High Resolution Airborne Geophysical Survey, Phase 1 Papua, Arafura Platform
The Centre for Geological Survey (CGS), of the Geological Agency (GA), Ministry of Energy and Mineral Resources, Republic of Indonesia, has conducted the High Resolution Airborne Geophysical Survey Project (HRAGSP) in Papua, Indonesia for the first time. Furthermore, this HRAGSP through the Geological Agency (GA), is planned as systematic geophysical mapping project to conduct a regional airborne geomagnetic and radiometric surveys of the whole of Indonesian territory.
The first phase of the HRAGSP covered an area of approximately of 135,522 sq km on a lowland of Arafura Plat-form. This area is located in the southern part of Papua (Fig. 1), covering eight sheets of 1:250,000 topographic map. Topographic feature of conducted 2010 survey as shown on digital terrain model (DTM) in figure 2. This project was funded by the Indonesian Government under the APBN of 2010 fiscal year and took about 5 months of field work that was commenced from September 2010 and finished on January 2011.
The survey was flown by using two Piper PA 31-350 Chieftain air-crafts owned by PT. Intan Angkasa Air Service and GPX Surveys was responsible for the data acquisition and processing of the airborne magnetic and radiometric data.
Geologicaly the survey area form as part of Arafura Platform that is included inside the North Australian Continent (Dow, et al, 1986). This area is covered by alluvial sediment and most part of it as a swampy area and makes the ground survey became difficult. Regional geology shows that this area is covered by thick Pre-Tertiary to Quarternary sediments (Dow, et al, 1986, Sukamto, et al, 1996). Tectonic process of two major plates, Australian Continent from the south and Pacific Plate from the northeast formed the Papua Central Ranges, whereas the Pre-Tertiary sediments became outcroped (figure 3). Drilling data from three locations in this area, show that the Pre-Tertiary sediments underlay this area. Gravity and magnetic data show the occurence of the basement undulation in the direction of north to south. Gravity anomaly ranging in between 15 to 40 mGal give an indication of the possibility the occurence of hydrocarbon prospect.
The airborne geophysics survey used magnetic and radiometric methods and flown the total lines of 120.000 km. A total of 108,000 km in lenght of N-S traverse lines with spacing of 1000 meter and 12,0000 km in lenght of E-W tie lines of 10,000 meter spacing, and both have 100 meter ground clearance has been measured. This regional airborne magnetic and radiometric survey is very good tools and plays important rule in constructing the geological map to aid in mapping lithology and structure in both hard rock environments and for mapping basement lithology and structure in sedimentary basins or for regional tectonic studies especially in the sediment covered area in which there is no rock outcrop exists.
There were two primary base stations installed during the survey by which their locations and coordinates shown in Table1.
Table 1. The Survey Base Stations
|Merauke Airport||140.4138° E||-08.5150° S||DGN'95 / WGS'84|
|Timika Airport||136.8931° E||-4.5324° S||DGN'95 / WGS'84|
Radiometric calibration tests were conducted to calculate attenuation coefficient, sensitivity, striping ratio, aircraft and cosmic backgrounds. These calibration based on The International Atomic Energy Agency (IAEA) 2003.
The units of measurement of a radiometric survey are count per second. The values can vary depending on the fligh clearence, type of spectrometer being used and the radiation background. To ensure the measurement value have geological significance, it has to be converted to make sure it reflects mean ground level abundances of the radioelements.
Gamma-ray spectrometry especially can be used for direct assistance to exploration many others commodity most obviously for Uranium(U), Thorium (Th) and Potassium (K). Therefore, the basic purpose of radiometric survey is to determine either the absolute or relative amounts of U(ppm), Th(ppm), and K(%) in the surface rocks and soils (figure 5). However, this method can also be used for determining Au, Sn, W, Nb and Zr associated minerals.
While magnetometer measures the total magnetic values as a representation of local disturbances in the earth's magnetic field that are caused by magnetic minerals in the upper regions of the earth's crust (underlying geology). Gamma ray spectrometry can be used for surface geological mapping to the depth of 5 meters approximately, while the regional magnetic expressing a deeper subsurface of geological features. The final product of an airborne magnetic survey consists of maps of the total magnetic contoured and shadow of those total field. These total magnetic values are also derived into reduction to the pole, vertical derivative and residual anomaly. The original data field in digital form shows the time, location and the value of each measurement.
The integrated method of gamma-ray spectrometric and magnetic usually the most cost effective tools available for both large regional reconnaissance surveys used as aids in geological mapping and for locating prospect areas. Although the survey area is mostly covered by Alluvium and swampy (figure 3), the methods which was applied is proved that some geological features has successfully interpreted (figure 6).
The main affected factor which inhibit the progress of the survey are the climate conditions, rapidly changing weather, extreme nature of the survey area. During the survey both aircrafts had to be monitored for full time duration by air traffic controller, which was located in the main base air port that are Merauke and Timika. Instrument Real Time Omnistar Tracking System were equipped on the aircrafts due to monitoring both aircraft while the survey was in progress.
Survey planning is neccesarily well designed to start with in order to anticipate the extreem climate in this area, even more in the central range that are predominantly very often remote, rough topography, wet and almost constantly cloud covered. This condition had caused hard challenges to the next HRAGSP, especially for the next 2011 HRAGSP purposing surveys of the Papua high land with steep slopes, rough terrain and selected helicopters will be used.
Dow, D.B., G.P. Robinson, U. Hartono and N. Ratman, 1986, Geologic Map of Irian jaya, scale 1 : 1,000000, Geolocical Researh and Development Centre.
---------, G.P. Robinson, U. Hartono and N. Ratman, 2005, Geology of Irian Jaya, Preliminary Geological Report, Special Publication, Geolocical Researh and Development Centre, 208 p.
IAEA, 2003, Guidelines for radioelement mapping using gamma ray spectrometry data, IAEA-Tecdoc-1363, 173 p.
Sukamto, R., N. Ratman and T.O. Simanjuntak, 1996, Geological Map of Indonesia, scale 1 : 5,000000, Geolocical Researh and Development Centre.
For further information, please contact Harry P. Siagian, the team leader of the airborne magnetic and radiometric mapping project,
The Centre for Geological Survey (CGS).