Airborne Geophysical Surveys
In June 2006 GSI carried out some pilot airborne geophysical surveys over parts of counties Kerry, Tipperary, Cavan, Leitrim and Monaghan.
The aircraft used was a Twin-Otter fixed-wing propeller-driven aircraft and it flew at an altitude of 55-90m in rural areas and 250m over urban areas. Survey lines were flown Northwest or Southeast and were spaced 200m apart.
Expected Survey Benefits
it is expected that data resulting from the surveys will help manage our environment and support sustainable natural resource development by providing more information about our soils, sediments, waters and rocks.
The aircraft carried a range of geophysical equipment that maps the geological characteristics of rocks and soils. Such information will improve the understanding of our natural environment. It will contribute to and therefore benefit sustainable land use planning decisions. The survey will also help identify potential natural hazards and map certain forms of contamination.
The three main systems on board the aircraft were magnetics, radiometric (gamma ray spectrometry) and frequency domain electromagnetics. These different systems measure different properties of the earth. In the case of the magnetic system, the instrument measures the strength of the magnetic field of the earth. The strength of the magnetic field provides information on the different rock types under the ground (in general the more iron in the rock the greater the magnetic field strength). It also allows geologists and geophysicists to identify faults and fractures in the earth. Such faults or fractures are often pathways where water, mineralizing fluids (to form mineral deposits) or radon travels through the earth.
In the case of the electromagnetic system the instrumentation measures the strength of the electromagnetic field set up locally by a current within special coils in the wingtips of the aircraft. The instruments on board the aircraft measure the electrical conductivity of the ground. Geologists and geophysicists interpret these measurements in terms of more or less conductive materials in the ground. Examples of conductive materials would be metallic mineral deposits, water-bearing gravels, water bearing cave systems in limestones (karst features), or leakage of contaminated water from landfill sites.
The radiometric system measures the natural radiation given off by the earth. It is known that several naturally occurring elements emit radiation, e.g., potassium, thorium and uranium. Almost all of the radiation is harmless in the amount emitted. Certain rocks are naturally enriched in some of these elements. For example, potassium is known to be associated with certain granites and uranium is known to be associated with certain shales and also certain granites. Uranium is unstable and it decays to among other elements radon. In forming radon gamma radiation is emitted and it is this radiation that the aircraft instrumentation system measures. Radon as we know can be dangerous as it is known to be associated with lung cancer. The radiation emitted has a telltale signature which when measured by the aircraft system and the data processed by geophysicists allows us to identify those areas with elevated uranium. Once the source of the parent uranium is known we must then identify how and by what route the radon travels to peoples houses. Both the magnetic and electromagnetic systems help us identify faults, fractures in the rock and cave systems, which are the most likely travel paths. The radiometric system also allows us to identify areas underlain by peat and to estimate its thickness. Using the system therefore allow us to produce an estimate of our peat resources.
A necessary part of the survey also required that magnetic and GPS base stations were set up (figure 3). The base stations were set up at Enniskillen airport for the Cavan-Monaghan-Leitrim survey and at Kerry airport for the Castleisland and Silvermines areas.
Selection of the specific survey areas was based on the following:
|Cavan-Monaghan-Leitrim ties in with work already underway by GSI where a groundwater protection scheme has commenced. In addition it will allow comparison with recent detailed GSI mapping in Monaghan and it will also allow a comparison of our data with that from the recently completed NI Tellus Survey.
The Castleisland area has recorded elevated radon levels in buildings. This area was covered by the airborne survey to gather an improved level of detail on the bedrock geology and identify geological units and or structures, which may have potential for radon. Analysis of the data is being carried out in conjunction with the RPII.
This is a former Base Metal mining area. This site was also part of a recent mine site study and is part of a proposed mine rehabilitation programme. A similar survey was flown here in the past and it should be possible to compare results of both systems and try to extract additional environmental information.
To view some preliminary imagery from the surveys please click - After the survey....