September 16, 2009
VTEM detects the Night Hawk Lake conductor from 705 m above.
Read the case study (PDF)
March 24, 2009
BENTON RESOURCES, ROYAL ROADS & GOLDEN DORY APPROVE 2009 PROGRAM FOR LONG RANGE NICKEL JOINT VENTURE, NEWFOUNDLAND – DRILL TARGETS IDENTIFIED
"... Among the targets selected for further work is a 600 metre long VTEM conductive anomaly described by Geotech Ltd. as “a relatively strong conductor consistent with massive sulphides”. This target has been selected for testing by diamond drilling, and it is anticipated that additional targets will be developed from results of the prospecting and ground geophysical surveys."
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News archive
Case Studies
September 16, 2009
VTEM detects the Night Hawk Lake conductor from 705 m above
Presentation: Exploration and discovery of blind breccia pipes
Exploration and discovery of blind breccia pipes - the potential significance to the uranium endowment of the Arizona Strip District, Northern Arizona.
This presentation includes results from the first Arizona Strip District Airborne VTEM Survey.
Author: Eugene D. Spiering, VP Exploration
Quaterra Resources
WATER RESOURCES APPLICATIONS WITH THE VTEM SYSTEM
Introduction
Airborne electromagnetics are increasingly employed to satisfy water resources objectives worldwide. These water resources objectives include the spectrum from locating small-scale near-surface groundwater aquifers to hydrogeological mapping at a large regional scale and depth. Airborne electromagnetic platforms that are already optimised for locating conductive bodies for the mineral industries due to their high-power output and low noise characteristics are well-suited for water resources objectives. Continuous technical improvements to the signal-to-noise ratio from both early to late time windows and data acquisition methodology are important elements to maintaining a comprehensive airborne groundwater survey platform.
Geotech modified their VTEM system and acquisition methodology to produce an airborne EM platform to meet a wide range of water resources objectives. The resultant VTEM dataset has very low noise characteristics maintained from early to late time windows which are optimal for successful use in layered-earth inversion modeling interpretation commonly used in water resource applications.
Recent water resources applications of the VTEM system include a large-scale mapping of the Okavango Delta region in Botswana for the Ministry of Minerals, Energy and Water Affairs and the regional mapping of an area along the Taoudeni Basin boundary in Mauritania for SNIM. The main objectives of the Okavango survey are to map the freshwater-saltwater interface throughout the delta region, identify freshwater resources for exploration and regional geological mapping. The main objective of the Taoudeni Basin boundary survey was to provide a reconnaissance of the hydrogeological framework in a geologically complex region. VTEM surveys in both instances yielded highly successful results. Both surveys have been conducted with COWI as a groundwater interpretation partner.
Authors:
Kerim Martinez COWI
Bob Lo, Geotech
Carsten Ploug, COWI
Doug Pitcher, Geotech
Pavel Tishin, Geo Equipment Manufacturing
DETECTING BASEMENT CONDUCTORS IN A CONDUCTIVE ENVIRONMENT: AN EXAMPLE FROM THE NEPEAN MINE VTEM SURV
Introduction
In March 2007 a VTEM test survey was flown over the Nepean Nickel Project, Coolgardie, WA, Australia. The results from this survey are compared to the known geology as well as responses from ground surveys and a Hoistem survey over the same area.
Authors:
Magdel Combrinck
Geotech Airborne Ltd.
121 Rosen Office Park
37 Invicta Rd, Midrand, 1686
South Africa
Paul Mutton
Southern Geoscience Consultants
8 Kearns Crescent
Ardross, WA 6153
Australia
Bob Lo
Geotech Ltd.
245 Industrial Parkway North
Aurora, Ontario L4G 4C4
Canada
PRELIMINARY REPORT ON HELICOPTER-BORNE ZTEM TIPPER AFMAG SURVEY RESULTS over SHEA CREEK TEST BLOCK
Introduction
In April 2009, ZTEM (Z-Axis Tipper Electromagnetic) surveys were conducted, on behalf of Areva Resources Canada Ltd. (Saskatoon, SK), over the Shea Creek Test Block that is situated in the Cluff Lake region of the western Athabasca Basin.
The ZTEM survey consists of airborne Tipper AFMAG (audio frequency magnetic) measurements, as well as aeromagnetics using a caesium magnetometer. The survey consisted of twenty four (24) 18km long, NE-SW oriented (N-060OE) flight lines, totaling 432 line-km, that were obtained at 400m nominal line spacings over an approximately 9 x 18km area. The area was chosen because it hosts the well known Shea Creek uranium deposits, is overlain by a significant thicknesses of Athabasca sandstone (650+ metres) cover, it hosts a major graphitic horizon and sandstone alteration zones at depth, and has been the focus of extensive airborne and ground geophysical coverage.
Authors:
Jean M. Legault, M.Sc., P.Geo.
Geotech Ltd., Aurora ON
Harish Kumar, M.Sc., P.Geo.
Geotech Ltd., Aurora ON
The Geophysical Response of the Tusker Gold Deposit, Lake Victoria Gold Fields, Tanzania
SUMMARY
The Tusker gold deposit is located in the Lake Victoria Goldfields of Tanzania. The deposit contains total estimated resources of 123.27 Mt at 1.15g/t Au, as at 5th September, 2006. Geophysical techniques trialled over the deposit include down hole measurements, airborne time domain electromagnetics, dipole-dipole induced polarisation and resistivity and airborne magnetics.
Petrophysical measurements suggest the deposit is associated with a conductive and chargeable signature. This is confirmed by airborne EM and dipole-dipole IP and resistivity data. Magnetic data map the unmineralised magnetic mudstone package overlying the deposit, and show that stratigraphy is deformed.
Key words: down hole, geophysics, induced polarisation, Lake Victoria Goldfields, magnetics, resistivity, sulphides, Tanzania, time domain electromagnetics, Tusker, VTEM.
Authors:
Karen Pittard
Barrick Gold Corporation
kpittard@barrick.com
Barry Bourne
Barrick Gold Corporation
bbourne@barrick.com
Comparison of ground TEM and VTEM responses over kimberlites in the Kalahari of Botswana
One hundred meter square in-loop ground TEM soundings were an effective way to screen aeromagnetic, ground gravity, and Falcon® gravity gradiometer signatures caused by kimberlite intrusions overlain by 40-120 meters of transported Kalahari sedimentary cover in the Kokong kimberlite field of Botswana. Ground TEM’s effectiveness in identifying kimberlite pipes led to the flying of the VTEM aeroTEM system over selected areas at Kokong. Ten kimberlites previously covered by ground TEM surveys were over flown by the VTEM survey. A comparison of the ground TEM and VTEM responses show that the VTEM effectively drill-screened nine out of the ten kimberlite magnetic or gravity signatures, whereas the ground TEM systems effectively screened all 10 kimberlite signatures.
VTEM vs. HeliGEOTEM II Data Comparison
A comparison of survey data from the VTEM and HeliGEOTEM II systems over the McFaud’s Lake area of the Superior Province in Canada is presented. Both VTEM and HeliGEOTEM II flew the same area, 220 line-km with the same line spacing 100 m, and the same north – south line direction.
Geophysical Report on Helicopter-borne ZTEM Tipper Case Study Results - Mt. Milligan test block
January 2009 Mackenzie Region, British Columbia on behalf of GEOSCIENCE BC and TERRANE METALS CORP
ZTEM (Z-Tipper Axis Electromagnetic) surveys were conducted over the Mt-Milligan Test Block, belonging to Terrane Metals Corp. (Vancouver, BC) and situated in the Mackenzie region of central British-Columbia, from October 15TH to 22ND, 2008, on behalf of Geoscience BC. The ZTEM survey comprised airborne Tipper AFMAG (audio frequency electromagnetics) measurements, as well as aeromagnetics using a caesium magnetometer. The survey consisted of twenty five (25) approx. 8.0 km long, EW oriented flight lines, totaling 200 line-km, that were obtained at nominal 250m line spacings over an approximately 6 x 8 km area (Figure 1). The area was chosen because it hosts the Mt Milligan copper-gold porphyry deposit, containing a Measured and Indicated Mineral Resource of 590.8 Mt at 0.193% Cu and 0.352 g/t Au containing 2.52 billion lb copper and 6.70 million oz gold (Terrane Metals Corp. 2008), that is not yet in production, and also because area’s geology is well known and available in the public domain - making it an ideal case-history example.
Geophysical exploration with audio frequency natural magnetic fields
Experience with the AFMAG method has demonstrated that an electromagnetic exploration system using the Earth’s natural audiofrequency magnetic fields as an energy source is capable of mapping subsurface electrical structure in the upper kilometer of the Earth’s crust. We resolved the limitations of this method by adapting the tensor analysis and remote reference noise bias removal techniques from the geomagnetic induction and magnetotelluric methods to computation of the tippers.
After a thorough spectral study of the natural magnetic fields, we designed lightweight magnetic field sensors capable of measuring the magnetic field throughout the year. We also built a digital acquisition and processing system with the ability to provide audiofrequency tipper results in the field.
This new instrumentation was used in a study of the Mariposa, California site previously mapped with AFMAG. This study once again demonstrates the usefulness of natural magnetic field data in mapping an electrically conductive body. Reoccupation of one of the sites in several different seasons proves the high level of repeatability of these data. As well as being repeatable,
the tensor data provide additional information on the geometry of the conductive body. Different electrical conductivity features can be distinguished using a broad band of frequencies.
by V. F. Labson, A. Becker, H. F. Morrison, and U. Conti
Z-TEM (Airborne AFMAG) as applied to Hydrocarbon Prospecting
Geotech’s latest implementation of its patented airborne AFMAG (Audio Frequency Magnetic) technology is the Z-TEM or Z-Axis Tipper Electromagnetic system. Z-TEM was introduced in mid-2007 and over 20,000 line kms have been flown commercially for minerals exploration. In theory, as with some other electromagnetic techniques, it has applications for petroleum prospecting. From numerical modeling of various hydrocarbon plays, Z-TEM has a depth of investigation for resistive targets to several kilometers.
Z-TEM uses the natural or passive fields of the Earth as the source of transmitted energy. These natural fields are planar and due to the manner in which they propagate, are horizontal. Any vertical EM field is caused by conductivity contrasts in the Earth. The Z-TEM system measures the vertical EM field in the air along the survey lines and makes use of a base station to measure the horizontal fields. The assumption is that the horizontal fields are relatively homogenous over the survey area and the base station location. The vertical EM field is remotely referenced to the horizontal EM fields.
Airborne VLF measurement over the Island of Gotland, Sweden
Airborne VLF data are on a routine basis collected by The Geological Survey of Sweden (SGU) as part of its bedrock mapping programme. Measurements are made from two different transmitters at the same time and a response so called tipper is estimated (Pedersen et al, 1994). The collected data are then transformed into apparent resistivity, assuming that the average background resistivity distribution is known (Becken and Pedersen, 2003). The resistivity maps has shown to be very useful for bedrock mapping and detection of faults and fracture zones. The data can also be used for mapping of quaternary deposits like sulphidic clays. Today, the information is effectively used for planning of infrastructures and to an increasing extent by the mineral prospecting companies.
Additionally, information about the resistivity variation at depth can be obtained along selected profiles using 2D-inversion codes. The direct inversion of the real and imaginary parts of the tipper provides more quantitative information about the subsurface resistivity distribution (Persson, 2001). 2D-inversion of airborne VLF-data along flight lines is now routinely carried using a module developed at SGU.
VES and VLF—an application to groundwater exploration
"Although the geophysical methods are being routinely used for exploration of groundwater, at times it becomes a challenge because of various factors such as geometry and depth of the aquifer and the yield of groundwater. Further, in the absence of surface manifestations of structures favorable for groundwater occurrence, instead of depending on one particular geophysical method, an integrated geophysical strategy plays an indispensable role not only in mapping and understanding the nature of aquifers but also ensures a better success rate of exploration.
...
VLF-EM has proved to be successful in identifying deep, water-bearing fractures in bedrock. Resistivity measurements using VLF-R were used extensively in Sweden by Mullern and Erikson in the 1980s to locate the saline water/fresh water boundary on the coast of the Bothnian Sea."
by
N. SUNDARARAJAN and G. NANDAKUMAR, Osmania University, Hyderabad, India
M. NARSIMHA CHARY and K. RAMAM, Central Groundwater Board, Hyderabad, India
Y. SRINIVAS, Manonmaniam Sundaranar University, Tirunelveli, India
Field Tests of Geotech’s Airborne AFMAG EM System
Geotech’s airborne Audio Frequency Magnetics (AFMAG) system was successfully flown in a helicopter towed bird configuration over selected test sites in Sudbury, Canada.
Airborne trials were conducted over Vermillion Lake, the Trillabelle deposit, north range of the Sudbury Basin, and over the Reid-Mahaffy test site. The results over Reid-Mahaffy show that exploration sized targets can be detected while the conductors seen in the Sudbury trials show that larger, and regional conductors are detected as well.
Research and development is underway to implement the system onto Geotech’s Grand Caravan, although the noise characteristics of a fixed-wing aircraft are challenging.
Key words: airborne, electromagnetic, AFMAG
AFMAG - Interpretation
In electromagnetic prospecting it is customary to use scale model experiments as the basis for interpretation of field data. The model data is compiled into a catalog of families of anomaly profiles where each family illustrates the effect of varying a single parameter such as depth, depth extent, dip, thickness, conductivity, etc. An interpretation diagram of quadrature secondary field versus in-phase secondary field is used in some systems to obtain the conductivity-thickness product of a thin semi-infinite sheet conductor.
by S. H. WARD, D. P. O’BRIEN, J. R. PARRY, AKD B. K. McKNIGHT
AFMAG - Airborne and Ground
The existence of natural magnetic fields in the audio and subaudio frequency range has been known for some time. The primary source of energy for these fields is usually considered to be distant and local thunderstorms. Because of this origin, the fields are quasi-random with both amplitudes and directions changing drastically over short periods of time. Hence, use of these fields in geophysical prospecting has been extremely limited.
A new development, AFMAG, however, essentially eliminates the time variance in recording these fields without any sacrifice of the intelligence of their space variance. Since the space variance can be correlated with geologic features, AFMAG provides a new method of exploration with particular application to prospecting for conductive mineral deposits.
by S. H. WARD; Paper read at the 28th Annual International Meeting of the Society at San Antonio October 14, 1958
AFMAG - Aplications and Limitations
The AFMAG method (Ward et al, 1958, Ward 1959, 1960) is an inductive electromagnetic method in which the source is a natural audio frequency magnetic field arising from atmospheric discharges. It was first made generally available in 1958 and since that time has been employed in many parts of the world. However, the literature on the method is still limited (Kellog, 1960; Shaub, 1960 and 1962; Hallof and Sutherland, 1962; Shaw 1962; Jewel and Ward, 1963 ;Karandeev and Grinevich, 1965; Lutsenko and Pushnoi, 1965; Afakoniecki, King and Cretchley, 1965) so that the advantages and limitations of the method are not generally appreciated. The inventors of the method (Mc-Laughlin et al, 1964) were provoked into considering its development by the desire to utilize an electromagnetic method with a depth of exploration considerably in excess of that attainable with conventional methods. The primary application of the method was considered to be the search for massive sulfides. While some success has been met in this application, certain peculiarities of the method have reduced its application in this regard, while other applications, especially fault tracing, have arisen.
It is the purpose of this paper to review the reported limitations and advantages of the method in various applications and to evaluate the reported limitations on the basis of improved knowledge of the characteristics of the inducing fields.
by S. H. WARD, J.O'DONNELL, R. RIVERA, G. H. WARE
A comparison between VTEM and AeroTEM III with examples from data collected in early 2008
A comparison of survey data from the VTEM and AeroTEM III systems over the McFaud’s Lake area (sometimes termed the “Ring of Fire area”) of the Superior Province in Canada is presented. Both VTEM and AeroTEM III flew the same area, 101 line-km with the same line spacing 100 m, and the same east – west line direction. The depth of the known conductors in the area is approximately 100 m (confirmed by drilling). The overburden thickness averages approximately 50 m.
Geotech’s continuing improvements of VTEM as demonstrated at Caber, Québec
Geotech’s continuing airborne tests over the Caber Deposit in Québec demonstrate some of the advances that have been accomplished with the VTEM system. The tests show that it is not all about peak dipole moment, although this appears to be the major yardstick by which airborne EM systems are evaluated.
A comparison of VTEM and AeroTEM data at the Delta Deposit, Raglan Belt, Québec
Geotech’s VTEM data over the Delta Deposit in the Raglan Belt demonstrates the advantages of using a low base frequency and calculated B-Field in the search for massive nickel sulphides and other excellent conductors.
VTEM vs. AeroTEM Data Comparison
VTEM MEGATEM data comparison for 800 metres deep conductors in Athabasca Basin
VTEM detects the Caber deposit
Noranda Inc./Falconbridge Limited accurately drilled from VTEM data
From 40 drills located using the VTEM data, all 40 intersected a conductor; 38 were within 10m of expected depth.