Network-magnetotelluric method and its first results in central and eastern Hokkaido, NE Japan

A new field observation technique based on the magnetotelluric (MT) method has been developed to determine deep and large-scale 3-D electrical conduct ivity distributions in the Earth. The method is named 'Network-MT', and emp loys a commercial telephone network, to measure voltage differences with lo ng dipole lengths ranging from 10 to several tens of kilometres. This obser vation configuration enables us to obtain the telluric field distribution w ith nearly continuous coverage over a target region. Response functions are estimated between the respective voltage differences and the horizontal ma gnetic fields at a reference point. Owing to the long electrode spacing, th e observed responses are relatively free from the effects of small-scale ne ar-surface heterogeneity with a scalelength shorter than the typical electr ode spacing. Therefore, physically meaningful direct comparison between the observations and model responses is feasible even if the fine-scale featur es of near-surface heterogeneity are ignored. This extensively reduces the difficulty, especially in 3-D MT interpretation. The first Network-MT experiment was performed in central and eastern Hokkai do, NE Japan, in 1989. It took about five months to complete all of the mea surements, and used 209 dipoles to cover the target area of 200(EW) x 200(N S) km(2). The long electrode spacing enabled us to obtain the voltage diffe rences with a high signal-to-noise ratio. For 175 dipoles, the squared mult iple coherency between the voltage difference and the horizontal magnetic f ield at Memambetsu Geomagnetic Observatory was determined to be more than 0 .9 in the period from 10(2) to 10(4) s. 193 MT impedances were computed in tensor form by linear combination of the response functions. The estimated impedances generally possessed smooth period dependence throughout the peri od range. No drastic spatial change was observed in the characteristics of the tensors for neighbouring sites, and some regional trend could be detect ed in the spatial distribution. Thus, we confirmed the merit of the Network -MT method, that its responses are little affected by small-scale near-surf ace structures. The regional feature of the response implied a significant influence of the coast effect, and was well correlated with the regional ge ological setting in Hokkaido. Conventional Groom-Bailey tensor decomposition analysis revealed that the t arget region is not regionally one- or two-dimensional. Therefore, we devel oped a 3-D forward modelling scheme specially designed for the Network-MT e xperiment, and tried to reproduce the Network-MT responses directly. In the 3-D model, a realistic land-sea distribution was considered. The resistivi ty of sea water was fixed to be 0.25 Ohm m and, as a first trial of 3-D mod elling, the resistivity of the land was assumed to be uniform and its value was determined to be 300 Ohm In by a simple one-parameter inversion. Overa ll agreements between the observations and the best-fit model responses ind icated the importance of the 3-D coast effect in the target region. However , there remained significant discrepancies, especially in the phase of the responses, which provide a clue to determining a regional deep 3-D structur e.