Investigation of a Tertiary maar structure using three-dimensional resistivity imaging

This paper presents the application of the electrical resistivity tomography (ERT) method to the investigation of the Tertiary maar structure of Baruth (Germany) known from previous gravimetric surveys. ERT was applied to support the optimum location for a palaeoclimatological drill hole.
  Special modifications of data acquisition, signal processing and inversion are introduced to adapt the method of ERT to the special requirements for the 3-D investigation of structures with horizontal extensions of 1  km or more. More than 5000 dipole–dipole combinations were recorded at three concentric circular electrode arrangements using stand-alone transient data acquisition systems (RefTek).
  We present a fast approximate imaging technique based on the simultaneous iterative reconstruction technique (SIRT). As the complete calculation of the inverse Frechét matrix is avoided, the algorithm is especially suitable for large data and model spaces, where complete inversion is beyond the limits of available computing hardware. The single-step method is applicable to arbitrary irregular electrode layouts. Synthetic tests show that the imaging procedure reconstructs the main features of the subsurface.
  A low-resistivity body could be interpreted as limnic sediments filling the interior of the Tertiary maar crater. Considering the horizontal resistivity gradient, estimates for the lateral and depth extents of the structure were made. An optimum position for a palaeoclimatological borehole was found, and was in good agreement with the gravimetric minimum.     

Broad-band power-law spectra of well-log data in Japan

For the purpose of revealing the statistical characteristics of P -wave velocity, S -wave velocity and density in the uppermost part of the crust, we analysed well-log data obtained from five deep wells in different tectonic regions in Japan: three wells through the mainly sedimentary rocks in the Kanto plain and two wells in the Kuju volcano group in Kyushu Island. In the Kanto plain, the power spectral density of fractional fluctuation of P -wave velocity and that of density are proportional to a power of the spatial wavelength from a few metres to 100 m. where the power index (slope of the power spectral density at double logarithmic scale) is 1.1-1.3. At the Kuju volcano group, that of P - and S -wave velocity and density also obey a power law, with a power index of 1.3-1.6 for wavelengths from a few metres to few hundred metres. Correcting the effect of the moving box-car observation window which corresponds to the separation of two receivers of the logging tool, we find that the power-law characteristics hold for wavelengths down to a few tens of centimetres. The 1-D sections of the elastic inhomogeneities follow a kind of band-limited self-affine random process. Comparing the power spectral densities, we find smaller values of the power index in stable areas and larger values in tectonically active areas. The difference in the power index arises from long-wavelength components.     

Using evidence of non-linear induced polarization for detecting extended ore mineralizations

The discrimination between electrolytic and electronic conductors is highly relevant to geological modelling as it allows conclusions to be drawn about the formation and mineral composition of rocks. The induced polarization (IP) method, which compares the electric current injected into the ground with the corresponding earth potential differences can be used for this purpose.
  This paper describes a new method based on the theory that non-linear electrochemical processes on the surface of electronic conductors are responsible for non-linear IP (NLIP) phenomena. This results in multiples of the fundamental frequency being observed in the telluric voltage spectra when a monochromatic current signal is fed into the ground. The non-linearity of the current–voltage characteristic is most effectively described by a spectral method.
  A laboratory experiment was carried out, using an electrolytic trough with a small graphite cylinder serving as an electronic conductor, which clearly demonstrated the validity of the method. A field experiment was undertaken at a borehole of approximately 450  m depth, located in the transition zone of the Tepla-Barrandium and Moldanubicum in East Bavaria. A sinusoidal current was injected into the ground using a logging tool at depths varying between 150 and 450  m. The corresponding potential differences were simultaneously observed along a profile on the surface. Field and laboratory results show a striking similarity. It can be concluded that an extensive electronic conductor—probably graphite—is steeply dipping southwards meeting the borehole at approximately 310  m depth.