A systematic investigation of self potentials (SP) shows that they vary considerably with time. To study the influencing parameters, repeated measurements were conducted every 14 days over a period of a year and a half along fixed profiles which extended over different lithological units. An analysis of the recorded SP suggests decomposition into three components of different wavelengths. Variations of the SP with wavelengths in the 0.1–1 m range and amplitudes of up to 150 mV are thought to be induced by vegetation. The mean amplitudes change with a one‐year period, and there is a rough correlation with the soil temperature. A long‐wavelength component (wavelengths, meters and some tens of meters; amplitudes, some 10 mV) is attributed to changes of the underground lithology. The long‐wavelength SP and its pronounced variation with time are assumed to be caused by water movements in layers of different permeability. A morphology‐dependent trend of the SP values which has been much disputed in the literature shows systematic variations with time, too, with gradients changing on the order of .5 mV/m difference in elevation. There is a correlation with precipitation with a one‐month time lag. This effect is assumed to result from depletion and recharge of perched water aquifers. Measurements at the surface and at shallow depth establish the existence of a vertical gradient of the SP which shows variations with time of up to 50 mV/m. With a three‐month time lag, the amplitude of the vertical gradient relates closely to the variation of the potential evapotranspiration. Groundwater recharge from gravitational water is considered the main cause of the vertical gradient. The results suggest that self potentials at the nearsurface must be regarded primarily as being of a streaming‐potential character. Their variation with time should not be neglected when the SP method is applied to problems in engineering geophysics. On the other hand, the results confirm that because of the close relation between self potentials and hydrogeologic parameters, SP measurements may become a valuable and standard technique in hydrogeology.
The state of substances under ultrahigh pressures and temperatures (UHPHT) now raises a special interest as a matter existing under extreme conditions and as potential new material. Under laboratory conditions only small amounts of micrometer-sized matter are produced at a pressure up to 100 GPa and at room temperature. Simultaneous combination of ultrahigh pressures and temperatures in a lab still requires serious technological effort. Here we describe the composition and structure of the UHPHT vein-like impact glass discovered by us in 2015 on the territory of the Kara astrobleme (Russia) and compare its properties with impact glass from the Ries crater (Germany). A complex of structural and spectroscopic methods presents unusual high pressure marks of structural elements in 8-fold co-ordination that had been described earlier neither in synthetic nor natural glasses. The Kara natural UHPHT glasses being about 70 Ma old have well preserved initial structure, presenting some heterogeneity as a result of partial liquation and crystallization differentiation where an amorphous component is proposed to originate from low level polymerization. Homogeneous parts of the UHPHT glasses can be used to deepened fundamental investigation of a substance under extreme PT conditions and to technological studies for novel material creations.
The Lairg Gravity Low may represent a buried impact crater c . 40 km across that was the source of the 1.2 Ga Stac Fada Member ejecta deposit but the gravity anomaly is too large to represent a simple crater and there is no evidence of a central peak. Reanalysis of the point Bouguer gravity data reveals a ring of positive anomalies around the central low, suggesting that it might represent the eroded central part of a larger complex crater. The inner or peak rings of complex craters show a broadly consistent 2:1 relationship between ring diameter and total crater diameter, implying that the putative Lairg crater may be as much as 100 km across. This would place the crater rim within a few kilometres of the Stac Fada Member outcrop, a location inconsistent with the thickness and clast size of the ejecta deposit. We propose that the putative impact crater originally lay further east, substantially further from the Stac Fada Member than today, and was translocated westwards to its present location beneath Lairg during the Caledonian Orogeny. This model requires that a deep-seated thrust fault, analogous to the Flannan and Outer Isles thrusts, exists beneath the Moine Thrust in north–central Scotland.
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