Field spectroscopy measurements of rocks in Earth observations

• Rocks are natural mineral aggregates and thus considered in the experiments;

• Hapke’s equation to model the reflectance () from an exposed rock (Hapke, 1981). The point of all of this is that with knowledge of known optical constants of various minerals and the angle of incident and emitted light, we can model the reflectance of a rock with mixed minerals and grain sizes.

Fig. 1. Physical processes in solid transparent parallel plate
What does the solar radiation do when it hits a rock? When the radiation reaching the object (in this case - the rocks) it simultaneously scattered, reflected off of the grains either away from the surface or onto other grains, refracted through a grain onto other grains and absorbed by a grain. Thus the physical processes occur simultaneously (Figure 1) and following physical laws and relationships are applied:
• Beers Law (exponential law of absorption):
(1)
I – observed light intensity [W/sr], – original light intensity [W/sr], k – absorption coefficient () [m^-1] and x is the distance traveled through the medium [m].
• Intensity of reflected solar radiation [W/sr]:
(2)
(3)
R is the reflection coefficient, is defined for two types of media,

and is not dependent on the direction of propagation and n is the relative refractive index of the reflecting medium.

represents a measure of the ability of a surface to reflect solar radiation which criterion is equal to the ratio of reflected solar radiation by the object for each wavelength [W/(m².sr)] to reflected solar radiation by the calibrated reference surface the same wavelength [W/(m².sr)] in the same conditions of these measurements. Resultant graph plotted as a functional dependence vs. is called the spectrum or the spectral reflectance characteristics, or the spectral signature.

- atmospheric correction;

- spectral data correction;

- radiometric data correction.

In Table 1 as a part of the preprocessing data the additional information is presented. In addition we have to take in account GPS-coordinates of the field points of measurement (GPS), weather conditions of the measurements in the field experiments (atm), the integration time for each acquisition (), the number of statistically averaged spectra () and the distance between the object and the device fore optics ().

On the road to the village Barziya in about 3 km after Petrohan pass are revealed magmatic rocks from Petrohan pluton. It is a complex magma body emplaced into rocks of Berkovska and Dalgidelska groups. It is made of several magmatic phases: the first is gabbro, widespread in northern part of the pluton near Berkovitsa; the second is represented by diorites, and the third - by granodiorites. The largest areas occupy diorites and granodiorites. Field spectrometric studies have been made of the diorites. They are gray, grey-greenish, medium-grained, and uniform-grained. Their structure is massive, and the texture – prismatic granular. The main rock-forming minerals are plagioclase (andesine) and amphibole, secondary – biotite and augite, and accessory - titanite and magnetite.

Since contamination of the standards material is often a problem, the spectral and spatial characteristics should not degrade over time and – if used frequently in the field – should be able to be washed or rinsed to remove contamination or particles such as mud, sand or others (Schaepman, 1998). The used reference standard is made from barium sulphate and its surface exhibits lambertian behavior (Figure 3).

red with similar data from the spectral libraries (USGS, 2013).

The spectral libraries represent the collections of spectra of numerous minerals and rocks obtained using the spectrometers with high radiometric and spectral resolution. Collected spectra could be compared with other spectra in order to be more accurately identified objects of comparative research. It is particularly important to take into account that data from different spectrometers should be compared only when they are converted to the criterion of comparability - the reflectance coefficient. Such spectral libraries are the basis for comparative remote sensing studies of rocks.

Conclusions
The procedure of the preprocessing of the field spectrometric data is presented and tested. Obtained with the spectrometer TOMS the reflectance spectra of the studied rocks correspond as form and values of spectral reflectance in the same range of EMS to the similar signatures received using other spectrometers and presented in the spectral libraries.
Acknowledgments. This research was supported by the Bulgarian Science Fund under contract DFNI – I01/8.

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Статията е рецензирана от проф. дгн Стефан Димовски и препоръчана за публикуване от кат. „Приложна геофизика.