We present and discuss the properties of a time-domain CSEM (Controlled Source ElectroMagnetic) technology that utilizes vertically oriented transmitters and receiver antennas. The data are recorded in transient mode, wherein voltage time-series are recorded after transmitter switchoff. A square pulse with an alternating polarity is followed by a silent period in which the response is measured. The response curves from many pulses are averaged in order to reduce noise, and then binned into time windows. From theory, the vertical electric field is sensitive to deep resistive layers. At late times the vertical electric field decays like E z (t) ~t-5/2 over a rock of uniform conductivity. Another model, with the same overburden resistivity but with a resistive layer gives rise to a faster temporal decay, with a maximum contrast occurring typically at t=2-10s depending on the depth of the resistivity layer (hydrocarbons).

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