MARE2DEM for the 2.5D wavenumber domain transforms may break down if the receivers are located too far down strike from the transmitter (gray regions); typically this may be 1 -2 km or more offline.

Geo-referencing to UTM coordinates

Note: UTM referencing is only used for plotting purposes. MARE2DEM completely ignores this for the 2D EM calculations.

To help with post-inversion plotting of well logs and SEG-Y seismic overlays on your 2D inversion models, the data file has a line for specifying the UTM zone, origin and rotation of the 2D model coordinates:

UTM0

The 2D strike angle is the geographic angle corresponding to the x direction of the 2D model, clockwise from north. The example below has a 2D strike of 20º.

UTMreferencing

Data geometry: vertical positions of transmitters and receivers

Receivers and transmitters can be located anywhere in conducting parts of the model domain, but some care is needed to make sure MARE2DEM will output meaningful results. Seafloor EM transmitters and receivers should be located at least a little bit above the seafloor boundary; this is so that MARE2DEM knows they are in the seawater rather than the seafloor below. Likewise, land EM receivers and transmitters should be positioned a tad below the surface. A small vertical distance such as 0.1 m is sufficient; make sure you remember that  is positive down, so seafloor receivers should have a  position slightly less than the seafloor depth.  You can also have borehole transmitters and receivers, but these have only received a small amount of testing so far.

 

Receivers and transmitters can be located anywhere in conducting parts of the model.
Receivers and transmitters can be located anywhere in conducting parts of the model.
RxGeometry2
Seafloor receivers should be positioned a small distance above the seafloor so that MARE2DEM will use seawater conductivity when computing the normal component of the electric field. Usually a small numerical value (such as 0.1 m) is sufficient. So if z_{seafloor} = 1000 m,  then z_{rx} = 1000- 0.1 m.
RxGeometry3
Onshore receivers should be positioned at least a small distance below the surface so that MARE2DEM will use a finite conductivity for its receiver calculations. Usually a small numerical value (such as 0.1 m) is sufficient. So if z_{surface} = 0 m, then z_{rx} = 0.1 m.