INTERPRETATION AND DEPTH OF INVESTIGATION OF GRADIENT MEASUREMENTS IN DIRECT CURRENT GEOELECTRICS*

Gradient measurements in a homogeneous electrical primary field can easily be interpreted for simple models. The simplified solution (conducting or resistant body in a homogeneous space in a homogeneous electrical field) is often sufficiently accurate, as comparisons with the exact solution (body of finite resistivity in a homogeneous half-space in a quasihomogeneous electrical field) show. The exact geometry of the body cannot be determined by gradient measurements; the same anomaly of apparent resistivity can be caused by different bodies. In particular, the similarity between a sphere and a cube of the same volume is very high. There is a distinct influence of the resistivity of the overburden: the higher this resistivity is, the stronger is the effect caused by a buried body. If a deviation of 10% of the apparent resistivity is assumed as the lower boundary at which a buried body can be detected by gradient measurements, the depth of investigation for a three-dimensional body is approximately equal to its width; in the two-dimensional case the thickness of the overburden can be twice the width. If the overburden has a resistivity which is higher than the resistivity of the substratum, these depths are greater. The greatest possible depth is approximately three times the width of the body.     

RESISTIVITY PROFILING WITH DIFFERENT ELECTRODE ARRAYS OVER A GRAPHITE DEPOSIT*

Motivated by several papers by Indian colleagues suggesting that a single-pole array is the most favorable one for locating electrically conductive targets in the underground, detailed measurements were carried out with different electrode arrays over a well-known graphite deposit near Pfaffenreuth, Germany. The following arrays were used: (1) Wenner array; (2) single-pole array; (3) half-Wenner array; (4) half-Schlumberger array. Each of the arrays established the location of the main graphite deposit. The half-Schlumberger array turned out to be the most reliable one since it showed details which were only very poorly indicated by the other arrays or not at all. Mathematical modeling of three-dimensional bodies simulating the geological situation for this deposit were carried out. These results show an unexpectedly good correlation with the measured effects.     

Prograde–retrograde P–T–t–deformation path of Austroalpine micaschists during Variscan continental collision (Eastern Alps)

A complete prograde P–T path, defined by 10 calculated P–T fields in succession, is recognized from metapelites by using geothermobarometry on garnet-bearing assemblages with microstructural control. Overstacking of several tectonic units during an early Variscan continental collision explains the complex prograde P–T history. Isostatic uplift and deformation controlled the retrograde P–T path. Deformation with changing character acted continuously during all stages of the evolution of the Austroalpine basement complex. After the intrusion of Caledonian granitoids, metapelites and magmatic rocks suffered a shearing deformation D₁–D₂, which produced sheath folds as well as the main foliation S₂. Spessartine-rich first-generation garnets, situated in microlithons enclosed by S₂, record the onset of shearing under increasing high-pressure–low-temperature conditions (7 kbar/380°C). Geothermobarometry on second-generation garnets which have been rotated during growth indicates isothermal decompression from 9 kbar to 5 kbar/500°C and subsequent recompression/heating during continuing shearing. This is explained by overthrusting of a tectonic unit (unit 2) from NE to SW upon the micaschist unit (unit 1), followed by isostatic uplift and further overstocking of a third unit (unit 3). The resulting P_max of 12 kbar at 650°C and further increasing temperatures up to 680°C accompanied by decompression have been calculated using a third generation of garnets. These high-pressure–high-temperature conditions may explain the occurrence of eclogitic metabasites in adjacent regions. Staurolite and kyanite first appeared under decreasing pressures at the last stage of prograde P–T evolution. Shortening deformation D₃ and simultaneous growth of typical amphibolite facies minerals (staurolite 2, kyanite 2, sillimanite, andalusite) occurred during the retrograde path. A final step of Variscan evolution was marked by an oppositely directed shearing D₄ (at T > 300°C and P > 3 kbar), possibly indicating backthrusting or extension. Apart from acid intrusions, no signs of a previous Caledonian thermotectonic history were found in the area to the south of the Defereggen–Antholz–Vals Line.     

Centennial-to-millennial-scale periodicities of Holocene climate and sediment injections off the western Barents shelf, 75°N

At the western continental margin of the Barents Sea, 75°N, hemipelagic sediments provide a record of Holocene climate change with a time resolution of 10-70 years. Planktic foraminifera counts reveal a very early Holocene thermal optimum 10.7-7.7 kyr BP, with summer sea surface temperatures (SST) of 8°C and a much enhanced West Spitsbergen Current. There was a short cooling between 8.8 and 8.2 kyr BP. In the middle and late Holocene summer, SST dropped to 2.5°-5.0°C, indicative of reduced Atlantic heat advection, except for two short warmings near 2.2 and 1.6 kyr BP. Distinct quasi-periodic spikes of coarse sediment fraction (with large portions of lithic grains, benthic and planktic foraminifera) record cascades of cold, dense winter water down the continental slope as a result of enhanced seasonal sea ice formation and storminess on the Barents shelf over the entire Holocene. The spikes primarily cluster near recurrence intervals of 400-650 and 1000-1350 years, when traced over the entire Holocene, but follow significant 885-/840- and 505-/605-year periodicities in the early Holocene. These non-stationary periodicities mimic the Greenland-[Formula: See Text]Be variability, which is a tracer of solar forcing. Further significant Holocene periodicities of 230, (145) and 93 years come close to the deVries and Gleissberg solar cycles.     

Dust Morphology Of Comet Hale-Bopp (C/1995 O1): I. Pre-Perihelion Coma Structures In

In 1996 comet Hale-Bopp exhibited a porcupine-like coma with straight jets of dust emission from several active regions on the nucleus. The multi-jet coma geometry developed during the first half of 1996. While the jet orientation remained almost constant over months, the relative intensity of the jets changed with time. By using the embedded fan model of Sekanina and Boehnhardt (1997a) the jet pattern of comet Hale-Bopp in 1996 can be interpreted as boundaries of dust emission cones (fans) from four — possibly five — active regions on the nucleus (for a numerical modelling see part II of the paper by Sekanina and Boehnhardt, 1997b). This revised version was published online in July 2006 with corrections to the Cover Date.     

Vertical profiling of convective dust plumes in southern Morocco during SAMUM

Lifting of dust particles by dust devils and convective plumes may significantly contribute to the global mineral dust budget. During the Saharan Mineral Dust Experiment (SAMUM) in May–June 2006 vertical profiling of dusty plumes was performed for the first time. Polarization lidar observations taken at Ouarzazate (30.9°N, 6.9°W, 1133 m height above sea level) are analyzed. Two cases with typical and vigorous formation of convective plumes and statistical results of 5 d are discussed. The majority of observed convective plumes have diameters on order of 100–400 m. Most of the plumes (typically 50–95%) show top heights <1 km or 0.3DLH with the Saharan dust layer height DLH of typically 3–4 km. Height-to-diameter ratio is mostly 2–10. Maximum plume top height ranges from 1.1 to 2.9 km on the 5 d. 5–26 isolated plumes and clusters of plumes per hour were detected. A low dust optical depth (<0.3) favours plume evolution. Observed surface, 1 and 2–m air temperatures indicate that a difference of 17–20 K between surface and 2-m air temperature and of 0.9–1 K between the 1 and 2-m temperatures are required before convective plumes develop. Favourable horizontal wind speeds are 2–7 m s⁻¹.     

INTERPRETATION OF RESISTIVITY MEASUREMENTS OVER 2D STRUCTURES1

Resistivity measurements were carried out in a survey area in the south of Germany. This area is characterized by complicated subsurface geology. Schlumberger full-arrays and their respective half-arrays were recorded simultaneously. The results obtained by the one-dimensional (1D) interpretation of the full-array measurements were incorrect because of a resistivity discontinuity. This discontinuity, under a relatively thick overburden, could only be located by the half-array soundings. Its exact location and the resistivity distribution in the subsurface were ascertained by comparing the sounding curves with 2D model curves, which are calculated by a finite-difference method.