Geophysical surveys

Project Description: 

Geophysical surveys conducted during the summer of 2014 followed on previous work that investigated the nature and spatial variability of ground penetrating radar (GPR) reflections in the Rio Icacos watershed (Figure 1a). GPR surveys using a variety of shielded (160 MHz) and unshielded (50, 100 and 200 MHz) antennas (Figure 1e) was combined with multi-frequency terrain conductivity measurements to upscale previous measurements.
Figure 1a shows a 2 km long transect (red line) across a trail in the Rio Icacos watershed. The transect in the northern edge had an approximately elevation of 640 m, and ended in the southern edge below 540 m elevation and close to the knickpoint. The GPR data along the transect revealed a series of vertical zones with presence of chaotic reflectors (Figure 1b, between 240-265m, 270-300 m, and 320-350 m along the transect; and Figure 1c, between 690-750 m along the transect). These areas repeated at several locations along the 2 km transect (white lines in Figure 1a). Other GPR reflector facies signatures (not shown here) included two landslide locations (yellow lines in Figure 1a); and an area of laterally continuous reflectors (blue line in Figure 1a) towards the end of the transect and close to the knickpoint.
Terrain conductivity surveys consistently depict a) increases in terrain conductivity; and b) decreases in magnetic susceptibility that coincide with the vertical zones of chaotic GPR reflectors described above (shaded areas in Figures 1b and 1c)
We attribute these areas of enhanced GPR reflections to vertical fracturing within the bedrock-regolith interface associated with the formation of corestones. Water infiltration may cause regolith wash off (resulting in a decrease in electrical conductivity) and concentration of corestones (resulting in increases in magnetic susceptibility). This preliminary hypothesis is confirmed by the presence of large corestones adjacent to the transect (Figure 1d) and following topographic valley areas (Figure 1a).
These results confirm the potential of hydrogeophysical measurements for understanding variability of bedrock-regolith interface in the Icacos watershed at large (i.e. km) scales and have direct implications for the controls on subsurface fluid circulation and presence of preferential groundwater flow.

Research Location: 
Core Area(s) and/or Keywords: 

GPR, ground penetrating radar, terrain conductivity, electrical resistivity imaging

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Contact Information
Person(s) Completing This Form: 
Miguel Leon
Investigators: 
Xavier Comas, Scott Hynek,William Wright, Susan L Brantley
Investigator E-mail Addresses: