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Discovery

Through the integration and interpretation of all geo-scientific data, Terra Resources can assist in giving you the best chance of discovery.

Geophysics

Specialising in geophysical survey design, acquisition, processing, modelling, inversion, data integration, interpretation and drill hole targeting.


Discovery

Through the integration and interpretation of all geoscientific data, Terra Resources can assist in giving you the best chance of discovery.

Terra Resources offers integrated interpretation products, combining geophysical modelling/data with surface mapping, geochemistry and drilling.

As a group Terra Resources combines expertise from all geo-scientific fields. We specialise in evaluating exploration targets and the exploration upside in your project area.

In addition, we specialise in target generation, identifying other commodity opportunities and targets both within and outside your current areas of interest.

 

Geophysics

Terra Resources specialises in geophysical survey design, acquisition, processing, modelling, inversion, data integration, interpretation and drill hole targeting.
Providing their clients with expertise, outcomes and ideas that can be easily tested within your budget constraints.

Terra Resources are highly experienced in all geophysical techniques from petrophysical analysis to processing, interpreting and modelling of data.

Our expertise includes:

  • Physical Properties (Core Measurement, Borehole Logging, Forward Modelling)
  • Potential Fields (Airborne/Ground Magnetics and Gravity)
  • Electromagnetics (Airborne, Ground and Borehole)
  • Electrical (IP/Resistivity, CSAMT/AMT/MT, MMR/SAM/PFE, MALM)
  • Radiometrics (Airborne, Ground)
  • Spectral (Landsat, Aster, Hymap)
  • High Resolution Seismic Surveys (On-Shore)

Geophysical Surveys

Magnetic Surveys

Magnetic surveys can be used to map lithological changes beneath the Earth’s surface, to delineate structures, and in some instances to directly detect mineral deposits (e.g. those containing magnetite or pyrrhotite). They are conducted by measuring very small variations in the earth’s magnetic field with a magnetometer which may be carried by hand (in ground magnetic surveys), or mounted on a helicopter or fixed-wing aircraft (airborne surveys). The strength of the magnetic field is given in gammas or more commonly nanoTeslas (nT) (1 gamma = 1 nT).

Gravity Surveys

Gravity surveys are also used to map changes in lithologies and to delineate structures. It is rarer that mineral deposits will directly give rise to gravity anomalies. However dense minerals such as those found in VMS or skarn deposits can indeed give rise to discrete gravity “highs”. Likewise zones of deep weathering over gold deposits may also be identified as discrete gravity “lows”. In essence gravity surveys reveal contrasts in the density of rocks below the surface. These variations are measured with a gravity meter, and usually described in milliGals (mGals) or gravity units (gu) (1 mGal = 10 gu). Ground gravity surveys are conducted by placing a gravity meter on the ground, and reading the Earth’sgravitational attraction at a particular point. This meter may be transported by foot, car or helicopter. Airborne gravity sensors (either gravimeters or gradiometers) mounted in a fixed wing or helicopter platform should also be considered – these surveys are best applied where ground access is limited or tight time constraints prevent acquiring a conventional ground survey.

Electromagnetic Surveys

Electromagnetic (EM) surveys are used to delineate changes in the conductivity of the earth’s surface.They are particularly useful for mapping changes in lithologies, and to delineate structures. They can also be used to directly detect mineral deposits, although generally a portion of a deposit must contain semi to massive sulphides in order for it to directly respond to EM surveying. A large variety of survey arrays can be adopted, depending on the objective of the survey. Commonly large “loops”, with high currents, operating at low frequencies are used to penetrate deep into the Earth. Similarly smaller systems operating at lower power and higher frequencies can be employed if depth of investigation is shallower. A large number of both ground and airborne systems are available. Airborne systems are attached to either fixed wing or are helicopter are heli-towed – technical specifications,terrain, budget and safety aspects should be considered during system selection. The strength of the electromagnetic field is usually measured in milliVolt/Ampere (mV/A). The properties of the rocks giving rise to the EM field can be measured in either resistivity units (ohm-m) or conductivity units (S/m) (resistivity = 1/ conductivity).

Induced Polarisation

Induced polarisation (IP) surveys are primarily employed to directly delineate disseminated and/or massive sulphides. This is done by transmitting a current into the ground through a pair of transmitting dipoles, and recording the response at a receiving pair of electrodes. A variety of survey arrays can be adopted, although dipole-dipole, pole-dipole and gradient arrays are most commonly applied. Again survey geometries and methodologies can be varied according to the size and depth of target. But it is possible to detect sulphides at depths of 300 metres or more. A variation in the earth’s IP(chargeability) response is either measured in percent frequency effect (PFE), milliseconds (msec) or milliradians (mrad). By far the most common is either msec or mrad (1msec = 1.8 mrad). Resistivity data is always collected with IP data.

CSAMT and MT

The Controlled Source Audio-Magnetotelluric (CSAMT) and Magnetotellurics (MT) techniques were introduced in the mid 1970’s as an EM method for finding mapping resistivity. MT is a passive technique which utilises natural energy sources (solar wind and lightning strikes), as opposed to CSAMT which involves use of a grounded dipole source.

Borehole Surveys

Borehole surveys are used to map the in-situ physical properties of the surrounding rocks (nominally 50cm around the hole). Properties that can be measured include density, magnetic susceptibility, induced polarisation, resistivity, inductive conductivity, acoustic velocity and natural gamma. Some specific types of borehole surveys can be designed to look up to 50m away from the borehole. Downhole EM (DHEM) is often used in base metal exploration as a follow up tool to surface EM.

Geophysical Planning - Airborne Surveys
Many factors must be addressed when planning airborne geophysical surveys.

Technique

  • Exploration objectives, including size and nature of the target, as well as properties of the host and surrounding rocks should be clearly outlined to the geophysicist, so that an appropriate geophysical technique can be applied to meet these objectives.
  • The most common airborne surveys are airborne-magnetic/radiometric and airborne-electromagnetic (AEM). Both are very good geological mapping tools, and can directly detect certain styles of mineralisation.
  • Airborne magnetic data are also routinely collected while acquiring AEM data. AEM data are usually ten times more expensive to acquire.

Instrumentation/Platform

  • There is little difference in the quality of the instrumentation used by contract airborne magnetic/radiometric acquisition companies. There is however considerable difference in AEM instrumentation.
  • The nomination of the best contract company to conduct an AEM survey should always be made in consultation with a geophysicist. Survey Direction
  • Flight lines should be oriented perpendicular to the strike of the local geology unless important structural or mineralisation orientations are at high angles to the strike. In this event, the survey orientation should be discussed with a geophysicist.
Survey Direction
  • Flight lines should be oriented perpendicular to the strike of the local geology unless important structural or mineralisation orientations are at high angles to the strike. In this event, the survey orientation should be discussed with a geophysicist.

Survey Height

  • Survey height will influence the ability to resolve responses from small sources or those that are close together.
  • Ideally surveys should be flown as low as possible – but the safety of the operator(s) is of paramount importance.
  • Large local variations in topography may also influence the lowest safe survey height.

Line Spacing

  • Line spacing will influence the ability to resolve/detect small sources and subtle features.  The closer the line spacing the better the resolution.
  • The objective of the survey and the size of the target should be clearly explained to the geophysicist, so that appropriate line spacing can be chosen.
  • As a very general rule of thumb the maximum scale that data can be usefully plotted is 100 times the line spacing at which it was acquired.  For example data collected on 200 metre lines can be plotted at 1:20,000, beyond this its integrity will be questionable.

Existing Data

  • The quality of any existing data in an area should be reviewed.  It may shed light on optimal survey parameters for the proposed survey.
  • Before finalising plans for any survey all of these aspects should be discussed with a geophysicist.

Safety and Logistics

  • Logistics, safety considerations and risk assessments should be considered when planning any airborne survey – the Terra Resources airborne geophysical standards and procedures, and other accompanying Terra Resources standards and procedures must be adhered to.
  • A Field Work Plan should be started during the planning phases which covers logistics, Emergency response procedures, and risk assessments (both Formal Risk Assessments and International Airborne Geophysical Safety Association (IAGSA) risk assessments).
Geophysical Planning - Ground Surveys
As with airborne surveys, many factors must be considered when planning ground geophysical surveys.

Technique

  • Exploration objectives, including size and nature of the target, as well as properties of the host and surrounding rocks should be clearly outlined to the geophysicist, so that an appropriate geophysical technique can be applied to meet these objectives.

Instrumentation

  • With the survey objectives clearly defined the geophysicist will be able to advise the most suitable instrumentation.
  • This will limit the number of contract companies who can potentially acquire the data.

Line and Station Spacing

  • The geophysicist will be able to advise the most suitable line and station spacing in order to meet the objectives of the survey.

Preparation of Grids

  • Where terrain and overhead canopy permit, GPS is commonly utilised to locate a survey grid. Where the above mentioned inhibit accurate GPS gridding, a compass and hip chain off a base line is an effective alternative.
  • Hence a surveyed grid must be installed prior to commencement of the geophysical survey.
  • Follow up of geophysical anomalies should be carried out on this same grid.
  • If significant time has elapsed between acquisition of the geophysical data and follow-up of anomalies, and the original grid can not be located, it may be necessary to repeat a small portion of the geophysical survey to ensure that the exact location of anomalous responses can be delimited.

Safety and Logistics

  • Logistics, safety considerations and risk assessments should be considered when planning any ground survey – the Terra Resources ground geophysical standards and procedures, and other accompanying Terra Resources standards and procedures must be adhered to.
  • A Field Work Plan should be started during the planning phases which covers logistics, Emergency response procedures, and risk assessments (Formal Risk Assessments).
Geophysical Planning - Borehole Surveys
When to Conduct a Borehole Survey
  • Downhole EM surveying should be performed on all deep drill holes (>200m) that have been drilled to test for massive sulphide mineralisation.   It is also desirable for holes where the likely target might include significant sulphide (e.g.: pyrite alteration) .
  • Down-hole logging surveys (e.g. resistivity, density or magnetic susceptibility) should be conducted on drill holes that have been drilled to test geophysical anomalies when there is uncertainty as to whether or not the source of the anomaly has been identified.
  • Hand held magnetic susceptibility measurements should be read on all samples (drill core, RC/RAB chips and outcrop) taken in the field.  Magnetic susceptibility measurements aid the interpretation of regolith, geology, magnetic modelling, and airborne magnetic data.  The most common form of magnetic susceptibility meter in Terra Resources is the KT-9 (Exploranium) or SM-30 (ZH Instruments).  The units are Six10-3.  Operating Procedures for the KT-9 and SM-30 have been included as Appendix 13-B.

Borehole Preparation

  • The downhole geophysics techniques of magnetic susceptibility, natural gamma, 3 component magnetics, resistivity and inductive conductivity do not ordinarily require the hole to be cased unless the ground is unstable.
  • PVC piping with a ‘Class 9’ rating is usually used for holes down to 500m; with classes 12, 15 and 18 for progressively deeper holes.   The higher class pipes have the same outer diameter, but the thicker walls reduce the inner diameter.  Note that the pipe classes do not correspond to any particular dimension of the pipes.
  • In order to conduct downhole EM surveys, the drill hole should be cased with 50mm PVC tubing. Most down-hole ‘slimline’ probes have an outside diameter of 30-38 mm and will fit down 40mm casing. This has a nominal inner diameter of ~46mm for class 9 with an OD of ~52mm at the coupling.   32mm piping has an ID of ~39mm for class 9 and although nominally large enough to take most slimline probes, the smaller margin means that probes tend to jam more readily and this size is not recommended.
  • It is quite common for a hole to become blocked after the drill rods are drawn and before the piping is in place.  Two approaches to overcome this are:
  • (1) To put an open ‘shoe-bit’ on the end of the rods which will allow the piping to pass through it. The rods are taken to the bottom of the hole and then withdrawn after the piping has been put in place.   A cap with a small hole in it, on the bottom end of the piping will help to control the rate of descent (which can be a problem in a long hole).
  • (2) A reverse thread metal ‘foot’, specially made up with two lugs to ‘catch’ at the end of the hole, is made up and replaces the drill bit.  The piping is then inserted and glued as the rods are lowered. At the bottom of the hole, a sharp twist on the rods releases the foot which remains in the hole. (Note this device can only be used in ‘good’ holes, since any turning of the rods as they are lowered will release the foot.)
  • Both of these procedures require drill rods of sufficient diameter to allow the rods to pass through.  The inside diameter of NQ rods is 60 mm which will take both sizes of piping.  The commonly used BQ is 46mm which is too small for either size.
  • If downhole IP/resistivity data is to be collected then the PVC tubing will need to be slotted to allow for contact between borehole fluid and surrounding rock.   Slotted casing can be purchased from field supply vendors or produced by drilling holes or sawing a series of overlapping slots – a higher class of pipe should be used to prevent kinking. 
  • Occasionally a hole may not be able to be cased, for example when extending the hole or drilling a wedge from it.  To survey an uncased hole in bad ground where the hole is at risk of collapsing, place the drill rods with a shoe-bit (or no bit) in the hole to a depth to cover the deepest bad ground.  The survey is then undertaken through the rods up to within 10-15m of the bit.  The rods are then withdrawn up to the next suspect section and from there down to the previous bad ground surveyed and so on.  Thus the hole is surveyed in sections with gaps at the bad ground.
  • Apart from keeping a hole open, piping can also be used to allow surveying of a shallow-dipping or even up-dipping hole. (Generally probes will stop in holes with slopes of less than about 23o .) ‘Bungs’ are used to loosely seal the probe and water is then pumped into the piping with the gap between the hole wall and the piping, forming the return.
  • A geophysicist should be consulted in order to determine suitable sample spacing and tool calibration techniques.
  • All drillholes (cased and uncased) must be dummy probed prior to dropping the logging tool into the drillhole.

Procedure for Running PVC Casing in Diamond/RC Drill Holes

  • When possible, run PVC through wireline drill rods.
  • Prior to running PVC, always make sure that all steel casing in the hole is free.
  • The piping comes in 6m lengths.  When gluing together make sure that (1) the glue is not past its use-by date and (2) that both surfaces are clean and dry (paper towels are better than rags). Gluing (say) six lengths together before lowering into the hole allows plenty of drying time and a manageable length.  The pipe down the hole can be held with a stillson wrench or a piece of rope. Pop riveting is not recommended.  Pipes left in the sun for any extended period become very brittle and may collapse under the weight of the pipe above. 
  • To overcome the problem of the casing kinking in a hole, the casing can be held up off the bottom so that it is suspended in the hole.  This can be done using a rope or by injecting expanding foam between the casing and the hole wall.  On deep holes, it is advisable to use a shoe to support the weight of the PVC.  The shoe has a left hand thread which is easily unscrewed at the bottom of the hole by rotating the rod string clockwise. 
  • When placing PVC in the upper part of a hole (e.g. PQ or HQ size) it is necessary to use a collar, tapered bit to fit the lower smaller hole, which will guide survey tools past the step in the hole wall.
Geophysical Data
It is important when accepting geophysical data that all forms of the data are received from the contractor.  Quite often the final data has been pre-processed and re-processed several times before the final product is reached.  More often than not a geophysicist will need to validate data integrity and will require raw data as collected in the field in order to do so.

Airborne Data

  • A contractor should provide line located data in an ASCII format.   An example of some things to included are as follows: flight line number, flight date (DDMMYY), fiducial number, time of reading, Universal Transverse Mercator (UTM) zone number, easting UTM (metres), northing UTM (metres), Latitude (WGS84), Longitude (WGS84), radar altimeter height (metres), GPS height (metres), corrected terrain height (metres), raw total magnetic intensity reading (nT), diurnal magnetic correction applied (nT), final leveled total magnetic intensity (nT), IGRF correction applied (nT), Final leveled, IGRF corrected total magnetic intensity (nT), barometric height (metres), barometric pressure (HPa), air temperature (degrees Celsius), humidity (percent), raw total count, raw potassium, raw thorium, raw uranium, raw cosmic, predicted radon correction, air absorbed dose rate, corrected total count, corrected potassium, corrected thorium, corrected uranium.
  • A contractor should also supply the results in gridded format.
  • A logistics report should be provided documenting survey specifications, digital data formats and all calibration tests performed throughout the survey.
  • All data should be provided on a disc, thumb drive or hard drive.
  • All data should be checked by a geophysicist before final payment has been made to ensure the data is in an acceptable format and it meets survey specifications.

Ground Data

  • A contractor should provide the data in both raw (instrument dump) and processed ASCII formats. 
  • As ground condition can quickly change it is advisable for a geophysicist to be present at the start of a ground geophysical survey to re-assess survey specifications and check initial data quality.  All ground contractors should have the ability to send digital data to a geophysicist on a daily basis.
  • A logistics report should be provided documenting survey specification, digital data formats and a hard copy plot of the final processed data.
  • All data should be provided on a disc, thumb drive or hard drive.
  • All data should be checked by a geophysicist before final payment has been made to ensure the data is in an acceptable format and it meets survey specifications.

Borehole Data

  • A contractor should provide the data in Log ASCII Standard format (LAS) for general downhole logging services.  Raw (instrument dump) and processed ASCII formats need to be supplied for downhole EM surveys.
  • A brief logistics report should be provided with the data documenting survey specifications, data formats and a hard copy plot of the final processed data.
  • As downhole logging data is acquired very quickly it is important to get it right.  For small surveys the hole should be logged both down and up (twice).  The two data sets from the same hole can be checked for repeatability.  For long term contracts a “calibration” or reference hole should be established so results of different contractors/tools can be compared against each other.
  • All data should be checked by a geophysicist before final payment has been made to ensure the data is in an acceptable format and it meets survey specifications.

Why use Terra Resources?

Combined experience

The Terra Resources team have a combined experience of over 25 years in the mineral exploration industry.

Guaranteed Project Delivery

The team at Terra Resources can deliver at all stages of your project pipeline, from tenement evaluation to resource definition.

Exploration success and leadership qualities

Constant exploration success and leadership qualities coupled with excellent geological/technical ability.