ArjunGUI is a graphical user interface (GUI) for ArjunAir (Wilson et al., 2006), a computer program for modelling and interpretation of geophysical airborne electromagnetic (AEM) data from single profile using a two-dimensional (2D) model of electrical resistivity and susceptibility. ArjunAir was originally developed by Drs. Glenn Wilson, Art Raiche and Fred Sugeng for the CSIRO/Amira consortia (project P223F), and it became public domain software in 2010 (http://p223suite.sourceforge.net/). The modelling method is called 2.5 dimensional, because the model is two-dimensional and the (dipolar) source-receiver system is three-dimensional.
Presently, ArjunGUI supports only time-domain (transient) EM data and measurement systems. Because the focus is on airborne applications, the source loop is approximated as a magnetic dipole, i.e., the actual size and shape of the transmitter loop is not taken into account. The receiver is either a small loop or a coil measuring the time derivative of the magnetic flux density dB/dt (T/s) or a magnetometer (flux-gate or squid) measuring the intensity of the magnetic flux density B (T). The response is the vertical (Z) component or the horizontal (X) component along the flight line or both Z and X components.
ArjunGUI was developed primarily for inversion, but it does suit forward modelling too. In forward modeling the topography is flat and flight altitude is constant. In inverse modelling both the topography and the varying flight altitude can be taken into account. Inversion uses either unconstrained (SVD) or constrained inversion method to update the model resistivity values so that the data error, i.e., the difference between the measured and the computed data is minimized. In constrained (Occam) inversion the roughness of the model, i.e. the variation of the resistivity between neighboring points is minimized together with the data error. Occam inversion is directed towards a smoothly varying resistivity model if all the cells below the profile are free to change. The initial resistivity (and susceptibility) values and the fix/free status of the parameters can be edited manually to incorporate a priori data in the model. Weights can be assigned to individual data points or stations or channels to either include them in or exclude them out from the inversion.