AIRBORNE MICROWAVE RADIOMETER SURVEY TO DETECT SUBSURFACE VOIDS Emory V. Dedman and John L. Culver, Resources Technology Corporation, Houston

Airborne radiometric measurements were obtained over an area in Kansas City, Kansas, during March 1970. The purpose of the measurements was to determine whether the radiometric temperature of the earth's surface could be used to detect and possibly map subsurface voids beneath thick soil cover so that potential highway foundation problem areas could be lo­cated. The microwave survey showed significant radiometric relative low­temperature anomalies corresponding to the presence of subterranean voids. The conclusion is, therefore, drawn that there is a high correlation between subsurface voids and radiometric low temperature.

•AN EXPERIMENT to determine the possibility of detecting subsurface voids by the use of an airborne microwave radiometer was planned for March 1970. The airborne data were collected by Remote Sensing, Inc., under contract to the State Highway Com­mission of Kansas and in cooperation with the Federal Highway Administration. The data were reduced and analyzed by Resources Technology Corporation under contract to the Federal Highway Administration and in cooperation with the State Highway Com­mission of Kansas.

The area surveyed is a 2,000-ft square approximately¼ mile northeast of the in­tersection of Gibbs Road and 42nd Street, Kansas City, Kansas. This area was par­tially mined in the past, and subsurface voids were left. Figure 1 shows the area surveyed; the depths to the underground cave ceilings are shown in the form of isopachs, previously determined by underground mapping and auger core holes. The experimental airborne microwave survey was to evaluate the potential of remote microwave radi­ometry in detecting and mapping subsurface voids by searching for correlations between survey measurements and the known cave in Figure 1.

MICROWAVE THEORY AND EMPIRICAL KNOWLEDGE

Electromagnetic theory shows that all objects above absolute zero emit radiation produced by the oscillations of charged atomic and molecular particles. In the micro­wave portion of the spectrum, generally agreed to lie at frequencies between 1 and 100 GHz, the Rayleigh-Jeans approximation to Planck's radiation law (valid to within 1 per­cent) expresses the microwave radiation of a body as equal to its emissivity (at the relevant frequency) times its thermometric temperature. The emissivity of an area of the ground is an extremely complex function of both electrical and geometrical prop­erties. These properties are in turn representative of soil type, soil moisture content, vegetation coverage, and surface roughness. Present knowledge of the exact mecha­nisms that result in ground radiation temperature is not sufficient to predict microwave temperatures, given all ground characteristics, except in certain ideal conditions.

However, experiments and theory have shown that soil water content is very influen­tial in affecting ground microwave temperature . The more moisture a soil contains, the cooler is its temperature; thus, any differences in microwave temperature because of underground voids will be related to the perturbation of the groundwater drainage system caused by the void. Moreover, the presence or absence of water in the cave itself will tend to lower or raise the radiometric temperature measured. The exact mechanisms that can make subsurface voids appear warmer or cooler radiometrically are at present not fully established.

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Figure 5. Topographic map of site.

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An experiment by Kennedy (1) indicated that cold microwave anomalies may be as­soci:iten with hurien k::iri:;t fP.::iturP.R . 'T'hP Pxperiment. was conducted tn an area similar geologically to the area surveyed in the present study. Measurements of microwave temperature were made at ground levels in the 1968 experiment and were made in the air in the present survey. The results of the ground measurement encouraged the ae­rial survey because of the rapid synoptic coverage aerial surveys provide.

DESCRIPTION OF EXPERIMENT

Ground truth in the form of surface moisture content and ground thermometric tem­peratures was sampled over the area. Figure 2 shows a summary of moisture content by means of contours and contains representative thermometric temperature measure­ments. Some ground radiometer data (8- to 14-µm range) were also collected.

Daytime and nighttime missions were flown at 15,000 ft above the terrain in a grid pattern. In the daytime, 2 east-west and 2 north-south flights were made. In the night mission, 11 east-west lines and 11 north-south lines at 200-ft separations were flown. The flight lines were controlled by the use of amber rotating beacons on the ground. The lines were flown from midnight until about 5 :00 a. m., when thermal equilibrium of the environment should be greatest. The data gathered were digital recordings of the aircraft roll, pitch, and heading angles versus time on a computer-compatible ½-in., 7-track digital tape; continuous data of brightness temperatures versus time on a 1-in., 14-track analog tape; and film strips from an airborne infrared scanner. The radiometer used was vertically polarized with a center frequency of 13.3 GHz.

The temperature data were digitized and processed by computer and resulted in profiles of brightness temperature versus ground-beam axis intercepts. Figure 3 shows an example of the resultant temperature profiles developed for the survey lines. Figure 4 shows a radiometric isotherm map of the area prepared from the temperature profiles.

Comparisons between radiometric temperature measurements made from an air­craft and existing landform voids are demonstrated by the profile shown in Figure 3. This graph reveals a striking correlation between the actual cave widths and the re­corded cool anomalies. However, these profiles are not representative of the entire set, from which the contours shown in Figure 4 were drawn, but rather are drawn from tho c,,nhoof- fhgf- ohn,uo.~ th.o h.ocf- ,i,n,-.,-..ol..,,Hnnc., f"\H"a,,. n,,.Al'Hoco C'lhnnrnrl lnc,c, nr,, .... "'"1,·d·.;,...,.,..

A visual comparison of the radiometric temperatJre contour map (Fi;. 4) and a topographic map of the area (Fig. 5) does not appear to show any significant correlation, nor f'!ln !lny ~nh~t~nti!ll rnr~Pl~tinn ht1hu~Pn ,.!ltiinmof-l'"ir f-ol'np~""-::ih,,.A '!:lnrl C!nil mni5t11-ro

content (Fig. 2) be shown. This lack of correlation may lend credence to the assump­tion that one of the factors affecting microwave temperature is the presence or absence of subsurface voids.

The radiometric isotherms show definite closures of cold anomalies. Superimposing the limits of the cave on the isotherm map, as shown in Figure 6, reveals that 3 of the anomalies occur over the known void. The fourth anomaly, detected in the northwest quarter of the area, does not appear to be connected to the known cave.

It is concluded that an airborne radiometer may have some potential in detecting voids. Further microwave field surveys over areas with known voids are now being conducted to more positively identify the cause of the anomalies.

REFERENCE

1. Kennedy, J. M. A Microwave Radiometric Study of Buried Karst Topography. Geological Soc. of America Bull., Vol. 79, No. 6, 1968, pp. 735-742.