One of the things highlighted at the recent FORC workshop at Santa Fe was a simple method devised by Ramon Egli to reduce the appearance of artefacts in FORC diagrams along the “remanence diagonal” – the line with slope -1 through the origin of the FORC diagram that represents the region where the applied field to the sample is zero (i.e. remanence measurements). In this region it is often the case that FORCs have a sigmoidal appearance that is not well described by the 2nd order polynomial fit used to smooth the FORC surface (e.g. due to the presence of SP particles). This can result in artefacts along the remanence diagonal and large anomalies where the remanence diagonal intersects the lower edge of the FORC diagram, especially when the smoothing boxes are not square and when large smoothing factors are used (as is the case for VARIFORC smoothing). The solution is simple but very effective: subtract the lower branch of the hysteresis curve from each of the FORC curves to obtain the ‘difference FORCs’. This reduces the sigmoidal component of the signal. The resulting FORC distribution is not affected, but the appearance of artefacts is significantly reduced (see below).
On the left we see the standard bush.txt FORC diagram processed using VARIFORC. The intensity has been enhanced to highlight the artefacts along the lower edge and the remanence diagonal. On the right we see the ‘difference FORCs’ obtained by subtracting the lower branch of the hysteresis curve. The processed FORC diagram now has the artefacts completely eliminated!
Go to our download site to get the latest version of FORCinel 2.03 that includes this new correction feature. See the ReadMe file in the FORCinel package for instructions on how to perform the correction. Highly recommended!