12MB PDF - Association for Mexican Cave Studies

from the sixth air space to avoidthe silt we had riled up along theshallow route going in. We discoveredanother isopod on the way out,and spent much of decompression inthe spring basin examining the remainsof a former railroad bridgethat according to Sr. Santos was dynamitedduring the revolution. Ialso discovered to my surprise thatI could not hear the waterfall whileunderwater above the falls, eventhough it is very audible above thesurface. We had previously supposedthat we would be forewarned as to theexistence of waterfalls beyond downstreamsumps from the noise underwater,but now it appears that this maynot be true. Sump divers beware!On dive #32 Jamie, Carol andFrank extended the deep line another45 meters to a depth of 43 meters anda penetration of 254 meters. Mostof the participants feel that theChoy definitely warrants further investigationand may soon come up intosignificant air-filled passage.Nacimiento del Rio FrioWe checked the headsprings andsnorkeled the lower spring, butnever found any cave here despitethe considerable volume of water inthe river.Mapping techniquesandExpedition notesNaturally, underwater surveyingdoes not produce the degree of accuracycommon to most AMCS maps.Nevertheless, the compass and "knottedline" method we employ is probablymore accurate than most caverswould suppose. One reason for thisis that significant errors in linearmeasurement can be detected easily.Each length of the line is measuredagainst a surveying tape and knotted'at precise 10 foot intervals, andthe resulting total supply of lineis of a known length. Thus, if a600 foot length of line was laid inthe cave, the survey shots should30total 600 feet if no errors are made.A compact Suunto diver's compassgraduated in increments of 5 degreesat intervals wide enough to estimatethe nearest 2-1/2 degrees is used fordirection. Ascertaining the depth ofeach station so we can convert ourdistances to "true horizontal" is imeasy, clinometer-less task, thanks toour depth guages. Getting accuratesurvey shots between stations in poorwater visibility is no problem becausewe can line up the edge of thecompass against the guideline to obtainthe proper azimuth.