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# Reciprocity for Faster Modeling of CSEM Data

Here we just state some quick tips about when to use reciprocity to speed up CSEM computations; see the Electromagnetic Reciprocity section for the theoretical background and more details.

The run time for MARE2DEM depends strongly on the number of CSEM transmitters being modeled and less so on the number of receivers. Thus, when the number of transmitters greatly exceeds the number of receivers (more specifically, the number of receivers times the numbers of receiver components being modeled) then electromagnetic reciprocity should be applied to the input data file so that MARE2DEM can solve the reciprocal problem more quickly.

## When should I apply reciprocity?

### Yes

• When the number of transmitters greatly exceeds the number of receivers times the number of components (per receiver). Reciprocity is almost always recommended for nodal seafloor marine CSEM data since there are usually far fewer receivers than there are transmitter shot points.

### No

• For land CSEM data, usually there are far fewer transmitters than receivers and so reciprocity should not be applied.

• For towed streamer CSEM data, the source moves with the attached towed receivers and so reciprocity should not be applied since it offers no speed up advantage.

## How do I apply reciprocity?

The geometry (location and angles) of the source and receiver wires must be maintained for EM reciprocity to hold; thus a dipping transmitter source becomes an identically dipping receiver at the same location, and likewise a real tilted receiver will become an identically tilted (and located) transmitter.

An easy way to think about this is to consider the actual transmitter wire and the actual receiver wire. Leave them as is (same location, same rotation and tilt angles), and just reverse energize them, so that the source wire becomes a receiver wire and the actual receiver wire becomes energized as the source wire.

The easiest case is for electric sources and receivers as shown in the image below. If the receiver was instead a magnetic receiver component (say the crossline magnetic field component), then the actual receiver becomes a magnetic source (in the same direction and location as the actual receiver component) and the actual sources become electric receivers.