Land Electromagnetics

Worldwide magnetotelluric and controlled-source methods

We offer worldwide land electromagnetic (EM) services, including passive magnetotelluric (MT) and active controlled-source electromagnetic (CSEM) methods. Our land EM group based in Milan has more than 25 years of worldwide experience in integrated EM solutions.

Land MT

MT is a broadband electromagnetic geophysical technique that uses natural time-dependent variations in the Earth’s magnetic field as the source and electric fields induced in the earth as output. These data are used to determine (usually via 2D or 3D inversion) the resistivity distribution within the earth to depths of many kilometers. This is then interpreted in terms of lithology.

Under some geological conditions, it is impossible or prohibitively expensive to acquire useful seismic data. These conditions include subthrust plays, areas of volcanic cover, and beneath shallow salt bodies. To MT, the conditions that cause problems to seismic data are largely transparent, and thus the method readily images the subsurface in terms of resistivity. Ideal targets include resistive carbonate reservoirs below shale or flysch and clastic reservoirs below salt bodies. Exploration environments we have worked in include the Alpine thrust belts of Europe and the Middle East; subvolcanic areas in North Africa, the Middle East, and the United States; and subsalt zones in Europe, the Middle East, and the Gulf of Mexico.

The results of an MT survey carried out in Greece in an area where seismic data obtained and processed with modern techniques gave almost no useful results. The objective was to map the 3D structure of multiple-thrusted carbonate and anhydrite units and the underlying autochthon. This region is an active petroleum exploration area where carbonates can represent both source and reservoir rocks.

Land CSEM

CSEM acquisition techniques cover

inductive source-inductive receiver
grounded source-inductive or grounded receiver
grounded source-seismic receiver.

Transmitters can provide up to 250 A of current, and our range of transmitter controllers is capable of providing both standard and specialized waveforms.

The scale of applications ranges from shallow ground-water exploration to direct hydrocarbon identification. Interpretation is based on imaging using pseudoseismic responses as well as 2D and 3D inversion.

Seismic section illustrating the improvements in poststack depth migration (PSDM) imaging using frequency-domain CSEM to accurately identify the top of a salt dome, where this was needed as a constraint for improved PSDM processing. The blue horizon is from inversion of CSEM data; the improvement in PSDM quality at target depth is described by den Boer et al. (First Break, 2000)