Airborne Geophysical Survey of Northland Under Way

One of the the largest airborne geophysical surveys to have been carried out in New Zealand will take place in Northland over the next two months.

The survey will produce seamless scientific information on subsurface geology across the Northland region. It will start in the Far North District and work south progressively during February and March.

The project, which originated from an economic development strategy completed by the Far North District Council, comes three years after a resource assessment of the region was completed by GNS Science.

A light aircraft of a type used for topdressing will carry out the survey using sensors in the tail of the aircraft. Geophysicist Paul Vidanovich explains: “The survey instruments take measurements of the Earth's natural background fields such as magnetism and hence are completely passive. These natural fields are quite weak so the aircraft needs to fly at an altitude of around 60 metres to make its measurements so there will be a noise nuisance as it flies overhead. It will cover the region by flying back and forth in straight lines which will be 200 metres apart.”

Paul has supervised several such surveys in New Zealand and notes “Only a small area around Kaeo has been surveyed in Northland to date. This is one of the biggest surveys of this type to have been carried out in NZ, although the whole of the Otago region was surveyed with support from the Otago Regional Council in 2007. This type of geophysical data is now available for many resource-rich countries throughout the world, including nearly all of Australia, much of Africa and our Pacific neighbours. New Zealand is one of few resource-rich countries to lack this information. We are now starting to catch up.”

The data has a wide range of applications including understanding the complex Northland geology, mapping faults, resource assessment (including water, aggregate, clays and other minerals), foundation conditions for infrastructure, primary industry applications and land-use planning. The results will be made public later in the year following an interpretation by GNS Science.

The results will add value to the extensive geological and resource data on Northland that has been built up over the last century.

Recent advances in technology have made these surveys very cost effective. This is why governments (and aid agencies working in developing countries) have carried them out extensively.

The survey is being funded by the Ministry of Economic Development, the Far North District Council and the Northland Regional Council.

The survey contractor is Aeroquest Airborne, a Western Australia-based company that specialises in surveys of this type. It will advertise details, in local newspapers, of where it is flying while the survey is in progress. An 0800 number will be included in the advertisement for anyone wanting more information about the survey. 

Surveys of this type have been funded by government agencies to improve the accuracy of geological mapping for a wide range of applications, for delineating subsurface resources and (in the case of ground water) monitoring changes.   

In general, publicly funded surveys:

Privately funded surveys:

Airborne geophysical surveys can produce seamless, region-wide information on subsurface geology.  Most widely used are magnetic surveys that measure variations in the earth’s magnetic field, and can detect subsurface geological features.  In addition to their use in exploring for minerals they have numerous other applications:

Infrastructure investigations: Land instability is often associated with zones of ground weakness caused by geological features such as contacts, faults, shear zones and folds. Such structures are very evident within geophysical data sets, particularly those using magnetic and electromagnetic (EM) methods.   EM surveys are not included in the 2011 Northland survey but could be carried out in the future.  It will use magnetic and radiometric methods.

Geothermal Energy: Areas of rock affected by hot water at surface and at depth such as at Ngawha Springs have a particularly prominent magnetic signature. The geothermal fluids tend to be quite reactive and destroy the magnetic minerals within the host rock, thus creating non-magnetic zones which are readily observable in the magnetic survey data sets. Also, EM surveying can yield detail within the magnetic depletion zones.

In addition, potassium tends to be enriched in geothermal settings as rock gets turned to hydrothermal clays so radiometrics would detect these at surface by their potassium response.

Aggregates: The basalts found in Northland create a perfect target for magnetic prospecting because of their magnetic minerals content. New resources for aggregate could readily be pinpointed using a survey with 200m line spacing.

Ground Water Resources: Northland is drought prone and identifying aquifers that can produce water during these times may be a solution. Various rock types can form very good aquifers, such as basalts, limestones, and other sedimentary sequences.  Gravity, magnetic and EM surveying can all aid in identifying these rock types. Geological contacts, structure, faults and shears play a big part in the overall plumbing and identifying these would also aid greatly in defining groundwater resources. The conductivity mapping derived from the EM surveying would significantly improve the identification of water bearing structures and indicate any areas of salt water intrusion into currently exploited aquifers.  

Soil Mapping: Radiometric and magnetic data would be useful for improving the soil mapping of the region. This could be particularly useful for farmers, horticulturists and viticulturists. Magnetics would help identify the fertile volcanic soils, while EM and magnetics could help identify the marl soils that vintners appreciate. Potassic-rich soils would be readily identified by radiometric surveying. By combining data from all survey types it would be possible to build a catalogue of signatures to match soil types.

Geophysical instruments are mounted in an aeroplane or helicopter, which flies in straight lines back and forth across the area being surveyed. The flight lines are usually 200 metres apart for region-wide surveys.   Measurements are recorded every few seconds by the equipment in the aircraft and the results are processed by computer after the survey has been completed.

The aircraft has to fly at quite low altitude, creating a temporary noise disturbance for those on the ground.   The effects of this can be minimised by pre-survey publicity for those affected.

Surveys of this type can’t find and prove up mineral deposits or energy resources on their own.  The survey results are used with other information (such as geological maps and drill hole data) to define areas with potential, that then need to be followed up on the ground.  Surveys using a 200 metre line spacing (a typical spacing for regional surveys) won’t locate a feature that isn’t at least 200 metres long - that is, the length of 2 football fields.

Yes, but they have been carried out in a piecemeal way in many parts of New Zealand, and much of the information is still confidential.  The methods that have been used and the amount of detail vary greatly.  This map shows most of the surveys that have been carried out between 1980 and 2009.  The 2007 Otago survey was carried out under an agreement between a company and the Otago Regional Council.  
  

Airborne geophysical surveys carried out by companies, 1980 - 2009

Mining in Northland began with the discovery of coal at Kawakawa in the 1860s, followed by cement-making trials at Portland near Whangarei in the 1870s. Since then, antimony, copper, diatomite, kaolinite clay, kauri gum, manganese, mercury, peat, serpentine, silica sand and silver have been mined in Northland, along with aggregate and sand for building, and limestone. 

Today Northland has a well established mining industry that includes:

The value of Northland’s mineral output is the fifth largest of the 15 regions of New Zealand, and the mining sector is a significant source of employment in the region at present.   In 2006 it was employing nearly 600 people, more than half of them in the Whangarei District. 

The rate of unemployment in Northland is about twice the national average.  The development of the region is impeded by inadequate infrastructure and it lacks economic diversity.  The survey can contribute to the economic development of the region by facilitating investment in the resource sector, and supporting infrastructure investment with better information.

Northland has diverse geology, has a wide range of types of mineral deposit, and is under-explored using modern methods.   If this potential is developed, studies carried out by GNS Science and the NZ Institute of Economic Research show it has the potential to increase the GDP of the region by about $280 million, with about $142 million of this increase occurring in sectors other than mining.  Total employment growth would be about 2,700 full time equivalent positions.

• • NORTHLAND AIRBORNE GEOPHYSICAL SURVEY - BACKGROUND
  • • Why is the Northland survey being funded by Government agencies?
    • Cover large areas – regions, states and entire countries
    • Use survey and data presentation methods that are aimed at the widest possible range of users
    • Make the data available cheaply (typically the cost of copying and distribution) and quickly, often using the internet
    • Actively promote the use of the scientific information by the private sector, government agencies and researchers
   
  • • Are designed to locate areas with potential for a particular mineral or deposit type, usually covering one selected geological environment
    • Give companies a competitive advantage, and therefore they keep the information confidential for their exclusive use for as long as it may be of value to them.  Where surveys are carried out under a Crown Minerals Act permit, the data can remain confidential for a period of up to 5 years
    • Use data collection and presentation methods that suit their purposes, and may not necessarily be in a form that can be readily adapted for other purposes
    • • What can the results be used for?
      • • How will this benefit Northland?
       
      •  
        • Primary industries
          • Improve land capability assessment for agriculture, forestry, horticulture, viticulture
          • Attract exploration investment
       
      •  
        • Infrastructure
          • Provide subsurface information for locating and designing buildings, wind farms, new roads, sewerage systems, electrical reticulation.
          •  
        • Energy investigations
          • Help assessment of geothermal energy resources and aid the search for new geothermal fields.
       
      •  
        • Regional and district planning
          • Provide state-of-the-art data for assessing geological hazards and subsurface resources
       
      •  
        • The Community
          • Flow-on increases in household wealth, jobs, exports, and GDP
        • • How is it carried out?
          • • What won’t it do?
            • • Have they been used in New Zealand?
              • • Why Northland?
                • Cement – the Golden Bay Cement plant at Whangarei is New Zealand’s’ largest, and makes more than half of the cement being used throughout New Zealand.
                • High quality china clay for export from Matauri Bay
                • Aggregate, being produced at more than 50 quarries throughout the region
                • Limestone, mainly used as fertiliser in farming from more than 20 quarries
                • Sand, both onshore and offshore, for building and industrial use