Gravity, GPA and Geomagnetic Data in India
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Proc Indian Natn Sci Acad 80 No. 3 September 2014 pp. 705-712 Printed in India. DOI: 10.16943/ptinsa/2014/v80i3/55145 Review Article Gravity, GPS and Geomagnetic Data in India V M TIWARI*1, B VEENADHARI2, V K GAHALAUT1, S MUKHERJEE2 and V P DIMRI1 1CSIR-National Geophysical Research Institute, Hyderabad, India 2Indian Institute of Geomagnetism, Mumbai, India (Received on 17 July 2014; Revised on 13 August 2014; Accepted on 17 August 2014) Gravity, Global Positioning System (GPS) and Geomagnetic data sets in India are acquired by different research, academic and government institutions, under various projects. These data sets have extensively been utilized for natural resources and lithopsheric explorations, earthquake studies, atmospheric and ionospheric studies, control surveys, aircraft navigation, etc. The data are archived at individual institutions and have different modes of procurement considering some of the data, e.g., gravity data are classified in nature. Some of these data sets are contributed to the international observational network for example IGS and INTERMAGNET and are available as open source for the scientific communities. Present article provides information about different types of available Gravity, GPS and Geomagnetic data, their archival and mode of availability to the user community. Key Words: Gravity; GPS; Geomagnetism; India Introduction geodetic measurements in India date back to last century. Earlier measurements were made using The Earth’s gravity field varies with time and space pendulum and these were very crude. In seventies, due to numerous reasons. The gravity variation with International Association of Geodesy initiated time are both periodic and non periodical. The non International Gravity Standardization Network periodic gravity variations are often utilized to map (IGSN, 1971). India has been a part of that initiative mass transport in the Earth system (Tiwari and and several absolute values of gravity field were Hinderer, 2012). However, spatial variation of gravity recorded by relative gravity meter connected to field corrected for topography and latitudinal Potsdam reference gravity station. These gravity variations, known as gravity anomalies are more stations tied to IGSN reference station have been commonly used by geophysicists and geologists to serving as primary reference stations in the country. infer subsurface density distributions caused by differences in the nature of the rocks. Therefore, Global positioning System (GPS) is a navigation knowledge of causative sources of gravity anomalies system, which receives signals from a number of offers several objectives e.g. exploration of natural satellites orbiting the Earth and provides information resources, lithospheric density structure and deep about position, velocity, and UTC time. GPS satellites density anomalies involved in the geodynamic were initially launched and maintained by United processes (Mishra et al., 2008; Tiwari et al., 2013). States. Now, due to the development of technology, Measurements of gravity field are made in several satellites are being launched by different combination of absolute and relative measurements. countries like Russia and India, which provide similar Recorded data are processed to derive gravity information and complement to GPS. In recent era, anomalies following known procedures. Gravity and recording of precise GPS data, their processing and *Author for Correspondence: vmtiwari@ngri.res.in 706 V M Tiwari et al. numerous applications to the Earth, atmospheric and Survey India (GSI), Oil and Natural Gas Corporation ionosphere are noteworthy. Considering its (ONGC), Oil India Ltd. (OIL) to collaborate with the importance, a large number of observations are being primary goal of updating and publishing gravity recorded and data are archived and made available anomaly map of India from the basic gravity data to users. collected under various projects by these five collaborative organisations. A virtual data bank At any point on the surface of the Earth, the combining data from all the organizations for entire magnetic field is a vector quantity. The magnitude of India totalling 143,786 are archived and grid was the field is represented by the Total field F. The three prepared at GSI, Hyderabad. For uniform coverage components of the magnetic field are Horizontal over entire country, an optimal spatial interval of 3 Intensity ‘H’, Vertical Intensity ‘Z’ and the minutes arc interval is chosen thereby only 51,356 Declination component ‘D’ of the location defining gravity observations are considered to prepare the the angle between the true north and the magnetic revised gravity anomaly map of India 2006 (Fig. 1). north. Observations of these recordings are done on Since data were heterogeneous in quality, type of a continuous basis at magnetic observatories with the corrections and their absolute value determinations help of magnetometers specially designed to monitor were always not based on International Gravity the sensitive variations in the earth’s magnetic field. Standardization Network (IGSN, 1971); all the data Like gravity anomalies, magnetic anomalies were reprocessed additionally incorporating detailed also play an important role in investigating subsurface terrain corrections. The corrected data were supplied information of rock properties. They are derived from to the GSI printing division for printing of maps on Earth’s magnetic field measurements with necessary the 1:2milion scale (Sundaram et al., 2007). The processing including International Geomagnetic Bouguer maps were superimposed on geological map Reference Field (IGRF) corrections. Magnetic published by GSI on the same scale, which reveal measurements intended for obtaining magnetic one to one correlation between a number of anomalies anomalies are carried out on ground and also by and geological structure/mineralized zone. Though aeroplane/helicopter in air. Since airborne data the sharing of digital gravity data are restricted as collection is expensive, only selected regions of per the Ministry of Defence (MoD) guidelines, the primary interest for exploration are covered for availability of 5 sets of gravity anomaly maps provide magnetic measurements in various parts of India vital information for the planning and studies ranging (Rajaram et al., 2006). from mineral/hydrocarbon exploration to tectonic studies (Mishra et al., 2008). These maps can be Gravity Data procured from GSI, details of which are stated at web- pages (http://ngri.org.in/pdffiles/aboutngri/ Over the last few decades, a large number of gravity gravitymap.pdf; http://www.gsi.gov). One of these data are recorded in the different parts of India by maps (GMSI, 2006), Bouguer Anomaly Map of India several institutions for their specific projects. These (Image) is presented in the Fig. 2. data were compiled in seventies and a map of 10 mGal –3 2 (1mGal=10 cm/sec ) interval on 1:5 million scale Global Positioning System (GPS) Data was prepared. However, such a map is not adequate enough for present day exploration requirements and In India, crustal deformation studies using GNSS was mostly utilized only for regional lithospheric and (mostly GPS) started with the establishment of a geodynamic studies (e.g. Tiwari et al., 2013). To permanent International GNSS Service (IGS) station improve upon the gravity anomaly maps prepared in at IISC Bangalore in early 1990’s. This was followed seventies and eighties, an effort was made in bringing by the development of a National Programme on five major research/government organizations namely crustal deformation studies in 1997-98. Several CSIR-National Geophysical Research Institute campaign mode observations of plate motion and (CSIR-NGRI), Survey of India (SOI), Geological crustal deformation using GPS were initiated. In Gravity, GPS and Geomagnetic Data 707 Fig. 1: Filled circle in different colours are locations of gravity observations recorded by different organizations mentioned in the index. Numbers mentioned against each organization are number of gravity observations contributed while preparing Gravity Map Series of India-2006. Triangles are GPS stations and Rectangles are geomagnetic observatories. Source: Gravity Map Series of India-2006 1997, another IGS GPS station was installed at NGRI the foundation which either reaches the firm ground Hyderabad. By the beginning of this century, the or at least 6 feet below the ground surface. The semi- programme expanded extensively. Presently, there are permanent stations are also made on the firm base about 100 permanent or semi permanent GPS stations (Fig. 3b). Locations of the VSAT connected in India maintained by different organizations such permanent stations operated by different as NGRI, SOI and CMMACS etc. Several campaign organizations are plotted in Fig. 1. All the permanent mode studies in the seismically active regions stations acquire GPS data at 30 second have been taken up through periodical GPS sampling interval and a daily file (called as the observations for a few days. All permanent stations RINEX observation file) is generated. This file is used are installed on elevated ground, away from trees and to compute the coordinates of the site which could buildings to avoid obstructions in tracking the be accurate at ~3 mm level. The time series showing GPS satellites. Generally hard rock site is preferred the daily variation of these coordinates provides an so that actual ground motion can be recorded. A RCC estimate of site motion, which when