Correlation between spectral signature of basalts & sandstones of Central India as obtained by Radiometric & Spectrophotometric techniques

Correlation between spectral signature of basalts & sandstones of Central India as obtained by Radiometric & Spectrophotometric Techniques

Dr. S. K. Soni, Dr. A. N. Patel
S.G.S. Institute of Technology & Science Indore (M.P.) India

Abstract
Spectral signature of a feature comprises of a set of values for the reflectance or radiance of the feature where each value corresponds to the reflectance or radiance arranged over a different well defined wavelength interval. The present work embodies an attempt to analyse the spectral signature of Basalts and sandstones which are major litho stratigraphical units of Central India using Radiometric & Spectrophotometric techniques. The study revaled that Laboratory reflectance studies as obtained by spectrophotometric has higher value as compend to insitu value as obtained by Radiometric, It was further observed that spectral signature of sandstone varies with chemical composition colour and texture white on the other head for Basalts there was no change in the percentage reflectance in the wavelenght ranging to from 550 nm to 950 nm.

Introduction
The word signature by definition is referred in general to a distinguishing characteristics. In Remote Sensing spectral signature of feature comprises a set of values for the reflectence or radiance of the feature where each value corresponds to the reflectance or radiance arranged over a different well defined wavelength interval.

Spectral Radiometers permits in situ measurements of reflectance from vegetation canopies using incident solar radiation as the illuminent. Measurement s can be made at different angles of view. Spectroradiometer provide data more closely stimulating the energy recorded by remote sensors.

Spectral Radiometers permits in situ measurements of reflectance from vegetation canopies using incident solar radiation as the illuminent. Measurements can be made at different angles of view. spectroradiometer provide data more closely stimulating the energy recorded by remote sensors.

Spectrophotometers measures the hemispherical reflectance. In the laboratory studies spectral signature is obtained from measurements using spectrophotometer. The measurements are the distribution of reflectance of a material as a function of wavelength, compared to that of a diffuse white surface. The spectral resolution of the measurements is generally 0.001 mm.

Area of study
The area selected for the study is the area around Central Narmada basin. The rocks in this area shows numerous Geological significance.

The co-ordinates of the selected area are bounded by the following limits:

77°00' E - 79°00' E

22°00' N - 24°00' N

Geological Succession:
The Geological succession of the area selected for the study is:

Pleistocene and Recent formations along the Narmada catchments.
Deccan Trap.
Upper Palaeozoic and lower Gondwanas.
Archean.

During the research work field studies were carried out for the earlier selected sites, field work comprised of:

To take the radiometric observation for the major litho stratigraphical units of the area.
To collect the rock samples for laboratory anlysis of the spots where the radiometric observations were taken.

Radiometric Studies:
Exotech Model 100A. Ground truth radiometer and SAC (ISRO) developed ground truth radiometer were used for obtaining field spectral data.

The above radiometers were taken in the field and was standardized by the lid cover for complete darkness or black of the gry scale and by using BaSO₄ layer on a thin aluminium plate for standard white. The time of observation was in between 10 AM to 2 PM on a cloud free dry thereby minimizing the shadow effects.

Observations were taken for Vindhyan sandstone and plateau basalts. One to three sets of observations were taken for individual outrops. The location of the rock out crops where these observation were taken are shown in Map1.

Petrographical and Petrological Studies of the Rocks:
As mentioned above, uring the field work samples from different Geological formations were collected. Major rocks were Deccan trap Basalts, and Vindhyan sandstones. These rock samples were studied megascopically and microscopically. The results of these studies are presented as.

Vindhyan Sandstones:
The Vindhyan sandstones exposed around Bhopal are light brown to dark brown in colour. The change in colour is due to the presence and/or absence of iron content. The textures of the roch changes from outcrop to outcrop. Most of the Vindhyan sandstones are fine grained, Coryer and hard in nature. Quartz and feldspars are essential minerals. Mica mostly muscovite is present as acessory minerals.

Angularity of the grains is distinctly seen. Feldspars mostly orthoclase shows carls bad twiest and cleavage plains are distinetly seen. Cementing material is Silica. Presence of Iron orides made on the basis of opaque nature . Quartz shows various shades of colour with wavy exinc Vindhyan sandstones exposed around Hoshangabad shows colour of different shades or pink and gryy. These rocks are hard, compact and quartzitic in nature. Silica is the cemanting material. Due to high silicious binding nature of the quartz and feldspar grains are giving rise to the compact and hard nature of the Quartzitic sandstone formations Texture is uniformly coarse and subhedral to Anhedral grains of Feldspars are little elongted as compared to Quartz and this is the typical property bsides Carlsbad twinning which was the basis of identification of orthoclase feldspars. Overall study of Vindhyan Sandstones concluded that they are of Arkosic nature.

Plateau Basalts:
Plateau basalts of the Deccan trap were seen around Bhopal. These basalts are dark black in colour. Samples of weathered basalts shows dark drownish tint alongwith the original colour Basalts are mostly fine to medium grain in size showing aphenetic texture in hard speciments. Microscopic study revealed that these basalts shows poikitic to ophitic texture. The plagioclase mostly laboradorite occurs as lath shaped subhedral crystals and in highly weathered conditions it shows anhedral nature oriented at random to yield and interrupted meah work. Mostly aaround plagioclese laths the pyroxenes are sutrrounded. The pyroxenes are filled up with the rest of the rocks. Aegirine and Iron oxides are also seen as accessory mineral. The Table No. 1 shows the megascopic study of rocks.

Chemical analysis of the rock samples:
The apparatus used in the study of chemical analysis were Philips 1410 X ray spectrometer with a chromium target, a 5 crystal changer, 4 sample changer and simple spinner. The spectrograph was interfered with 32 K wards core memory P-852 Philips computer with P-833-001 cassette tape unit with the ability to transfer data to or from cassette tape and with P 841-101 type writer attached. The results obtained are presented in Table No. 2

Table no.1

Sample No.	Colour	Texture	Mineral composition	Specific Gravity
HS₂	Reddish Brown in colour	Medium grained rock compact in nature shows stratification	Quartz, Feldspars Iron oxide	1.71
HS₄	Reddish Brown in colour	Fine grained	Quartz, Feldspars	1.70
BB₁	Dark gray in in colour uniformly distributed	Fine grained, Plagioclase feldspars Surrounds the smaller grains of Pyroxenes	Plagioclase Augite, Hornblende	3.32
BB₂	Dark gray in colour uniformly distributed	Fine grained rock	Plagioclase Feldspars Augite Hornblende	3.30
BB₃	Light gray in colour uniformly distributed with greenish tint	Fine grained rock little coarser than BB1 BB2	Plagioclase Feldspars Augite, Hornblende Iron Oxide.	3.35

Table no. 2

Element	Sample Nos.
Name	HS₂	HS₄	BB₁	BB₂	BB₃
Na₂O	.258	.260	.185	2.135	.1934
MGO	.914	2.863	.185	4.0333	2.863
Al₂O₃	2.451	1.375	13.914	11.837	12.508
SiO₂	95.900	96.072	51.869	49.404	48.521
P₂O₅	.028	.174	.156	.232	.173
K₂O	.558	.136	.574	.154	.779
CaO	.030	.179	9.582	10.598	9.340
TiO₂	.073	.052	2.210	3.724	2.456
MnO	.155	.137	.038	.068	.028
Fe₂O₃	.886	1.795	13.627	16.016	14.211

Spectral Reflectance studies:
The absorption by and emission from substances in the ultra violet (wavelength from (200nm-400mn)) and visible (400nm-800nm) region and in near infra-red (800nm-1100nm) was obtained by using spectrophotometer model AMINCO D.W. 2 a model. The scannery speed of the spectrophotometer was 5 nm/sec and slit opening was 3 nm. Rock samples which were analyzed were of size 5 cm x 5 cm and they were slightly polished and edges were made smooth on low grinding and polishing machine. Reflectivity as a function of wavelength was recored and is shown in Fig. (1 & 2).

Analysis on spectral reflectance data:
The analysis on spectral reflectance data was observed in the field and the laboratory are discussed below:

Vindhyan Sandstone:
Fig. 1 represents the spectral reflectance curve for laboratory and the field observations for the Vindhyan sandstonne. HS₂ and HS₄ represent the laboratory observation using spectrophotometer.

The above figure shows that

Laboratory and field reflectances are quite comparable.
Secondly, laboratory reflectance curve of the sample HS₂ and HS₄ are exactly similir, The maximum and minimum percentage reflectance is 63 and 52 respectively, while in the case of HS₄ it is 80 and 65. The Silica, Aluminium and Iron conctent for HS2 and HS4 is 95.99, 2.45 and 0.886 and 96.07, 1.37 and 1.79 respectively. This variation in the chemirl composition has given rise to change in the percentage eflectance.
The variation in the field and the laboratory observations is due to manifold reasongs. Firstly, it's due to change in the field and the laboratory environment secodndly, due to different libit sources.

Megascopic and Microscopic study reveals that rock sample HS₄is reddish brown in colour, fire. Grained rock containing quartz, felspars; while HS₂ is also reddish in colour having same essential minerals but texturally it is coarse grained rock. This variation in grain might have caused the variation in the percentage reflectance among the laboratory observations. The nature of laboratory and field curves indicate that the percentage reflectance slightly dereases with the increase in the wavelength. Fig. 1. represents the average spectral reflectance curve.

Plateau basalts:
Fig.2 represents the spectral reflectance curve drawn simulteneously for field & laboratory studies.

These curves are drawn by averaging number of ields and laboratory data. It is observed that the nature of both laboratory and field observation is similar but the laboratory observation shows that rocks have got higher percentage reflectance. The reasons of this higher value may be attributed to-

Field environment is entirely different than the laboratory.
Surface of the rock samples analyzed in the laboratory is smoother than the field.
In the field the sum is the light source while in the laboratory monochromatic light is the sources of illumination.

The variation in the laboratory reflectance value among the three samples may be due to variation in the chemical composition as seen in Table -2. Thus, the nature of both the curves i.e. laboratory & field indicate that the percentage reflectance remains more or less constant with the increase of wavelength. The nature of curve show that there is abrupt decline in the percentage reflection in 500-800nm.

Conclusions:
The present study lead us to conclude:

Laboratory reflectance studies by spectrophotometer indicates higher value of reflectance of rock as compared to field Radiometric observations.
Effects of weamering on the rock surface significantly influences the spectral signature of the rock.
The spectral signature of sandstones varies with chemical composition, grain size, colour and textural variation.
Basalt specimen which were studied, revealed that there is no effect of wavelength on the percentage reflectance in the wavelength range from 550 nm to 950 nm.

Acknowledgements:
The authors are thankful to Director S.G.S. Institute of Technology & Science Indore, India, for providing the facilities for this work and to Dr. Baldeo Sahai Group Director. Remote sensing area, space application centre (ISRO) Ahmedabad India for providing the intrumentation facilities.

Reference:
Soni S.K. (1985), Application of Remotely Sensed visual & digital data in the field of earth Science, Ph. D. thesis, (Devi Ahilya Vishf avidhyaly University of Indore) India.