Time Domain Electromagnetics (TDEM) and nano TDEM

Time Domain Electromagnetics (TDEM) is a geophysical exploration technique to assess the subsoil electrical resistivity, especially for stratified geology. TDEM measurements are widely applied in hydrogeology and especially in groundwater detection. This technique uses a transmitter Tx and a receptor Rx connected to conductive cable loops. The Tx is connected to the external loop and cause a current pulse at specific frequency and for a determined time interval, inducing an electromagnetic field propagating through the stratified subsoil. The Rx is connected to the internal loop and is configured to record the subsoil electromagnetic behavior when the current pulse in the external loop is suddenly shut off. This behavior provides information about the subsoil electrical resistivity distribution. The loops can also be concentric and coincident.
The differences between normal TDEM and nano TDEM are basically loop size, current frequency and timing, resulting in different investigation depth (DOI): the TDEM presents higher DOI but less resolution at shallow depth, (and starting from about 30-40 m depth), while the nano TDEM investigates a minor DOI but with much higher shallow resolution, starting from a few meters depth.