Geophysics: The Tools of the Trade

Exploration activity in Canada last year fell markedly from the 1989 level, which was about 70% or so down from the record 1987 and 1988 levels.

Taxation changes have greatly affected the ability of the junior mining companies to raise money, resulting in a generally depressed level of grassroots activity.

Companies offering ground geophysical surveys have been hit particularly hard. But, rahter than sitting idle, these companies have been busy developing newer techniques as you’ll discover in the following pages…

The opportunity for prospectors and exploration personnel to become familiar with the use of ground geophysical equipment under actual field conditions is rather infrequent. In order to provide such a training opportunity, the Haileybury School of Mines in Haileybury, Ont., organized a 4-day field school in Kirkland Lake in June, 1990. Introductory lectures were presented each morning, and the rest of the day was spent using the various geophysical instruments on a test range about seven kilometers south of Kirkland Lake. More than a dozen instrument and contracting companies supported the field school by demonstrating their equipment and presenting discussions that included the compilation of the day’s survey results.

CORPORATE HIGHLIGHTS

A new company called CGI Controlled Geophysics, based in Mississauga, Ont., was formed in 1989 from the exploration geophysics and data-processing service divisions of A-Cubed of Toronto. The company carries out the planning, execution, data-processing and interpretation of ground and airborne geophysical and remote sensing surveys.

In June, 1990, Mississauga-based Crone Geophysics was sold to its employees and the company renamed Crone Geophysics & Exploration. J.D. Crone remains president. The new company will continue to make ground geophysical instrumentation and to conduct geophysical contract surveys and consulting work.

Dataplotting Services and Geotech have merged their operations at a single location in Markham, Ont. A new company, Geocorp, markets airborne geophysical surveys, geophysical instruments and (HEM systems, magnetometers and ground EM) data-processing services. Geocorp has also opened an office in Thailand.

Global Earth Sciences of Borehamwood in the U.K. completed airborne geophysical and remote sensing projects in Asia, Africa and a number of European locations. Last year also saw an expansion of the company’s range of services, and Global signed an exclusive agreement with Prospex to use that company’s new exploration sensors which operate in the visible spectrum.

Intera Information Technologies Corp. of Calgary completed the acquisition of the assets of Aero Service from Western Atlas in June, 1990. In addition to geophysical surveys to industry and GIS services to governments and petroleum companies. Aero provided remote sensing services and will continue to be based in Houston, Tex.

MPH Consulting of Toronto offers consulting and contracting services for geophysical techniques applicable to surface, borehole and underground exploration. In addition, MPH provides expertise in geological, geochemical and mining engineering. Projects in 1990 varied from single survey contracts to full project management and were completed in various countries.

Questor Surveys of Brantford, Ont., reports that 1990 was an excellent year for the company, which flew surveys in South America, Central America, the U.S. and Canada for base metals, precious metals and hydrocarbons.

Sander Geophysics of Kanata, Ont. has established a division for the evaluation and interpretation of airborne magnetometer and gravity data. This consolidates Sander’s expertise in forward modelling, Werner deconvolution, derivative calculation, and interpretation of these.

In 1990, Terraplus of Richmond Hill, Ont. opened an office in Denver, Colo. to provide geophysical equipment and service to the U.S. market. Terraplus U.S.A. will also market products of GEM Systems, ABEM, Pylon Electronic Development, Geosoft, Exploranium, and BRGM Instruments.

AIRBORNE GEOPHYSICAL SURVEYING

Aerodat of Mississauga has undertaken a number of overseas airborne surveys, including contracts in South America, Southeast Asia, the U.K. and the U.S. Aerodat operations in Canada during the last three quarters of 1990 decreased in line with the general reduction in exploration.

In Sarawak, Aerodata Pty. of Perth, Western Australia, completed the flying of a 45,000-line-km, high resolution magnetic and radiometric survey for the Geological Survey of Malaysia. The survey was flown along north-south lines at a basic line spacing of half a kilometre. Final maps and imagery are being prepared in Perth.

During 1990, Controlled Geophysics provided airborne electro-magnetic (EM) surveys, the data collection phase of which was subcontracted. The data-processing, presentation, and interpretation phases were carried out by Controlled Geophysics. Exploration companies without in-house airborne geophysical expertise have been particularly receptive to this policy. Each time-domain EM compilation included a time-constant processing technique called “Output” to extract and present transient decay information. During 1990, Geosoft personal computer (PC) processing software and the I-Power and Geopak RTI-CAD image analysis packages were acquired to expand Controlled Geophysics’ data compilation and processing tools.

Dighem Surveys & Processing of Mississauga reported a moderate level of domestic and international helicopter geophysical survey activity in 1990. The co-operative sales agreement between Compagnie Generale de Geophysique and Dighem resulted in the awarding of a helicopter EM survey in excess of 8,000 line km in the Republic of Yemen. Survey results will be used to locate base metals, precious metals and ground water.

Dighem reported a notable increase in helicopter-borne geophysical surveys in the Cordillera, especially in British Columbia and the western U.S., which offset a substantial decrease in survey demand in the Precambrian Shield. Survey interest in B.C. is being stimulated by the precious metal discoveries in the Eskay Creek region.

Terraquest of Toronto reports a significant downturn in the total aeromagnetic line kilometres flown by the company in Canada. Compared with the same period in 1989, it has experienced a 60% decrease in surveys for base metals, gold and industrial minerals, and a 75% decrease in kimberlite exploration surveys.

In 1988, Urquhart Dvorak of Toronto completed the compilation of all high-resolution magnetic data for the Abitibi greenstone belt. Processed data are gridded (50-m. grids) on PC-compatible media. All the Input EM anomalies are also available, as well as a full suite of anomalies selected recently by Geodatem Airborne Consultants. The data set can be imaged by the Geopak RTI-CAD software package. As might be expected, the trends of the EM anomalies follow those of the aeromagnetic data quite closely.

Airborne Data

Acquisition, Processing

After a long gestation period, the Global Positioning System (GPS) is finally operating. It has reached the stage where it can be used as the prime navigation technique along with Doppler or Inertial Navigation System (INS) (and with flight path camera backup) in aeromagnetic surveys.

Most of the airborne geophysical contractors are now using GPS in their survey operations. In Canada, this includes Geoterrex, Intera Kenting and Sander Geophysics.

As of November 1990, 14 satellites were operational in orbit with additional GPS satellites being launched about every two months. The full constellation should be in place by 1993.

Glonass, a similar 24-satellite system in three orbital planes, is planned by the U.S.S.R. and should be fully deployed about the same time as GPS. Trimble Navigation and Ashtech, both of Sunnyvale, Calif., are building combined GPS-Glonass receivers.

GPS can provide fixes with an accuracy of a few metres. Unfortunately, the most recent series of satellites launched have a built-in capability of degrading the accuracy by a com
bination of dithering the satellite clock and data manipulation of the ephemeris. This degradation is referred to as “selective availability” and was switched on March 25, 1990, to reduce accuracy to civilian users to no better than about 100 metres. Full accuracy can be restored by employing a second fixed ground receiver to record the dithering so that it can be removed by a differential process. As a result of the Middle East crisis, selective availability was switched off so that full GPS accuracy was restored in early September, 1990.

GPS benefits aeromagnetic surveying in several ways. Surveys can be carried out in any part of the world; and a much more even network of traverse and control lines can be flown because the pilot does not have to read a map continuously. Also, the flight path recovery process can be automated and a minimum of point-picking is required to verify the position of the flight lines.

Dighem’s I-Power VGA imaging workstation sales continued to grow in 1990 and were helped by a new bilateral distribution agreement with Geosoft of Toronto and the release of a new menu-driven user interface for the I-Power VGA imaging system. These advances will also be offered on

the more powerful Imagraph-based

I-Power workstations.

Dighem has developed a dedicated PC-based workstation for use in the field during airborne survey operations. At the end of the survey flight, digital data is loaded into the field computer for purposes of displaying the geophysical profiles for quality control, plotting the electronic navigation, and copying.

I-Power, in co-operation with Scintrex, delivered a complete mapping and imaging system to the Natural Resource Authority of Jordan. The latest PC 386 hardware was supplied along with Calcomp and Fujitsu plotters, tape devices and local area network. This system will be used to reprocess and interpret previously flown airborne magnetic surveys and process new gravity surveys.

Geopak Systems of Toronto has devised a graphic presentation and interpretation system with sophisticated grid file imaging features and drawing capability. This PC-based system is called RTI-CAD, for real time image, computer-aided design. It gives the user the capability to carry out a complete geological interpretation directly on the computer screen.

Geopak’s RTI-CAD system can display large grid matrixes and create a composite image of up to four grids simultaneously. More than 1,000 layers of graphic line and text information can be overlayed. Real-time color and shadow manipulation can be performed. Published file formats and interfaces to related software such as AutoCAD are available.

The main feature of Geopak software is integration. Combining the processing modules with RTI-CAD creates a complete processing and presentation environment. Geopak’s processing software can be divided into four groups: line data processing, random data processing, induced polarization (IP) pseudosection processing and filtering.

MPH Consulting offers aeromagnetic data enhancement services, including depth estimate ranges based upon the Euler deconvolution method. Werner deconvolution, derivative and analytic signal techniques are also available with other standard numerical processing methods to provide comprehensive lithologic and structural interpretations as well as to evaluate the data for specific target models. These processing techniques are also applicable to larger ground magnetometer data sets.

As of Sept. 1, 1990, Paterson, Grant & Watson (PGW) of Toronto has taken over all responsibilities for development, marketing and support of an extensive library of software from Geosoft. The library consists of a full range of programs for imaging, frequency and space domain processing, as well as the plotting of geoscientific data. The software incorporates an memory management system for use with extremely large data sets.

PGW is offering an alternative to traditional tablet methods of digitizing contour maps or point data. Such maps are now scanned into a raster image and digitized on a workstation screen in one of four modes — grid, contour, flight line or random. Real-time verification of the digitizing process significantly reduces errors. A second-pass editing system provides for a very clean final product.

The Compu-Drape method for continuation of magnetic data between arbitrary surfaces, introduced two years ago by PGW, has been considerably improved. It has been used extensively for reducing barometric or loosely draped data sets to a perfect pseudo-drape in order to bring out finer details. It has also been applied to height correcting profile data and draping over the magnetic basement or seismic bedrock surface.

The Ontario Geological Survey’s (OGS’s) project to recompile all of the government aeromagnetic data into a 200-metre master grid has been completed by PGW. A unique approach to levelling survey blocks together was utilized that built on the high quality 812.8-metre grid previously prepared by the Geological Survey of Canada (GSC). Preparation of the 200-metre grid was facilitated by implementation of the minimum curvature gridding algorithm. New dithering and color-scaling methodology, developed jointly by Geosoft, PGW and the OGS greatly enhance the quality of the final 1:1,000,000 scale maps, which consist of shaded color pixel maps of the total magnetic field and color pixel maps of its first vertical derivative.

Picodas Group of Richmond Hill, Ont. has begun to supply its PDAS-1000 data acquisition systems with 386 processors. The company is also manufacturing and marketing a navigational aid instrument, PNAV 2001, intended to improve real time grid flying for geophysical surveys. The unit interfaces to any navigation receiver including GPS, Loran C and Range Range transponder systems.

Scintrex, which markets Picodas instrumentation, reports excellent sales of the PDAS-1000 and MEP series real time software magnetometer compensator and other airborne instruments.

The Robertson Group PLC in the U.K. completed the compilation of a color magnetic map of Yemen by standardizing and integrating 25 surveys of different specifications. A set of 1:250,000 geological maps (34 in total) of the country are to be produced from satellite imagery under the same contract. The project also involved the completion of processing of 110,000 km of radiometric and magnetic data from the former Yemen Arab Republic (North Yemen).

The Robertson Group’s processing centre has completed the conversion of the powerful COGS potential fields software to Unix and enlarged its plotting capacity.

RMS Instruments of Mississauga has incorporated a microcomputer module in its DAS8 and DGR33A data acquisition systems.

The module has improved about 50% on throughput but, more significantly, has a turn-on capability that is six times faster than the earlier model. This enables the system to be ready for data-collecting quickly. In addition, recording on a variety of media is now available via the SCSI (small computer standard interface) module.

TerraSense of Sunnyvale, Calif. has released the first major upgrade of its TerraTools software package for compiling and interpreting potential field and gamma-ray spectrometer data. This software is being used in Australia for mineral and petroleum applications and by the German government for Antarctic research. All the functions to take raw input and edit, correct, calibrate, level, filter, interpolate, display and interpret the data are contained in TerraTools.

TerraSense continues to provide airborne and ground geophysical surveys, data-processing and interpretation. In the international arena, the U.S.S.R. Institute of Terrestrial Magnetism, Ionosphere and Radio Wave Propagation requested that TerraSense evaluate the ability of its magnetic survey aircraft to carry out commercial geophysical exploration in the U.S.S.R.

Tesla-10 Pty. of Perth, Western Australia, is now processing with three MicroVax II computers, each of which has 850-megabyte discs. The entire system is ethernetted with image processing and PCs also on th
e network. A Vax-based dual screen Intergraph land information system has been acquired for resource inventory mapping.

Urquhart Dvorak completed the delivery of a comprehensive radiometrics-processing software package funded by the International Atomic Energy Agency of Vienna, Austria. The package was installed in Teheran, Iran, and is currently used to process older data and produce maps for geologic interpretation. Recent acquisition of a Calcomp 58000 electrostatic color printer enhances the company’s capability to provide color pixel imagery.

Aeromagnetic

Anglo American Corp. of South Africa recognized the need to conduct future mineral exploration by using combined horizontal and vertical airborne magnetic gradiometry. Providing the necessary finance and guarantees, Anglo American signed agreements with Poseidon Geophysics and Aerial Surveys Botswana, both of Gaborone, Botswana, to develop such a gradiometer system.

After evaluating available systems, Poseidon decided to build what must be the world’s first 4-sensor triaxial gradiometer. This system is installed in a Cessna Titan 404 aircraft, belonging to, and suitably modified by, Aerial Surveys. Magnetically, the aircraft is exceptionally clean, and the RMS Instruments AADC compensator was utilized to obtain a very low figure of merit. Scintrex strap-down H-8 cesium sensors are mounted one on each wingtip and with two mounted 2 metres apart vertically on a tail stinger of the aircraft in order to provide the total field, the lateral, vertical and longitudinal gradients of the total field every 0.2 seconds.

Gem Systems of Richmond Hill has developed an optically pumped potassium magnetometer using sensors provided by the Vavilov Optical Institute in Leningrad/St. Petersburg. The GSMP20 potassium magnetometer uses a single narrow spectral line so there is virtually no heading error, although there are 30deg polar and equatorial dead zones. The Larmor frequency ratio is about 7 Hz/nT, i.e. about twice that of cesium, permitting a high sensitivity although the relationship between frequency and total field deviates slightly from being exactly linear. However, the absolute accuracy is within 0.2 nT. The noise envelope of the GSMP20 is 1 pT for one reading per second reducing to 10 pT for 10 readings per second and its range is 10,000 to 100,000 nT.

EG&G Geometrics of Sunnyvale, Calif. has completed development of its G-833 metastable helium magnetometer, based on technology originated at the Ministry of Geology in China. Airborne tests of the single-cell system have yielded RMS noise levels of 0.005 gamma. The sensor uses a swept frequency technique, so the polar dead zone common to self-oscillating cesium vapor magnetometers is eliminated, although there are still 30deg equatorial dead zones. This allows survey flights to be made in any direction without re-orienting the sensor in field inclinations of greater than 50deg.

G-833s have been installed on the Dornier 28 survey aircraft for use in the German Antarctic program and in the Twin Otter aeromagnetic survey aircraft in Canberra, Australia.

Geoterrex of Ottawa, Ont., reports another successful year flying aeromagnetic surveys around the world. Major surveys have been completed in Canada, Argentina, Madagascar, Australia, Malta and Trinidad.

Intera Kenting of Ottawa has replaced the KDSS data acquisition system in its Cessna Titan 404 with a Picodas PDAS-1000 system and will update its other aircraft as they become available. Intera Kenting has purchased the Geopak data processing package to change over from minicomputer processing to PCs. This changeover now enables Intera Kenting to offer electronic flight path recovery and data processing in the field up to the preliminary contour stage. Field data-processing has also been facilitated by the almost universal use of differential GPS by Intera Kenting.

Intera Kenting had a very busy year in 1990, flying aeromagnetic surveys in Ontario, Saskatchewan, Alberta and the Yukon. Overseas, two surveys in Zimbabwe were undertaken, the largest being a 145,000-line-km aeromagnetic survey funded by the Canadian International Development Agency.

Questor Surveys is providing a new mapping service in which the known geology and interpreted aeromagnetic data are integrated to provide geological map coverage. These Geomag maps provide insight into structure, lithology and intrusives, particularly in poorly exposed areas. Their production has been aided by the application of new image-processing methods such as the Questor enhancement technique. This technique enhances low amplitude, high-frequency localized anomalies and suppresses the high amplitude, low-frequency regional anomalies. The resultant imagery is plotted as a shadow relief presentation of aeromagnetic data having no directional bias that prominently displays all of the magnetic information, even subtle sub-gamma lineations on the flanks of major anomalies.

RMS Instruments completed development of its automatic aeromagnetic digital compensator (AADC). The 27-term unit will now compensate up to four sensors and the associated gradient. The AADC functions with all of the current high sensitivity magnetometers. The AADC hardware has been made 25% lighter by the incorporation of the power supply into the main chassis. Remote control of the AADC via an RS232 port is now standard, as well as serial control and communications to the RMS’ GR33A graphic recorder.

In 1990, Sander Geophysics of Kanata, Ont., experienced an increase in the number of aeromagnetic surveys flown overseas and in Canada especially for oil exploration. Improvements in the design of optically pumped magnetometers and in the design of aircraft magnetic compensation systems have certainly contributed to the greater popularity of the aeromagnetic method. However, in addition to better magnetic data, the main improvement appears to be in the use of GPS navigation. Sander carries out surveys using differential GPS combined with INS and has started to offer real-time differential GPS.

By the end of 1990, Scintrex of Concord, Ont. had completed the development and prototype testing of its highly sensitive cesium sensor for airborne, base station and portable magnetic applications. The new design, called the CS-2, uses advanced optical and electronic designs, for reliability and low cost. Production engineering, with rigorous ground and airborne testing, is scheduled for completion early in 1991.

The orientation range of the CS-2 with respect to the optimum direction compared to the magnetic field vector has been increased to +/- 35deg. Within this range, the heading error is no more than +/- 0.1 nT and the instrumental noise is below 0.0001 nT RMS. This orientation specification along with an on-board polarity switch, permits the worldwide use of the CS-2 in a strapped-down (non-orienting) mode with a minimal heading error. This means that survey lines in opposite directions as well as tie lines can be flown, without changing sensor position. The Scintrex CS-2 is also designed to follow steep magnetic gradients at high sampling rates.

Terraquest has reported on the efficacy of vector horizontal gradiometry in diamond exploration. Terraquest measures the horizontal gradients using two wing-tip-mounted Scintrex cesium magnetometers with the compensated data being recorded on a Picodas PDAS-1000 data acquisition system. In a particular instance in west-central Canada where the total field data indicated that the anomalies were situated along the flight lines, the use of vector horizontal gradiometry identified isolated anomalies due to kimberlites as occurring between the flight lines.

This has decreased ground follow-up time for clients and allowed the use of more widely spaced flight lines. In volcanic and sedimentary terrains, horizontal gradiometry has also improved the resolution of geological contacts and structures.

Airborne Electromagnetic (AEM)

Aerodat has completed the development of a 5-frequency helicopter electromagnetic (HEM) system using fixed-frequency HEM technology. High resolution data can be acq
uired at 500 Hz, 4,175 Hz and 33,000 Hz in the horizontal coplanar mode and at 930 Hz and 4,600 Hz in the vertical coaxial mode. The enhanced DCSP-99 analog-digital receiver can process up to six frequencies of HEM data simultaneously. It also incorporates a number of new features: all-new signal processor modules with fully automatic nulling, reduced internal noise and greater bandwidth for improved spheric rejection; an interfrequency crosstalk removal module; and remote operation capability.

A new type of HEM survey technology was used by Aerodat for the first time in April and May 1990. The system was designed primarily for sea ice measurement and was packaged in a bird less than four metres long and weighing about 100 kg. Data can be acquired in the horizontal coplanar or vertical coaxial modes and at rates of up to 50 samples per second for five (or more) odd harmonics of the system’s base frequency. Base frequencies tested so far range from 1.8 kHz to 31.3 kHz.

The data acquisition and initial processing, which includes coherent noise rejection, are performed in the time domain, while further data reduction is carried out in the frequency domain. Processing occurs in a digital receiver inside the bird.

Another novel feature of the Aerodat wideband system is its continuous in-flight precise internal calibration system, which permits it to operate without the usual manual calibration techniques based on Q-coils and ferrite bars. This capability, plus a new drift removal technique, facilitates the real time inversion of the HEM data for ice thickness and sea water conductivity. The system supports both modified image method and singular value decomposition inversion. Extension of the real time processing capability to geological mapping for overburden and other layered structures has already been accomplished.

Spectrem, a wideband digital towed-bird AEM system, has completed its second year of production surveys. The system, owned by Anglo American Corp. Botswana, is an extensive modification by Anglo American of the prototype Prospect I system built by A-Cubed. The Spectrem system was designed with two main objectives, namely to detect massive sulphides at depth in conductive environments and to carry out accurate EM sounding to depths in excess of 300 metres below surface. The results obtained to date confirm that these objectives have been met.

The digital Spectrem system, which is mounted in a DC-3, is capable of operating in either the time or frequency domain. In time domain operation, step response delay times from 0.47 to 6.7 milliseconds are covered. The RMS dipole moment of the transmitter is 120,000 A/m2 and is currently being upgraded to 300,000 A/m2. A high drag bird for fixed transmitter-receiver operation is used that allows accurate EM sounding and results in reduced noise levels.

CSIRO in Sydney and the Perth-based World Geoscience Corp. have been awarded a 2-year grant under the Australian Generic Technology scheme to develop a shallow penetration AEM system for determining ground salinity and for mineral exploration. The research will focus on digital data acquisition systems for rapid optimal signal analysis, electronic noise suppression, and colored information presentation.

Dighem delivered a HEM system with real-time spheric removal to Dowa Mining Co. of Japan last year. The hardware consisted of a Dighem IV EM system with frequencies of 385 Hz, 900 Hz, 3,800 Hz and 28,800 Hz, a Scintrex cesium vapor magnetometer, GPS navigation package, Picodas PDAS-1000 data acquisition system and MP-3 base station magnetometer, field workstation and I-Power imaging workstation. The system will be used in environmental and archeological applications as well as surveys for mineral resources in Japan and elsewhere.

Dighem also flew an EM-magnetic survey in excess of 8,000 line-km for a major mining company in the western U.S. The resistivity mapping capabilities of the Dighem IV system were an important factor in its being used to search for precious metals.

The resistivity mapping approach has proven successful since Dighem’s 1989 helicopter survey in the Getchell Trend of Nevada helped define lithology and structure for gold exploration along the trend.

In co-operation with the Bundesanstalt fur Geowissenschaften und Rohstoffe, Dighem has developed a computer program for displaying resistivity sections in color using airborne data. Applications of the technique were presented to a workshop at the Society of Exploration Geophysicists annual meeting in San Francisco in September, 1990.

Geotech is now offering a digital HEM having single transmitter coil electronics that is capable of generating a wide range of frequencies. Single transmitter and receiver coil pairs in each plane of the system used for all frequencies has reduced weight substantially. Digital signal processing throughout has allowed the introduction of self-calibration, nulling and phasing. This instrumentation is compatible with all existing ancillary systems.

Geoterrex reports that the two new Geotem systems introduced into the Canadian and Australian markets in 1989 have been extremely busy. Airborne EM surveys have now been conducted using the new Geotem systems in Canada, Botswana, Niger, Burkina Faso and Australia. Improvements in sensor technology, data acquisition and data processing were made by Geoterrex during 1990.

World Geoscience’s airborne digital transient electromagnetic (TEM) system, called Questem, is now operating in a Skyvan in Canada and a Trislander in Australia.

The real-time signal processing software achieves noise levels that hitherto have been unattainable from analog systems, enabling Questem to be flown successfully in areas affected by powerline, VLF and spheric noise. Integration of Questem and other remote sensing data is providing valuable mapping and interpretational aids for mineral exploration and water resource programs.

AIRBORNE RADIOMETRIC

Development of the GR-820 airborne gamma-ray spectrometer by Exploranium of Bolton, Ont., was completed in early 1990 and many units are now in service. The system’s ability to provide almost automatic operation with minimum operator involvement has been valuable in many different applications. The automatic gain stabilization on potassium has been considerably enhanced to reject interference from localized uranium and thorium sources which could distort the accumulating spectrum used for gain analysis.

The GR-820 has the ability to display the complete individual crystal or combined pack spectra. The display also has a chart recorder format that shows the basic 4-window data on the screen in an analog presentation. Exploranium has obtained a set of the new transportable concrete test pads which allow very accurate determination of stripping ratios for airborne systems as well as all calibration and sensitivity factors for small portable detectors.

The Exploranium GR-256 spectrometer has now been modified to allow use with up to 8.4 litres of detector for small airborne surveys. The system automatically controls gain drift and provides 4-window data acquisition. This system allows many users to fly reasonable quality airborne radiometric surveys where gamma-ray data are not the prime data being collected.

Remote Sensing

During 1990 Global Earth Sciences flew a number of multi-spectral scanner surveys for environmental monitoring, exploration and research projects. The annual 3-phase program for the U.K. Natural Environment Research Council was completed well ahead of schedule, including the transcription of high-density tapes to computer-compatible tapes prior to enhancement and interpretation.

Three services have been added to the remotely sensed imagery offered by Hunting Technical Services of Hemel Hempstead, U.K. A method of combining exploration data using the weights of evidence technique developed by the GSC has been implemented on the company’s image-processing and GIS equipment. The method allows the measurement of the spatial correlation between known mineral occurrences and exploration data, the comparison
of the strength of these correlations and the production of prospectivity maps based on the weighted combination of the data.

A new information extraction technique based on principal components analysis of multi-spectral imagery is also being offered by Hunting in association with Geological Consultants (Ireland). The method can be applied to both satellite and aircraft imagery and has proven effective at locating and mapping alteration zones in arid and semi-arid terrain. During development of the technique, several important western U.S. gold discoveries were highlighted on historical thematic mapper (TM) and airborne TM imagery acquired before discovery or disturbance of these deposits. A pilot study of northern Chile based on TM imagery has recently been completed.

Intera Kenting recently studied available remote sensing data and techniques for the long-term monitoring of tailings sites. This work was funded by the Atomic Energy Control Board. It was concluded that satellite-based remote-sensing techniques supplemented with airborne surveys could provide a solution for long-term monitoring of uranium tailings. A program based on annual coverage of Landsat Thematic Mapper data was recommended. Since the study, airborne remote sensing techniques have been used successfully on an operational basis for identifying problem areas at mine tailing sites.

GROUND SURVEY TECHNIQUES

Data Processing

Geosoft has improved the CPlot program to allow the quality of color and shaded maps to approach the resolution of photographic work. CPlot has also been upgraded to include color scale bars and support for HP Paint Jet, HP Paint Jet XL, and Tektronics 4697 and 4693-DX printers.

Working together with major instrument manufacturers, Geosoft has also made available data conversion programs for reformatting instrument dumps to Geosoft input format. The instrument makers include Apex Parametrics, BRGM, Crone, Exploranium, Gem Systems, Geonics, Phoenix, Picodas, Scintrex and Zonge. Modelling programs including Geosoft’s Magmod3 and Gramod3 for potential field data inversion, Interpex’s Emix MM Plus and Resix IP2D for horizontal loop EM and IP resistivity modelling. Northwest Geophysical’s GM-Sys for forward modelling of gravity and magnetic profiles have been customized for input and output in Geosoft formats.

Other new developments include the upgrading of Geosoft utilities to perform X and Y cartographic projections, shaded profile maps and volumetric grid calculations. A new program for minimum curvature random gridding that applies tension to more closely fit the surface to the data points has also been developed for both geophysical and geochemical data.

Drill Hole Methods

ABEM AB of Bromma, Sweden has introduced the Wellmac digital well-logging system, designed to measure and store geophysical and geotechnical data from up to seven different probes simultaneously. Each probe has its own alarm for leak detection and microprocessor for data transfer. Probes available include resistivity (SPR, normal, lateral), gamma-ray, density, magnetic susceptibility and temperature. Also included will be fluid resistivity, self-potential (SP), neutron, caliper, flowmeter and hole deviation.

ABEM has developed an ore delineation logger that uses a multi-periumeter probe combination to measure different drillhole perimeters. This is a single-operator system designed for blastholes, but it has also been used in exploration boreholes. The choice of probe perimeters that can be selected includes the same given for the Wellmac.

IFG Corp. of Brampton, Ont. reports the development of a downhole combination orientation-vector magnetometer tool. This tool provides borehole dip to 0.1deg accuracy and azimuth to 1deg in holes that dip up to 80deg from the vertical. The fluxgate sensors provide a 3-component magnetic readout to 0.1 gamma.

Reflex Instruments of Sweden has incorporated a verticality probe option to the Maxibor borehole dip and direction indicator. This allows for the measurement of the deviation of boreholes that are less than 2deg from vertical. With the Maxibor, readings can be taken every five seconds instead of every minute as with its predecessor, the Fotobor. For the wedging of drillholes, the stored data can be transferred directly into a computer for an immediate display. Therefore, more time can be given to the wedging of a given drillhole, rather than to processing the data.

Software that provides tomographic imagery is available from Urtec Instruments of Markham, Ont., for the RWL radio wave borehole logging system. This capability executed on standard PCs provides ray path attentuation perimeters that outline cavities, fault zones and ore zones between drillholes. Major testing and introduction in Canada are planned for this year.

Electromagnetic (EM)

Apex Parametrics of Uxbridge, Ont. has developed of the MaxMin I-10 ground EM system and the MaxMin computer (MMC) during the past year. The MaxMin I-10 is equipped with 10 octavely spaced operating frequencies, ranging from 110 to 56,320 Hz. Three switch selectable sets of 11 coil separations each are built-in, ranging from 12.5 to 400 metres, 10 to 320 metres or 50 to 1,600 ft. Coil configurations include maximum coupled horizontal and vertical coplanar coils as well as two minimum coupled perpendicular coils. The expanded frequency and coil separation ranges provide for an increased number of applications, from mineral and groundwater exploration to applications such as pollution-monitoring involving layered earth soundings.

The newly developed MMC displays and records the measured in-phase and the out-of-phase components and also computes, displays and records apparent ground conductivity values ranging from 0.1 to 3,276 millisiemens per metre. Additionally, standard deviation is displayed and recorded for the measured component values, thus indicating the reading accuracy. The computer comes with coil-control and correction programs for rough terrain surveys, as well as with data transfer and formatting programs, which perform best fit computations for the apparent ground conductivity values from multi-frequency data.

John Betz Ltd. of Etobicoke, Ont. has developed a software algorithm for computing an approximate bedrock topographic picture from MaxMin EM data. A first-order picture of the bedrock topography can be a useful aid both in geological mapping and in choosing conventional and basal till drilling sites.

The Betz algorithm will adjust a set of calculated depths to be in closer agreement with drilling results and reconcile the calculated depths at orthogonal grid line intersections to permit contouring.

The demand for deep penetration Crone pulse EM (PEM) surveys remained steady in 1990 because the mining companies continued to explore for deep base metal deposits in the vicinity of existing mines with diminishing reserves. The demand for borehole and surface PEM surveying is split about evenly. Crone is continuing the joint-venture agreement to offer PEM surveys with Val D’Or Geophysics. Similar agreements have been entered into with Exsics Exploration in Timmins and Scott Geophysics in Vancouver.

Crone, supported by Noranda Exploration, is carrying on with research into the development of a cross-component receiver probe for the downhole PEM system. The additional information obtained with this probe should provide more directional information and improve the resolution of multiple conductors. Also with the support of Noranda, Crone has set up an EM scale model to simulate various surface conductor shapes and orientations, conductivities and depths, as well as multiple conductors.

Gem Systems has incorporated a field strength measurement to the non-oriented, 3-component VLF option on its GSM-19V combined VLF-metreagnetometer system.

Geo Instruments of Ramsgate, Australia, has successfully introduced the Mark 3 version of Sirotem. All existing contractors now have Mark 3 units and some new contractors have started up because of the popularity of this model. In particular, the graphic representation of the apparent resi
stivity versus time curve on the liquid crystal display (LCD) on the new main console greatly speeds up data-checking in the field.

Feedback from users has already resulted in other desirable features being incorporated into Sirotem software.

As the Sirotem console now has three receiver channel inputs, this has given impetus to the development of a 3-component downhole receiver probe.

Geoprobe of Mississauga has been performing contract geophysical surveys for the past three years for deep massive sulphides in northern Ontario and Quebec, using the new portable mini-model of the Maxi-Probe EM system. Test surveys performed over known discoveries such as the Louvicourt deposit of the Aur-Louven joint venture and the 1100 lens at Mobrun of Audrey Resources and Minnova have proven that these types of massive sulphide bodies, situated at depths of 600 metres, can be detected from surface using the Maxi-Probe.

The depth-section, made from Maxi-Probe data, of apparent resistivities on line 64+00W over the Louvicourt deposit shows that the most anomalous low resistivity of this deposit is detected at depths of 518 to 610 metres, even penetrating a conductive medium consisting of disseminated and stringer zones above it. Besides deep massive sulphides, the system is routinely used for detecting tight shear zone type targets and alteration zones in gold exploration.

Geoprobe has also developed a non-grounded 4-channel EM resistivity instrument, called EMR-101, for shallow conductivity and/or resistivity measurements at four depth levels down to a maximum depth of six metres. The instrument consists of four transmitters and four receivers housed on a rigid boom.

Transmitter-receiver combinations measure the ground resistivity so that the depth of an anomalous target may be determined from the measurements. The four channels of EMR-101 permit measurements of vertical gradient resistivity directly because gradient is an important element in the search for small targets.

The EMR-101 is manufactured as either a 4-channel model or as a less expensive one-channel model. The range of conductivity measurement is 0.5 to 5,000 mS with a measurement accuracy of 2%. The data output is compatible with Geosoft software.

Lamontagne Geophysics, now of Kingston, Ont., has continued to develop the UTEM inductive source resistivity (ISR) method. The main advantages of the UTEM ISR method over conventional resistivity techniques is that the source is a horizontally oriented, large loop, inductive source. This transmitter arrangement produces a primary electric field that is largely unaffected by horizontal structure, including overburden.

The ample signal strength enables the survey to proceed rapidly, using a small crew with considerable cost savings over the conventional DC resistivity methods.

Capacitive electrodes for the UTEM ISR technique have been developed and tested over the Windjammer gold prospect near Matheson, Ont. Care was taken to lay the electrodes in the snow in the same manner at each station as the depth of burial and the contact areas were found to affect the signal significantly.

However, the survey was able to proceed at more or less the same production rate as the summer survey. The results of the survey using these non-contacting electrodes compare very well with those obtained in a summer survey using the grounded electrodes.

Geochemical

From June to September, 1990, Scintrex and Rudgeofizika of Leningrad collaborated to test novel geoelectrochemical exploration methods. The program was noteworthy in a number of respects, not the least being that it was financially supported by a consortium of most of the major exploration companies active in Canada.

Of a family of five geoelectrochemical methods developed and widely used in the U.S.S.R., three were selected for the 1990 test program. These methods, named MPF, TMGM and CHIM, are used in the initial stages of exploration to detect and localize the occurrence of ore metals.

All of these Rudgeofizika methods are based on the movement of mobile metallic ions upward from mineral deposits in the earth. While methods based on similar procedures have been attempted in North America and elsewhere, the key to Rudgeofizika’s success lies in its unique extraction technology. All three of these methods have demonstrated their ability to detect gold, lead-zinc, copper-nickel and copper deposits under a wide range of conditions in the U.S.S.R.

The purpose of the Canadian test program was to provide data for a range of deposit types, sizes, geometries, depths and overburden conditions to determine the applicability of the methods in the Canadian Precambrian Shield. Based on the results, Scintrex and Rudgeofizika are moving ahead with the formation of a joint company for the provision of geoelectrochemical services in Canada and elsewhere.

The remaining two methods, to be tested in 1991, offer the possibility of helping determine deposit tonnage and grade with very few drillholes, thus offering great potential savings in diamond drilling costs.

Gravity

Under a contract with Petro-Canada made late in 1989, Scintrex conducted a program to establish gravity (and magnetic) computer databases for the Natural Resource Authority of Jordan (NRA). Existing gravity data were compiled into Bouguer gravity anomaly maps at scales of 1:250,000 and 1:500,000. A gravity database was prepared and installed on computer hardware in Jordan.

Quality control, mapping, analysis and quantitative interpretations of gravity and aeromagnetic data covering nearly all of the country was carried out, as was the training of Jordanian geoscientists.

From the point-of-view of geophysical instrumentation, the project provided an opportunity to use a Scintrex CG-3 Autograv gravity meter to help establish the 1990 Jordanian Gravity Standardization Network (JGSN90). Although these instruments are now in use in more than 23 countries, a full test of the CG-3, alongside the well-known La Coste and Romberg (LCR) gravity meters, had not previously been documented.

The GSC processed the data from measurements made by both types of meters and determined the difference between measured gravity intervals and intervals obtained from the solution to network equations.

The CG-3 had a standard deviation of only 0.018 mGal. Its performance is impressive, given the fact that no special arrangements were made for its protection against shocks during transportation.

Last year, SIAL Geosciences of Montreal completed a semi-regional gravity survey of more than 3,000 stations over an area of Quebec encompassing the Aur-Louvem sulphide discovery. This survey helped define the geology and main structures present in that area. The work was followed by ground and innovative borehole seismic work to better define the geometry of the orebody.

Induced Polarization

The accompanying table lists the IP equipment available from various instrument manufacturers. Androtex of Mississauga has developed a 2-channel 5-frequency IP phase receiver, model FDR-2. Two of its unique features are the incorporation of automatic sensitivity adjustment and self-calibration. The FDR-2 receiver is provided with a 1,200-station memory and the data output is 100% compatible with the Geosoft IP data-processing format.

In addition to this receiver, Androtex has completed the development of a portable time and frequency domain and phase angle IP transmitter for fast affordable surveys in areas where access with larger systems may be prohibitive. The STX-300 offers a wide output voltage range and stabilized output current.

BRGM Instruments of Orleans, France, augmented its IP line by the addition of the VIP 3000 time- and frequency-domain IP transmitter. The transmitter can provide 3,000-V and regulated 5-A outputs that are displayed on an LCD.

An RS232 port as well as direct external synchronization connections allow for complete external control capability either for computerized system use or for simple remote operation. Used with the BRGM Elrec 6 receiver, the VIP 3000 provides a fully automated mult
i-dipole IP prospecting system.

Zonge Engineering and Research Organization of Tucson, Ariz. makes a number of transmitters for time and frequency domain IP, resistivity, complex resistivity, EM and controlled-source audio-frequency magnetotellurics (CSAMT) surveys. The GGT series transmitters will all drive wire loops or grounded dipoles. Zonge offers two multi-channel digital receivers to operate with the foregoing transmitters in the various applications. The more versatile multi-purpose GDP-16 receiver is software-controlled. The TIP-16 receiver is a less sophisticated receiver that may be used for time and frequency domain IP and complex resistivity surveys.

Magnetic

Bartington Instruments of Oxford, England, makes a magnetic susceptibility system with a measurement range of 1-9999 x 106 cgs units. The MS-2 gives digital readings in about one second on a large LCD. The MS-2 has a range of coil sensors that produce an alternating one-oersted magnetic field at the sample. The change in frequency due to the change in inductance of the coil is measured. The MS2B laboratory sensor can accept one-inch cubes and cores and can operate at two frequencies. The MS2D field sensor is used for in situ volume susceptibility measurements in the top 2-metre subsurface.

Gem Systems has introduced an option to the 0.1 nT GSM-19 Overhauser magnetometer-gradiometer series that allows the operator to record magnetic and/or gradient field data every 0.5 seconds as the operator walks along the survey line. The GSM-19 has undergone various software improvements, such as increased cycling time of five or 10 readings per second; decimal station spacing; and 60 Hz or 50 Hz filters. Additionally, the memory capaciy has been standardized to 128 kilobytes.

This latter improvement became necessary to accommodate the volume of data that can be accumulated by the GSM-19 in the “walking” option. A second “hip-chain” option will automatically assign grid co-ordinates to readings measured and stored by the walking option. This provides tighter ground control to the magnetic data collected using the walking mag option. The GSM-19 can also use other magnetometers, such as the Omni Series and GSM-10/18, as diurnal base stations.

Geoscan Research of Bradford, U.K., is manufacturing three microprocessor-controlled fluxgate gradiometers having 0.5-metre separation of the sensors. The FM9 basic version has a 5 nT resolution and a range of +/- 20,000 nT. The FM18 has a 0.5 nT data storage resolution and a 4,000-reading memory. The FM36 has a 0.05 nT data storage resolution and a 16,000 reading memory. Both the FM18 and FM36 keep track of the survey position. The ST1 sample trigger permits rapid sampling of up to eight readings per metre for detailed archeological work.

Geotech has produced two new versions of its proton precession magnetometer. The Model M100S is a 1-nT ground magnetometer that can be converted to a base station. The data for the M100S can be recorded by connecting it to Geotech’s miniature hand-held data acquisition computer.

Geotech has also developed a 0.1-nT proton magnetometer, the M1000 that incorporates auto-tuning and can be used with a wide range of sensors. Versions are available for airborne, portable marine and base station use. The ground memory magnetometer version will interface to portable GPS and VLFEM receivers.

Radar

ABEM distributes the Ramac borehole radar system, which operates over the range 10-80 MHz and has an effective depth of 300 metres. A directional antenna for use in single-hole reflection surveys has been introduced to obtain data such as the dip and strike of fracture planes and their angle of intersection to the hole. The antenna incorporates an electrical rotation technique that enables information to be stored in three independent images. The system may also be used in cross-hole reflection and tomographic surveys.

ABEM has made available a number of software programs, such as Crosshole, Radarplot and Tomocg, to interpret reflection data and enhance the quality of data-processing and presentation of borehole radar and tomography data.

Canpolar of Toronto distributes the Geophysical Survey Systems (GSSI) line of ground-penetrating radar (GPR) systems in Canada, including the direct-print SIR-3, the SIR-8, and the new digital SIR-10. Data can be interpreted directly or fed to a portable computer for further analysis with GSSI’s Radan software.

Canpolar has also developed software to enhance interpretation of GPR data, particularly combined with data from other sensors such as an EM conductivity meter. Canpolar has conducted a number of demonstration GPR surveys over the past year for applications as varied as measurement of coal seams to detection of subsurface contamination.

Sensors and Software of Mississauga introduced an upgraded version of its ground penetrating radar system. The pulse Ekko IV radar is a digital system that includes 12.5 MHz and 25 MHz antennas to augment the 50 MHz, 100 MHz and 200 MHz capabilities of the Mark III system. The Ekko IV system may be used to map bedrock, water table, soil stratigraphy and placer deposits as well as to detect tunnelling hazards.

Sensors and Software has also signed a joint-venture agreement with Seismic Imaging Software of Calgary to market Vista GPR, a version of the Vista seismic software package. With Vista GPR, advanced seismic processing such as deconvolution, filtering and migration techniques can be applied to GPR data.

Ground penetrating radar services are available from Multiview Geoservices of Mississauga, an associated company at the same location as Sensors and Software.

Radioactivity

The Exploranium GR-256 portable spectrometer has a new optional central processing unit (CPU) that controls the recording times using a quartz clock. The start time, sample rate and stop time can be programmed into the unit. The main ap plication is intended to be gamma-ray base station recorders for precise control of ground and airborne surveys. The first use was sea-bottom (2,200 metres) vent monitoring of seismic activity by the measurement of radon-radium emissions on the Juan de Fuca ridge off Vancouver Island.

Geosoft recently completed a user interface so the Exploranium GR-256 data memory can be output into a Geosoft data-processing package for contouring, profiling and data integration.

Resistivity

In 1990, ABEM introduced the microprocessor-controlled Terrameter SAS 300C resistivity system, which retains the advantages of the SAS 300B model plus a serial input-output interface. The signal current is switched from positive to negative in a regular pattern, thus eliminating electrode polarization. The transmitted signal plus SP and ground noise is measured when the eddy currents, the IP effect and the EM cable transients have decayed to low levels. The signal averaging system takes consecutive readings automatically, and provides a continuous calculation and display of the running average until the preset number of cycles has been completed. The Multimac multi-electrode cable system complements the SAS 300C and is based on the Schlumberger array.

Geoscan is introducing a microprocessor-controlled resistance meter in the spring of 1991. It will be available in two versions, basic and advanced, both having built-in data loggers with memory capacities from 4,000 to 32,000 readings. The modular design will allow upgrading from the basic to the advanced version, increase of memory capacity, or addition of more complex functions.

Geoscan’s advanced RM15 is the version required for tomography surveys and deep penetration. The expansion port can be used with all the multi-probe arrays. It is also the version required for addition of other options such as IP and SP measurement. The advanced RM15 provides current ranges of 10 mA, 1 mA and 0.1 mA at output voltages of 40 V or 100 V, giving a maximum resolution of 0.0005 ohm (unstacked). Frequencies may be programmed in the range 0.1 to 150 Hz.

Oyo Corp. of Tokyo, Japan, is making a data acquisition system suitable for tomography, resistivity mapping and 2-
dimensional profiling. It consists of the McOhm-21 main unit, the geoelectric node unit that switches surface electrode arrays, or the geoelectric scanner that switches the electrodes in a borehole array. The McOhm-21 can be programmed to measure three potential as well as injected current waveforms by using three arrays with a maximum of 250 electrodes each. The inversion for simple 2-dimensional profiling for the data measured by 30 electrodes in a pole-pole array is possible using an existing model and the results may be printed out using the internal thermal printer.

Tesla-10 has now added a Geonics EM-31 conductivity-metre to complement its vehicle-borne mobile magnetometer system used to map saline areas and diabase dykes in Western Australia. The dykes form subsurface walls against which the groundwater table may be dammed. The combined system is called Geoliner and consists of three pairs of heavy-duty bicycle wheels on an articulated wooden and aluminum chassis. The geophysical instrumentation is mounted on the chassis and is towed by a vehicle.

Urtec is introducing two microprocessor-controlled DC resistivity systems. Designed in co-operation with Androtex and targeted primarily for shallow surveys, the Aries 1000 and Aries 2000 combine the transmitter and receiver into a single portable package. Features include automatic calculation of apparent resistivity from any electrode array, automatic removal and recording of SP and simultaneous measurement of IP chargeability. Aries applications include ground water assessment, environmental surveying, overburden stratigraphy and archeological studies.

Seismic

ABEM has a software processing package called Vesna for shallow reflection surveys using the ABEM Terraloc Mark III seismograph. The program functions include spectrum analysis, filter design, common depth point sorting, normal move-out corrections, and graphics for the presentation of seismic data.

Compagnie Generale de Geophysique of Massy, France, carried out numerous high-resolution surveys for a wide range of applications including geothermal, groundwater, potash, mining and construction. In-seam seismic surveys done inside operating coal mines continued to be popular in Europe.

Oyo Geospace is manufacturing a 24-channel, signal-enhancement seismograph for use in mining and engineering. The microprocessor-controlled McSeis-160MX has a built-in CRT display, a thermal printer and a 3.5-inch disc drive. Up to four sets can be stored in memory. The McSeis-160MX incorporates a precision function generator to evaluate seismic channel performance and to support extensive diagnostics.

The McSeis-160MX has been used by Sial Geosciences in the Val D’Or, Que., area to define the shear zones associated with gold mineralization. Six hundred per cent coverage was used to permit common depth point stacking of reflections to a depth of 800 metres.

Oyo Corp. has also developed a 24-channel seismograph, the McSeis 16000, that is equipped with 16-bit analog-digital converters providing a maximum 100-microsecond sampling for shallow seismic reflection and refraction data acquisition and data-processing for analysis on site. The system consists of two electronic units — the Model 1118A for controlling the data acquisition and processing and the Model 1231A for displaying the interactive operation software menu, seismic data and interpretation results. There is also a thermal printer and 3.5-inch disc. The DAP-01 software provides automatic functions for refraction surveys. The DAP-02 software is developed for reflection surveys.