Electromagnetic Geophysical Mapping of a Stream Channel in a Tropical Montane Rainforest in Costa Rica

Headwater basins play a critical ecohydrological and geological role in shaping downstream systems. Streamflow generation processes shape the hydrology of tropical montane forests but are poorly understood. Stream channels are complex systems comprising fractures, sand, exposed bedrock, knickpoints, and woody debris, all of which control downstream flow patterns. While geomorphological observations are useful to understand streamflow generation processes and ecological implications, the subsurface component is often unknown. A field study was conducted to characterize the near-surface geology of a montane stream in Costa Rica. Electromagnetic induction (EMI) geophysical profiling at three frequencies (1,8,15 kHz) yielded data constraining the electrical conductivity of the streambed. The apparent electrical conductivity response of the streambed decreases, as does the signal complexity, moving upstream towards the headwaters basin. Geomorphological observations close to the headwaters include more exposed, less-conductive bedrock and less exposed, more-conductive sand; a steeper, narrower stream channel; more fallen logs; and increased interflow into the steam channel from rock seepage faces. Such along-stream variations indicate changes in the hydrological processes affecting streamflow generation from upstream to downstream. Horizontal (HMD) and vertical (VMD) magnetic dipole EMI data provide different insights on the nature of the subsurface due to their differing depth sensitivity functions. The EMI data (HMD-only, VMD-only and joint HMD-VMD) were inverted in terms of two-layer geoelectric models of the subsurface. The EMI geophysical method is shown to be a useful tool for reliably mapping the conductivity-thickness product of the upper layer of tropical forest stream channels.