Test of airborne fluorometer over land surfaces and geologic materials

Response of an experimental Fraunhofer line discriminator(FLD) to a wide range of surficial deposits common in deserts and semideserts was tested in the laboratory and from an 1-1-19 helicopter. By design the instrument sensed radiation from the ground specifically at the sodium D2 Fraunhofer line (5890 angstroms). It is capable of detecting a fluorescence component emanating from ground targets irradiated by sunlight along the line of flight, provided the aircraft is nearly level. However, no signals attributable to fluorescence were recorded during 540 miles of aerial traverses over southeastern California and west-central Arizona. It is concluded that exposed surfaces of target materials throughout the traverses were either non-luminescent at 5890 A or not sufficiently so to be detectable. It cannot be ruled out that the lack of fluorescence is partly attributable to surficial coatings of non-luminescent weathered material.

The principal route surveyed from the air was from Needles, California to Furnace Creek Ranch, Death Valley and return via the Amargosa River valley, Silurian Lake (dry), Silver Lake (dry), and Soda Lake (dry). Principal targets traversed were unconsolidated elastic sediments ranging from silty clay to cobbles, and a wide range of evaporite deposits. Evaporite minerals thought to have been viewed in high concentrations are halite (sodium chloride), gypsum (hydrous calcium sulfate), probably other sulfate minerals, borax and probably ulexite (hydrous sodium borates), possibly colemanite (hydrous calcium borate), and probably some carbonate minerals. Extensive manganese-rich outcrops midway between Needles and Prescott had to be by-passed because of low sun-angles, cloudiness, and a heavy load in relation to altitude (above 5,000 feet).

Several of the more common evaporite minerals were tested under the FLD on the ground, while being irradiated by bright sunlight. Minerals tested were chiefly from salt fiats (salars) in the Atacama Desert of northern Chile, and included halite, gypsum, ulexite, thenardite (sodium sulfate), aragonite (calcium carbonate) and nitrate ore containing soda niter (sodium nitrate). Crude oil from the Santa Barbara leak was tested in its natural state and in solutions of methyl-isobutyl-ketone (MIBK). Petroliferous sandstone samples containing both high and moderate concentrations of oil were tested on both fresh and weathered surfaces. No response definitely attributable to fluorescence at 5890 A was noted from any of the foregoing geologic materials, although a possible very weak response to one oil sample in MBK solution appears on the record.

Although no fluorescence was detected, the mechanical operation of the FLD was satisfactory throughout 8 hours of flight, with attendant vibrations, and during a wide range of sun angles, from 13° to 33°. In accord with design, the FLD showed no appreciable response to moderate contrasts in reflectivity among targets, although there was a marked response to strong contrasts in reflectivity. Airborne tests also established that a minimum instrumental warm-up time of seven minutes is required, and that response is not entirely independent of either the vertical angle (tilt) or horizontal angle (flight direction) of the aircraft.