X-ray microtomography for studying 3D-textures of speleothems developed inside historic walls

En este artículo se muestra una aplicación de la microtomografía computerizada de Rayos X (microCT-RX) como técnica no destructiva útil para la caracterización del interior de estructuras sin necesidad de perder la muestra. Gracias a la sensibilidad de la técnica ha sido posible distinguir diferentes tipos de crecimiento espeleotémico dentro de una estalactita localizada en las bóvedas interiores de la muralla histórica de la isla de Nueva Tabarca.

Identificación de minerales mediante difracción de rayos X utilizando bases de datos online de libre acceso

El artículo proporciona una metodología sencilla para la identificación de minerales mediante la técnica de difracción de rayos X de polvo utilizando bases de datos mineralógicos de libre acceso y online. Las bases de datos utilizadas son la base de datos mineralógicas webmineral y la base de datos de estructuras cristalinas de la American Mineralogist Crystal Structure Database, AMS. En el presente trabajo se han elaborado 3 actividades resueltas de estudios reales y en orden creciente de dificultad. Se ha pretendido hacer hincapié en puntos donde el profesor puede interactuar con el alumno y promover la capacidad de análisis, síntesis y razonamiento crítico del alumno ante un problema de investigación en geología. Finalmente se ha elaborado un Anexo donde se recogen recomendaciones para que el profesor desarrolle sus propias actividades.

Ultrasonic and X-ray computed tomography characterization of progressive fracture damage in low-porous carbonate rocks

This paper studies the fracturing process in low-porous rocks during uniaxial compressive tests considering the original defects and the new mechanical cracks in the material. For this purpose, five different kinds of rocks have been chosen with carbonate mineralogy and low porosity (lower than 2%). The characterization of the fracture damage is carried out using three different techniques: ultrasounds, mercury porosimetry and X-ray computed tomography. The proposed methodology allows quantifying the evolution of the porous system as well as studying the location of new cracks in the rock samples. Intercrystalline porosity (the smallest pores with pore radius < 1 μm) shows a limited development during loading, disappearing rapidly from the porosimetry curves and it is directly related to the initial plastic behaviour in the stress–strain patterns. However, the biggest pores (corresponding to the cracks) suffer a continuous enlargement until the unstable propagation of fractures. The measured crack initiation stress varies between 0.25 σp and 0.50 σp for marbles and between 0.50 σp and 0.85 σp for micrite limestone. The unstable propagation of cracks is assumed to occur very close to the peak strength. Crack propagation through the sample is completely independent of pre-existing defects (porous bands, stylolites, fractures and veins). The ultrasonic response in the time-domain is less sensitive to the fracture damage than the frequency-domain. P-wave velocity increases during loading test until the beginning of the unstable crack propagation. This increase is higher for marbles (between 15% and 30% from initial vp values) and lower for micrite limestones (between 5% and 10%). When the mechanical cracks propagate unstably, the velocity stops to increase and decreases only when rock damage is very high. Frequency analysis of the ultrasonic signals shows clear changes during the loading process. The spectrum of treated waveforms shows two main frequency peaks centred at low (~ 20 kHz) and high (~ 35 kHz) values. When new fractures appear and grow the amplitude of the high-frequency peak decreases, while that of the low-frequency peak increases. Besides, a slight frequency shift is observed towards higher frequencies.