Measuring rock moisture using different techniques in the sandstone area of Saxony

<p>Rock moisture is an understudied factor governing weathering and rockfall. Many weathering processes like hydration, thermal and frost cracking are governed by moisture availability. However, weathering studies have primarily focussed on temperatures. The role of moisture supply has not been given the same attention, also because there is no humidity sensor that meets all requirements for application in rock.</p><p>In the sandstone area of Saxony in eastern Germany ("Saxonian Switzerland"), climbing on wet rock poses a safety problem as the sandstone loses stability when saturated. Visitor guidance measures ('rock traffic lights') were implemented to temporarily stop climbing at rocks that are too wet. To accompany this measure, we carried out a pilot study at the Gohrisch sandstone massif, involving moisture measurements in the four cardinal directions at the rockwall base and near the summit of the massif. We used a combination of (a) electrical resistivity electrodes, combined with wind-driven rain collectors; (b) 2D-electrical resistivity (ERT); (c) microwave sensors (MW) with four sensor heads for different penetration depth and (d) Schmidt Hammer (SH) measurements to assess rock stability. All techniques were accompanied by laboratory measurements at rock samples.</p><p>Electrical resistivity, MW readings and SH rebound all showed very good correlations with rock moisture in laboratory samples. However, the range of values measured in the field strongly differed from laboratory values so that the calibration curves could not be applied to field data. Presumeably this is due to lithological differences between the fresh quarry samples and the pre-weathered rock faces.  </p><p>ERT profiles using adhesive electrodes showed good reliability (RMS error 5-14%). Most sites were slightly wet at the surface, drier at 5-15 cm depth and moderately wet at 20-30 cm depth (1000 – 8000 Ohmm). The site Bottom North was much wetter than all others, and the two top positions were dried out at the surface probably due to wind. This distribution was roughly confirmed by microwave sensor data. Direct correlation between MW and ERT measurements was poor as measurement principle and geometry are very different.</p><p>Schmidt Hammer data was very consistent with microwave moisture in the lab (lower rebound at wetter surfaces); however not in the field, where the wetter Bottom North site showed highest rebound values. The summit positions showed significantly lower rebound which we attribute to stronger weathering (more dry-wet cycles). Lab results show that the sandstone loses stability (SH rebound) mainly between 60% and 100% pore saturation. Currently we cannot reliably determine if this saturation was actually reached in the field.</p><p>The combined interpretation of all measurements, even if imperfectly calibrated, points to surface-parallel weakness zones that have developed at all sites except of Bottom North which is almost never hit by sunlight. Water supply by rainfall is weak at the almost vertical sites; water rather seems to infiltrate in flat areas and to seep out at the base of the massif. The results help to understand the distribution of dampness in the rock and will be supplemented by continuous monitoring and numerical simulations.</p>

Application of the Electrical Resistivity Method for Site Investigation in University of Anbar, Ar-Ramadi City, Western Iraq

The 2D resistivity imaging technique was applied in an engineering study for the investigation of subsurface weakness zones within University of Anbar, western Iraq. The survey was carried out using Dipole-dipole array with an n-factor of 6 and a-spacing values of 2 m and 5 m. The inverse models of the 2D electrical imaging clearly show the resistivity contrast between the anomalous parts of the weakness zones and the background resistivity distribution. The thickness and shape of the subsurface weakness zones were well defined from the 2D imaging using Dipole-dipole array of 2 m a-spacing. The thickness of the weakness zone ranges between 9.5 m to 11.5 m. Whereas the Dipole-dipole array with a-spacing of 5 m and n-factor of 6 allocated the geoelectrical stratigraphic layers sequence in low-accuracy of weakness zones, but deeper than the inverse model of 2 m a-spacing. This survey was made to explain the correlation between the weakness zone and the deeper layers in the study area. It points out that the deeper layers were not affected in the weakness zones. The inverse model was produced using the Standard Least-Squares Inversion Method and the Robust Inversion Model Constraints Method. The first method had a gradational boundary of the weakness zones and the second had sharper and straighter boundaries of fractures and voids within the weakness zones.

Influence of salt structures on the location of glacial valleys within the Pripyat Trough (Belarus)

It was found that glacial valleys in the Pripyat Trough are connected with the location of the halokinetic structures of the Upper-Famennian salt strata. Valleys were noticed over salt structures, which developed during the Quaternary. The most part of valleys is over salt diapires, pillows and swells, rarer they occur within synclinales and troughs. Within local anticlinales valleys focus mainly in its tops and limbs, which have incline down glacier. Within salt synclinales valleys lie over its axis and limbs, which are connected with adjacent anticlinales. The most typical regularities of the glacial valleys localization within the salt structures is their close connection with active fault systems. These faults propagate upwards from the tops and flanks of diapires across the Upper-Famennian and supra-salt overburden into the Quaternary, where underlie the valleys. The developing of salt structures and faults during the Narev and Dnieper glaciations created weakness zones in glacial bed that facilitated erosion by the glaciers and subglacial melt water.

Towards understanding the roll-back subduction of narrow oceanic domains: inferences from the modelling of Carpathians subduction zone

<p>Numerous subduction systems in the Meditteranean realm are derived from the subduction of narrow oceanic domains, which are too narrow to generate the means of a fully coupled two-dimensional thermo-mechanical numerical model that takes into account the visco-elasto-plastic properties of different lithospheric domains. The results show that the narrow extent of the Ceahlau-Severin Ocean commonly assumed by paleogeographic reconstruction cannot generate roll-back upon subduction, in particular for models that must assume that slabs do not penetrate the 660 km discontinuity. Therefore, we propose that the subduction of the Carpathians system must have an inherited component from a previous orogenic evolution, which will ensure sufficient slab-pull to generate roll-back in the Carpathians realm. The model is constrained by recent results in terms of mantle structure and geodynamic reconstructions, while multiple compositional, thermal distribution and geometrical scenarios are tested in successive models. In all of our models, roll-back is achieved, which indicates that the proposed inherited component can sufficiently explain the roll-back subduction of the aforementioned narrow oceans. The subducting oceanic slab does not penetrate the 660 km discontinuity, this is in agreement with seismic tomographic results from various Mediterranean subduction zones. The exact onset and dynamics of the roll-back are mostly controlled by the thermic age of the ocean and the convergence kinematics of the continental slabs. An outlook on possible future improvements to the model, such as taking into account pre-existing rheological weakness zones in the lithosphere, is discussed and the opportunity of a seismo-thermo-mechanical modelling to investigate the seismic cycle in the Vrancea-zone is highlighted.</p>

Why 3D seismic data are an asset for exploration and mine planning? Velocity tomography of weakness zones in the Kevitsa Ni-Cu-PGE mine, northern Finland

Kevitsa is a disseminated Ni-Cu-PGE (platinum group elements) ore body in northern Finland, hosted by an extremely high-velocity ([Formula: see text]) ultramafic intrusion. It is currently being mined at a depth of approximately 100 m with open-pit mining. The estimated mine life is 20 years, with the final pit reaching a depth of 500–600 m. Based on a series of 2D seismic surveys and given the expected mine life, a high-resolution 3D seismic survey was justified and conducted in the winter of 2010. We evaluate earlier 3D reflection data processing results and complement that by the results of 3D first-arrival traveltime tomography. The combined results provide insights on the nature of some of the reflectors within the intrusion. In particular, a major discontinuity, a weakness zone, is delineated in the tomography results on the northern side of the planned pit. Supported by the reflection data, we estimate the discontinuity, likely a thrust sheet, to extend down approximately 600 m and laterally 1000 m. The weakness zone terminates prominent internal reflectivity of the Kevitsa intrusion, and it is associated with the extent of the economic mineralization. Together with other weakness zones, a couple of which are also revealed by the tomography study, the discontinuity forms a major wedge block that influences the mine bench stability on the northern side of the open pit and likely will cause more issues during the extraction of the ore in this part of the mine. We argue that 3D seismic data should routinely be acquired prior to commencement of mining activities to maximize exploration efficiency at depth and also to optimize mining as it continues toward depth. Three-dimensional seismic data over mineral exploration areas are valuable and can be revisited for different purposes but are difficult to impossible to acquire after mining has commenced.