IMAGE-D7.05: Analogue field work

Including in the WP7 for developing exploration methods, this task is dedicated to the analogue field work and how the study of these analogues can be useful for the deep geothermal targets. 

On the field, numerous data and information on the rock mass are available can be measured by the geologist, but knowing which are useful for local models and numerical simulation is not often obvious. In particular, discontinuities use to play a role in the rock mass behavior (mechanical and hydrogeological behavior), and numerical simulation need to take into account the discontinuities, even explicitly (by example for hydraulical (transport and flow), mechanical and/or thermal modellings of complexes zones such as fault zones) using a Discrete Fracture Network approach (DFN), even by homogenizing the medium using continuous approach. We will focus and detail the development of practical methods to transfer field data though quantifiable value to local model with DFN approach which need the most detailed and complex data. 

The key question is which data are needed to be transferred from field data though quantifiable value to local model, and how can be transfer. For a long time, it has been well known that the discontinuities properties that have the greatest influence on the rock mass behavior are: 

- orientation, 

- size (here size refers to the extension of the discontinuity), 

- frequency, 

- surface geometry, 

- genetic type, and 

- infill material. 

Although this kind of information uses to be measured by the geologist, there is still a gap between the descriptive methods of the geologist (whatever the substrate of the data: aerial photographs and satellite images, outcrops, borehole logging…) and the needed quantitative data for modeling. This gap may be bridged by applying mathematical methods sort in three main themes, corresponding to the three main steps of the workflow, will be developed here: upstream work, data acquisition on field and post-treatment. 

A short application has been done in the Eclépens quarry, in Switzerland, where the rock faces are easily accessible. This site is belonging to the internal edge of the folded Jura. This edge is mainly constituted by calcareous rocks with marls from lower Cretaceous. The anticline structure of Mormont outcrops Mesozoic rocks within the Molasse Basin. The main interest of this study is the La Sarraz fault system, which outcrops in this quarry. This is a dextral strike-slip fault conjugated to the most important N-S dextral fault of Pontarlier. In this quarry, the vertical strike-slip faults linked to the main fault oriented NW-SE and probably currently active, offers an interesting site to acquire structural data in limestones. These fault zones are probably continuous under the Molasse Bassin and can be constituted potential geothermal target. 

An area of the quarry has been chosen in order to compare two fracture measurement methods: the scanline, directly on the outcrop, and the photogrammetry. The both acquisition methods give similar results for orientations, spacing and density. However, some differences can be linked to the scale observation. Therefore, the scanline method brings more detail for the small structures, whereas the photogrammetry method allows to better sample large structures and their relations with the whole outcrop. The combination of both methods allows to overly the whole fracture data acquisition. Then, for each type of data, it is important to identify the most appropriate method and their limits. 

As a way forward, we recommend to make a good screening of the need for modelling and adapt transfer method. The acquisition by photogrammetry can be used to give data in non-accessible areas, particularly in the highest part of the cliffs or dangerous outcrops. Whereas scanline give more precise data, particularly about mineralization, movement of fault …