LCZO -- Geophysics, Ground Penetrating Radar (GPR) -- Luquillo Mountains -- (2012-2015)

OVERVIEW

Description/Abstract

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.

GPR data found here in the second link are raw data, data was processed and interpreted in Orlando et al. 2016 ((DOI: 10.1002/esp.3948):

“GPR data processing was performed using ReflexW by Sandmeier Scientific. Steps were limited to: (a) a ‘dewow’ filter over a 10 ns time-window; (b), application of a time-varying gain; (c) a bandpass filter; (d) a static correction; and in some cases, (e) Kirchhoff migration based on a single EM wave velocity as determined from the CMP profiles.”

Creator/Author

Comas, Xavier|Hynek, Scott|Wright, William|Brantley, Susan L.

CZOs

Luquillo

Contact

Xavier Comas, xcomas@fau.edu Miguel Leon, leonmi@sas.upenn,edu

Subtitle

Geophysical surveys

SUBJECTS

Disciplines

Geophysics

Topics

Geophysics|Ground Penetrating Radar (GPR)

Keywords

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

Variables

ground penetrating radar|terrain conductivity|electrical resistivity imaging

Variables ODM2

Resistivity, electrical

TEMPORAL

Date Start

2012-05-30

Date End

2015-11-01

SPATIAL

Field Areas

Northeastern Puerto Rico and the Luquillo Mountains

Location

Luquillo Mountains

North latitude

18.32232903

South latitude

18.26143335

West longitude

-65.85692813

East longitude

-65.74067507

REFERENCE

Citation

Comas X., Hynek S., Wright W., and Brantley S.L. 2014. Geophysical Surveys of Luquillo. Florida Atlantic University.

CZO ID

4730