Dense vegetation, slippery trails and almost constant rain slowed down the survey; the MTU-5A is protected beneath a plastic sheet and leaves.
The AMT section above the “tunel de descarga” (discharge tunnel) shows a thick cover of tuffs atop the metamorphic basement. The tuff cover is gently inclined (15–20 degrees), probably due to tectonic tilting. According to the geophysical model (calibrated with some deep boreholes), most tunneling will be in soft rocks (tuffs).
The ~3D model (obtained from parallel 2D lines) for the “casa de máquinas” (underground power station). This model, calibrated with a 600 m borehole, indicates the underground power station will be only in soft rocks (tuffs), with resistivity ranging from 100 to 250 ohm-m.
Phoenix MTU-5A equipment has been used successfully for a 300-site AMT survey in Ecuador.
The six-month survey, performed by two Italian geophysical firms, Techgea Servizi and GDTest, ran from September 2010 until February 2011; it was part of a feasibility study for a hydroelectric power development plan that includes several deep tunnels (500m to 1000m) and an underground power station.
The survey objective was geological prediction for deep tunneling with a tunnel boring machine (TBM). TBMs are used as an alternative to drilling and blasting in rock and conventional “hand mining” in soil. A fundamental requirement of successful tunnelling is accurate prediction of rock type and conditions along the tunnel route.
The specific objectives of this survey were to identify the main geological contact between soft rock (volcanic tuffs) and hard rock (granodiorite intruded in schists and quartzitic schist) as well as to detect the main faults (some of them with water circulation and fault gouge).
The 300 AMT soundings were closely spaced (50 or 100m apart) on a line along the discharge tunnel route (2D data processing) and on a regular grid above the underground power station location (3D data processing). The survey was slowed by the difficult terrain (dense vegetation, slippery clayey soil and irregular ground) and the tropical weather (almost constant rain), but the lack of noise allowed shorttime data acquisition that produced very stable results.
The data, processed with WinGLink, provided consistent 2D resistivity sections. Resistivity of soft rock was usually fairly low (<200ohm-m), while hard rock (granodiorite and metamorphic rocks) showed higher resistivity. Lower resistivities were also measured in the fault zones, due to water content and rock softening.
From their experience in the Ecuador project, the Italian team drew the following conclusions about the use of AMT in geotechnical/engineering studies:
- AMT is useful for such deep investigations. The small-footprint system provides a significant logistic advantage. AMT uses a natural source, so there is no need to inject energy into the ground; this is a significant cost saving.
- In difficult terrain such as that encountered in Ecuador, or in places where blasting is forbidden, AMT is a useful alternative to the seismic reflection method.
- Depending upon the resistivity contrasts expected, AMT could be useful in any similar project that involves deep tunneling.