Weathering variations in a granitic massif and related geotechnical properties through seismic and electrical resistivity methods

Abstract

Mapping and distribution with depth of alteration in rocks is critical in engineering planning because it has a fundamental impact on the geotechnical properties of the materials. Lateral heterogeneity on a weathered rock massif makes boreholes inadequate for its complete characterization. Geophysical methods increase spatial sampling along the study area and can be related to geotechnical parameters, so subsoil conditions can be better understood.In order to determine its geotechnical qualities and variability along two different profiles, we attempt to characterize a granite massif in north‐west Spain by the integration of results from seismic refraction, multichannel analysis of surface waves (MASW) and electrical resistivity tomography methods (ERT). The study area, the so‐called Carlés granite, shows all the weathering grades from sandy soil to fresh rock. A reference borehole where samples were taken and laboratory measurements were made, serves as a direct check for the results of one of the profiles, the other being interpreted without any direct information. This approach has permitted the evaluation of the advantages and limitations of each geophysical method and created an accurate geotechnical model of the massif, correlating physical and geotechnical parameters such as rock quality designation, weathering grade, or standard penetration test.The field seismic velocities have been compared with the ultrasonic measurements at the laboratory, permitting an evaluation of the field and laboratory elastic constants. The trend in the values of these parameters agrees with the field and laboratory test for the shallow parts of the massif. However, unrealistic elastic constants have been obtained for fresh rock based on the results of the field experiments. This is related to an apparent underestimation of the velocity of seismic S‐waves for the deepest layers. This fact suggests that the methodology followed throughout this work is able to provide a full geotechnical model of an altered rock massif for the first tens of metres, discriminating between different weathered levels. It is also useful and reliable when inferring elastic constants for depths of up to 20 m. However, its validity becomes doubtful with depth, so care must be taken when calculating elastic moduli and trying to extrapolate directly to a rock massif.

Groundmagnetic survey used to identify the weathered zone, in Blang Bintang, Aceh, Indonesia

“…Total field and gradient are the two common types of magnetic measurements [4]; [18]; [19]. Total field measurement responds to the total magnetic field of the earth, natural and cultural magnetics, and any changes caused by a target [20]; [21]; [22].…”

Seismic Refraction and Electrical Resistivity Tests for Fracture Induced Anisotropy in a Mountain Watershed

“…This would be a valid assumption in I must mention again that the "closure depth" is velocity dependent. I chose 3500 m/s for closure (from Olona et al, 2010). Such an assumption could be wrong.…”

“…Most CZ-hydrogeophysics surveys are collected using only a single azimuth (Olona et al, 2010;Befus et al, 2011;Holbrook et al, 2014;Yamakawa et al, 2012;Yamakawa et al, 2010). The results might vary dramatically depending on the direction of the survey with respect to the fracture orientation.…”

“…The velocity I chose for fracture closure is 3500 m/s, based on Olona et al (2010). I admit this assumption could be wrong.…”

“…is the Vp of the background material, in our case we assumed a value of the velocity of fresh granite of = 3500 m/s, taken from Olona et al (2010).…”