Affordable Magnetotellurics: Interpretation in Natural Environments
Areal magnetotelluric (MT) surveys and their 3-D interpretation are expensive. It therefore can be valuable to know when a line profile and a 2-D (or quasi-2-D) interpretation may suffice. Some factors that determine this are the effects of electric charges that arise at resistivity contrasts, the similitude in the electromagnetic (EM) responses of small and large structures, and the immunity of certain MT tensor elements to 3-D effects. The most important task in designing a 2-D survey, of course, is to determine geoelectric trends accurately from existing information. Electric dipoles should be laid out parallel and perpendicular to a profile that is orthogonal to the strike of the terrane; most of the dipoles should be oriented across strike for adequate sampling of the more rapidly varying response. Two-dimensional modeling should concentrate on the impedance in the transverse magnetic (TM) mode because of its low sensitivity to the effects of finite strike. The vertical magnetic field and the phase of transverse electrical impedance should be checked for consistency with a 2-D model. Impedances in the Groom-Bahr decomposition can help fix geoelectrical trends, but care is needed to preserve longer spatial wavelengths of the response amplitudes. Profile interpretation is not applicable to all MT prospects. Successes and limitations are illustrated with MT transects over the Juan de Fuca subduction zone and the Long Valley caldera magmatic system.