Comment on “Cooperative constrained inversion of multiple electromagnetic data sets” (Michael S. McMillan and Douglas W. Oldenburg, 2014, Geophysics, 79, no. 4, B173–B185)

McMillan and Oldenburg (2014) develop a method for cooperatively inverting multiple electromagnetic data sets to produce a “consistent” 3D resistivity/conductivity model with improved resolution. Their data set includes airborne-time-domain electromagnetic data, controlled-source audio-frequency magnetotellurics (with inline electric-field and orthogonal magnetic-field measurements for frequencies ranging between 2 and 2048 Hz), and DC resistivity data. Because (1) conductivity/resistivity is known to be frequency-dependent especially in the presence of disseminated metallic particles (Wong, 1979; Marchant et al., 2013), (2) the range of frequencies investigated by McMillan and Oldenburg (2014) is rather substantial (⁠$∼four$ orders of magnitudes), and...