Research on Fiber Bragg Grating Sensor Group for Three Dimensional In-situ Stress Measurement

The observation and estimation of deep crustal stress state is a key and difficult problem in in-situ stress measurement. The borehole wall strain gauge based on the overcoring stress relieving method is one of the main methods of in-situ stress measurement. In this paper, a strain sensing array based on FBG is designed by using the main structure of the classical hollow inclusion cell, and its layout scheme on the hollow inclusion is studied. According to the layout scheme, the in-situ stress inversion algorithm of hole-wall strain to stress is deduced; then, the triaxial loading and unloading experiment platform is built, and the calibration experiment of FBG strain sensor is designed; Finally, Abaqus finite element software is used to simulate the in-situ stress measurement process of the overcoring stress relieving. The FBG strain values of each measurement direction before and after the overcoring process are extracted, and the stress inversion equation is used to carry out the stress inversion. Through the comparison of the inversion results, it is proved that the FBG strain sensor group is feasible and reliable. The quasi-distributed FBG sensor module designed in this paper can invert the three-dimensional in-situ stress by measuring the hole-wall strain, which lays a theoretical and experimental foundation for the development and application of FBG hole wall strain gauge. It fairly makes up for the deficiency of the existing hole-wall strain gauge based on resistance strain gauge, provides direct and accurate observation way for hole wall strain measurement, and has important practical value for the development of in-situ stress measurement technology.

Tectonic stylolites as a valuable stress archive – new insights from Late Cretaceous intra-plate stresses in Europe

<p>The importance of paleostress analysis is dramatically increasing due to its application in diverse fields, such as sustainable exploration of resources, reservoir potential or storage sites. A good understanding of the subsurface geology, the geological stress-history and associated fracture and fault networks is essentially for these applications. Understanding of the complete paleostress history is not only of interest for applied research, but also for an understanding of the dynamics of geological processes in general. In recent years a diverse toolbox of stress inversion methods has been developed including stress inversion from tectonic stylolites (and slikolites). The pressure solution structures not only preserve the direction of the largest principle stress – they are an archive for the complete stress tensor and the absolute stress magnitude at the moment of their development. Here we present the first results of a systematic study of this upcoming method. For comparison we preformed roughness analysis of tectonic stylolites from Mesozoic limestone from SE Germany. In late Cretaceous the area was affected by shortening in a NE-SW direction, which is clearly illustrated by fault-slip analysis and the orientation of tectonic stylolites. During this tectonic event the stress regime changed from thrusting to strike-slip, with the sampled stylolites persevering the transition between these two stress events. With our preliminarily results we show that roughness analysis of tectonic stylolites enables us to record short time intervals during phases of contraction, and therefore offers crucial insights into stress history and tectonic processes with pulsating stress fields.</p>

Tectonic stress patterns along the Vrancea subcrustal zone from the inversion of focal mechanisms data

<p>The Vrancea zone is an unique area with both crustal and intermediate-depth seismic activity and constitutes one of the most active seismic area in Europe.  An intense and persistent seismicity is generated between 60 and 180 km depth, within a relic slab sinking nearly vertical in the Earth’s mantle due to the increasing of the stress state within this volume. At intermediate-depths, large magnitude events are frequent, i.e. four earthquakes with moment magnitudes (Mw) >7 occurred in the last century. An unique slab geometry, likely preserved until the present, causes stress localization due to the slab bending and subsequent stress release resulting in large mantle earthquakes in the region.</p><p>In this study, we evaluate the current stress field along the Vrancea subcrustal region by computing the fault plane solutions of 422 seismic events since January 2005. The continuous development of the National Seismic Network allows us to constrain the fault plane solutions and subsequently to evaluate the current stress field.</p><p>The main style of faulting for Vrancea subcrustal events presents a predominant reverse one, with two main earthquakes categories: the first one with the nodal planes oriented NE-SW parallel with the Carpathian Arc and the second one with the nodal planes oriented NW-SE perpendicular on the Carpathian Arc. The main axis of the moment tensor may indicate a predominant compressional stress field (Tpl>45<sup>0</sup> Ppl<45<sup>0</sup>). Another characteristic of  the Vrancea subcrustal zone is the tendency of the extension axis T to be almost vertical and the compression axis P being almost horizontal.</p><p>The results of stress inversion indicate a dominant reverse faulting style, with an average stress regime index of 2.9. Other tectonic regimes were observed in the present dataset as normal and strike-slip but they are retrieved for a restrained number of events.</p><p>The stress patterns obtained from formal stress inversion of focal mechanism solutions reveal many features of the current stress field that were not captured by large-scale numerical models.</p>

Seasonal stress inversion trends of RTS in Song Tranh2 reservoir, Vietnam

<p><span><span>The Song Tranh 2 hydropower construction is located in the Quang Nam province (central Vietnam), it has a reservoir volume of 740 million cubic meters of water and a dam height of 96 m. The reservoir was filled to capacity for the first time in February 2011. The seismicity in the vicinity of reservoir is example of reservoir triggered seismicity(RTS). The natural seismic activity of the Song Tranh 2 reservoir is very low. After the reservoir was filled, the seismic activity increased, and the number and frequency of the tremors also changed as the water level changed. Water level changes are accelerating the tectonic process leading the critically stressed faults to slip. Data suggest that reservoir exploitation stress field changes as triggering origin of this seismicity. The stress inversion method was used to check if there were any seasonal trends. The inverted stress tensor and, in particular, the stress ratio, which is very sensitive to data quality and scope and difficult to accurately retrieve, can be influenced by porous pressure changes. Has been checked, how the average annual seismic activity is related to the change of the water level and if it implies the orientation of the principal stress during high and low water levels in the reservoir. The pore pressure changes and the stress ratio changes were also estimated in relation to the high and low water level periods.</span></span></p>

Structural evolution of the Moroccan Central High Atlas Syncline-Topped Anticlinal Ridges: Insights from micro-structural analysis of Tirrhist and Anemzi ridges

<p>The Moroccan High Atlas mountain range is an aborted Mesozoic rift basin that was moderately shortened during the Late Cretaceous‒Cenozoic inversion. The range is currently featured in its central part by the presence of conspicuous S-shaped open gentle synclines where Middle Jurassic strata crop out, with sub-horizontal bottom, separated by 15-to-80-km narrow faulted anticline ridges with two distinct directions: ENE and NE. The tight anticline ridges are cored by Triassic continental red-beds intruded by the CAMP basalts and subsequently by Upper Jurrasic‒Lower Cretaceous alkaline magmatism. Regional cleavage with very low-grade anchi- to epi-zonal metamorphism are depicted along several structures of the High Atlas, particularly the NE-trending anticlines. The sedimentary layers thickness, on the other hand, gets thinner towards the faulted anticlines with the development of intraformational truncations. The structural history of the High Atlas syncline-topped anticlinal ridges remains a controversial matter. Any attempt to reconstruct the evolutionary process of such folded structures must take into consideration the following circumstances:</p><ul><li>After a Triassic rifting episode followed by the establishment of Liassic carbonate platform, the High Atlas basin underwent a wide spread exhumation event at the time interval between the Middle Jurassic and Lower Cretaceous leading to the deposition of continental detrital series and sedimentary hiatus;</li> <li>The upward motion was accompanied with the emplacement of alkaline magmas in the Central High Atlas;</li> <li>A complex halokinetic history characterizes the Central High Atlas salt province during both pre-orogenic and orogenic stages;</li> <li>During the Late Cretaceous‒Cenozoic, the High Atlas experienced a moderate crustal shortening which was focused essentially within the range’s borders;</li> </ul><p>In order to bring new insights to the structural history of the High Atlas folded structures, a structural investigation was carried out in Tirrhist and Anemzi ridges. In each station, fractures measurements were taken, and oriented samples were collected for micro-structural analysis. First paleo-stress inversion in some stations reveals the presence of pre-folding bedding-parallel maximal horizontal stress oriented NE to NNE. For a deep analysis of pre syn and post-folding stresses history, we use a calcite stress inversion technique, namely Etchecopar’s method, to unravel the paleo-stresses orientations and to quantify the differential stresses during the different episodes of deformation. The present work is a preliminary attempt to quantify tectonic stresses in the hinterland of an arguably weakly deformed orogenic belt.</p>