CHIRP FISH FINDERS ARE AN INEXPENSIVE ALTERNATIVE FOR FIRST ORDER BATHYMETRY MEASUREMENTS IN FRESHWATER CENOTES

Taking measurements in remote karst environments can be an expensive undertaking. Easily accessible and inexpensive equipment helps to make investigating the remote areas possible. We tested the efficacy of measuring cenote bathymetry using Compressed High Intensity Radar Pulse (CHIRP) Fish Finders in two cenotes located in the Yucatan Peninsula, Mexico. The CHIRP Fish finder collects geolocated depth measurements using a 47° (100 kHz), 20° (240 kHz), or 7° (675 kHz) beam.

We measured the bathymetry of each cenote using three separate fish finders and the wider and narrower beam options. In addition, we tested the accuracy of the instruments in a swimming pool of consistent depth. Data were interpolated using the Kriging method. Each dataset was processed to only include data in the area that overlapped with the other datasets from each CHIRP and scan. The interpolation was completed over the same area for all datasets facilitating calculation of differences between the contour maps. Each contour map was then compared to every other scan completed at the location by differencing the interpolated grids.

The bathymetry from CHIRPs is data limited. Multiple scans of an area to increase the amount of data points collected is highly suggested. CHIRPs were fairly consistent between each other; however, there can be malfunctions that can be identified by collecting data with multiple units at each location. Narrow beam scans tend to be more precise, but can create large discrepancies in areas with large variability in depth. In high variability locations, wide beam scans create more consistent but less detailed interpolations. Results of this study indicate that CHIRP Fish Finders provide an inexpensive first-order estimation of bathymetry in freshwater cenotes. For best results, multiple fish finders should be used and multiple scans should be completed. The best beam size to use is determined by the variability in bathymetry gradients and amount of detail desired.