VLF-EM for geological exploration is a well known method (McNeill & Labson 1991) and is commonly used in the early stages of a geological characterization program for remote areas. The signals propagate deep in the earth and can help in understanding subsurface fractures, faults and contacts between rock types. The measurements are relatively inexpensive and can be useful in the early stages of an earth characterization project.
The geophysical user community for VLF is sparse and sometimes there are gaps of several years where changes to the VLF transmitters are not common knowledge. The dismantling of VLF station NSS (transmitting at 21.4-kHz) in 1999, the transference of this frequency to NPM in Hawaii, the arrival of the new 500 kW VLF transmitter station NML (25.2-kHz) in La Moure, North Dakota are fairly recent changes. Interference between NML and NLK, only 400-Hz apart, is worth noting. There are important implications to these changes to geophysical surveyors.
Reference:McNeill, J.D. and V.F. Labson. 1991. Geological Mapping using VLF Radio Fields. in Electromagnetic Methods in Applied Geophysics, Volume 2, Applications part B, page 521-640 Society of Exploration Geophysicists.
VLF Transmitter List*1
1. LF transmitter list courtesy of Stanford University Solar Centre -
http://solar-center.stanford.edu/SID/docs/SID_Manual.pdf
Figure 1. Nulling and reading using an EM-16 on a frozen lake.
Figure 2. Nulling and reading using an EM-16 on a frozen lake.
Figure 3. Local VLF transmitter.
Figure 4. Automatic VLF-EM, magnetics, and GPS being read.