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A number of analytical inversion schemes of the one-dimensional magnetotelluric response Z ( T ) = &w.L~~,(T)/T exp i [n/4 -@(T)] require input data in the form of smooth single-valued functions of apparent resistivity pa and phase Q, versus the period 2' . It is generally assumed that one-dimensionality is guaranteed if the observed surface impedance Z ( T ) reduces to a complex scalar, rather than a tensor, and if the derivatives of pa@) and $(T) and the value of @(T) remain within certain bounds. This is not sufficient, however, since apparent resistivity and phase are related through integral dispersion relations similar to those which in optics connect modulus and argument of the complex reflection coefficient. Inversion schemes operating on data which do not satisfy the dispersion relations can never lead to model structures of which the computed response function agrees with the original input data. But since real field data always exhibit a certain amount of scatter, the dispersion relations may profitably be used to construct pa@) and @(T) functions compatible both with these relations and with the scattering range of the field data. In the present paper the assumption is made that measurements of pa( 2' ) are usually more reliable than those of @(T); a method is therefore described with which to compute @(T) from a smoothed single-valued function pa(T). This calculated phase, and its corresponding apparent resistivity, are thus compatible with a tabular structure and suitable as input for one-dimensional inversion schemes that need both p a ( T ) and $ ( T ) data. Such schemes are usually more efficient than those working only with the apparent resistivity. The same dispersion relations are also shown to connect amplitude and argument of the principal impedances of two-dimensional structures.

S U M M A R YWith a diameter of 40 km the Araguainha Dome is the largest known impact site in South America. The main characteristics of the structure is its appearance as a set of imbricated circles and a central uplift with a diameter of 9 km. 25 magnetotelluric (MT) soundings were completed across the structure, approximately defining five radial sections. The MT responses in the frequency range from 1000 to 1 Hz show 1-D behaviour, in contrast to the longer periods where data indicate a strong anisotropy and a decline of the resistivity. 1-D modelling of the high-frequency data reveals a well-defined symmetric crystalline basement ring at 1000 m depth between the radii of 9 and 20 km, which seems to drop deeper further away from the centre. Inside the ring and toward the centre, the top of the granites rises, outcropping at 1.5 to 2 km from the centre.

An analytic one-dimensional magnetotelluric inversion scheme is described. The main characteristics of this new scheme are its simplicity and its minimal requirements in computer time and storage space. The basic idea of the scheme is that for a given period T only the structure above a certain depth matters. The scheme therefore starts with the shortest periods of the available data set and tries to explain the observed response, specifically the apparent resistivity pa(T) and the phase @(T), in terms of a two-layer structure. Shifting successively to longer periods, discrete new layers are introduced at progressively greater depth. Some stabilizing features simultaneously keep the inversion process from diverging and hold the necessary number of layers to a minimum. The properties of the scheme are analysed by studying its performances when working on synthetic and real field data. The scheme can handle fairly scattered data and is very stable. It has, in fact, never been observed to diverge. Examples are given of how the scheme might be handled in practice, especially regarding the use of the causal dispersion relations with which it is possible to ensure that the model proposed by the inversion scheme will return a response function in agreement with the initial data.

The research is aimed at delineating the post-impact structural characteristics across the Serra da Cangalha impact crater in Brazil using a combination of magnetotelluric (MT) and aeromagnetic data. The MT survey was carried out along three radial MT profiles trending NW-SE, ENE-WSW and NNE-SSW across the crater. For MT sites located further away from the centre of the crater, isotropic MT responses were observed, suggesting a 1D conductivity distribution in the subsurface in the frequency range of 100-10 Hz. For sites located in the vicinity of the inner ring of the crater, anisotropic responses were observed for the same frequency range. We believe that this zone probably represents the areas of structural disturbance. A 2D resistivity inversion of these data reveals a four-layer model, representing a thin resistive layer underlain by a conductive layer, a weathered basement and a resistive crystalline basement. The depth to the top of the basement is estimated to be about 1.2 km. This is in good agreement with the estimation of depth to the basement of about 1.1 km, calculated using the aeromagnetic data. However, in view of the circular geometry of the crater, we have carried out a 3D forward modeling computation to supplement the derived 2D model. The 3D resistivity forward model, fitting the MT responses by trial-by-error revealed a five-layer model, showing a significant reduction in the basement resistivity. This, perhaps, could be due to the structural disturbances that have been caused by the impact on the crater, resulting in brecciation, fracturing, alteration and shocked zone filled with weak-magnetic materials and fluids. We have calculated the effect of the impact on the overall structural deformation beneath the Serra da Cangalha crater, following the classical crater scaling relation of Holsapple and Schmidt [Holsapple, K.A., Schmidt, R.M., 1982

Knowledge about the hydraulic connections between submarine groundwater discharge (SGD) and its terrestrial coastal catchment is relevant with regard to the management of marine and coastal waters in karst areas. This study applies different methods and monitoring approaches to trace SGD between the Burren Limestone Plateau and Galway Bay in western Ireland, via an excavated sinkhole shaft and deep conduit. Areas of potential SGD were first delineated based on sea surface temperature anomalies using Landsat satellite images. Two fluorescent dyes and solid wood chips were then used as tracers. Solid wood chips were tested as potential means to circumvent the problem of high dispersion in the sea, impacting on the fluorescent dyes to yield readings below the detection limits. Sampling was conducted at 10 different terrestrial locations and in the sea at Galway Bay. Offshore sampling was conducted in transects over a period of four successive days onboard of a vessel using an automated field fluorometer and a conductivity-temperature-depth sensor. No wood chips were recovered in the sea but both fluorescent dyes were successfully sampled. The estimated travel times are in the order of 100 to 354 m/h, and localised tracer readings correlate well in space and time with low conductivity readings. By confirming hydraulic connections between the two karst features and Galway Bay, the study substantiates the hypothesised importance of Variscan veins with regard to regional groundwater flow in the region.