Posts Tagged gravity

Factors of Modern Discoveries

Posted by on Wednesday, 8 April, 2015
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“INTEGRATING NEW TECHNOLOGY WITH HISTORICAL DATA, GEOLOGICAL INTUITION AND A LITTLE LUCK, COMPANIES ARE FINDING SUCCESS WHERE OTHERS HAVE NOT”

E&MJ News

Mineral Exploration Strategies

E&MJ News published an article by Steve Fiscor where some factors of new mineral deposits discoveries have been outlined – these factors include applying modern geophysical technologies. The author provides some examples – airborne EM (VTEM, ZTEM), airborne gravity gradiometer, and airborne radiometric surveys which played key role in discoveries of mineral deposits.

“All of the discoveries were in the zone of other major discoveries, but most of them had been overlooked for one reason or another. The motivation could only be attributed to a geologically motivated hunch. New technology in the form of deep-penetrating airborne surveys allowed ground-based geophysical surveys to target undiscovered anomalies. Combining the new information with what they knew historically, exploration geologists were able to improve the drilling programs to quickly determine a resource.”

The next discoveries made with airborne geophysical technologies:

Albany Ultra-pure Graphite – VTEM survey

The Balboa Discovery at Cobre Panama – ZTEM survey

Kennady North Kimberlite Discoveries – airborne gravity gradiometer survey

PLS High Grade, High Techand Contrarian – airborne radiometric survey

Details..

 


Novel technologies for greenfield exploration – GEOLOGICAL SURVEY OF FINLAND 2015

Posted by on Wednesday, 1 April, 2015

The Geological Survey of Finland (GTK) reports about new mineral exploration and mapping methods which were developed in the project ‘Novel technologies for greenfield exploration’ (NovTecEx) carried out in 2012–2014. The project was a part of the Green Mining Programme funded by Tekes. The research partners of the project were the Geological Survey of Finland and the University of Oulu. The main study areas were in the SavukoskiPelkosenniemi area and in the Lätäseno area in Finnish Lapland.

The methods include a tool for Audiomagnetotelluric (AMT) measurements, GUI developing for 2D MODELLING AND INVERSION SOFTWARE FOR AIRBORNE TIME-DOMAIN EM DATA, and description  THE EQUIVALENT SOURCE METHOD for CALCULATION OF THE DERIVED BOUGUER ANOMALY.

The report pdf


Unlocking Australia’s hidden mineral potential with geophysics

Posted by on Friday, 1 August, 2014

Dr Richard Blewett:

“It is becoming increasingly difficult to discover near-surface mineral resources in Australia. New and innovative products and techniques are being developed as part of the UNCOVER Initiative to help attract mineral exploration investment that has the potential to lead to the discovery of new resources.”

One of the focus regions for the UNCOVER Initiative is the Thomson Orogen: “The Thomson Orogen is a large area that lies to the north and west of the Lachlan Orogen in New South Wales, South Australia, the Northern Territory and Queensland. Much of it is under the cover of younger sedimentary basins, with some up to several kilometres thick, and it is therefore a poorly understood element of Australia’s geology.

The southern Thomson Orogen is true ‘greenfields’ country. Although the mineral potential of the region is largely unknown, the northeastern Thomson Orogen (for example Thalanga, Charters Towers) and the similar-aged Lachlan Orogen to the south are well mineralised (for example Cadia, Northparkes, Lake Cowall Cobar). In order to attract exploration investment into the southern Thomson Orogen, and also to improve the geological understanding of the area, Geoscience Australia, the Geological Survey of Queensland and the Geological Survey of New South Wales have commenced a collaborative project to collect new (and synthesise existing) pre-competitive data.

One of the first steps in this collaboration is to acquire airborne and ground geophysical data including airborne electromagnetics (AEM), gravity and magnetotelluric (MT) data. Regional AEM data has now been collected to map cover thickness and assess the geology and prospectivity of the Southern Thomson Orogen across the New South Wales-Queensland border around Hungerford and Eulo. These data will be interpreted using existing borehole stratigraphic data and a new solid geology compilation of the region developed between Geoscience Australia, the Geological Survey of Queensland and the Geological Survey of New South Wales.

 

The Geotech VTEM FullWaveForm
airborne electromagnetic acquisition
system used in the Southern Thomson
Orogen airborne electromagnetics survey.
Image credit: Geotech Airborne Limited.

 

 

 Geoscience Australia is a leading promoter of AEM surveying for regional mapping of cover thickness, under-cover basement geology and sedimentary basin architecture. Geoscience Australia flew three regional AEM surveys during the 2006-11 Onshore Energy Security Program (OESP): Paterson (Western Australia, 2007-08); Pine Creek-Kombolgie (Northern Territory, 2009); and Frome (South Australia, 2010) [1]. The surveys were primarily designed to provide reliable, fit-for-purpose pre-competitive AEM data for mapping critical features of uranium mineral systems.

Results from these surveys have now produced a new understanding of the architecture of critical mineral system elements and mineral prospectivity for a wide range of commodities of these regions and includes details on the thickness and character of the regolith, sedimentary basins and buried basement terrains. The data have since been found suitable not just for uranium, but for mapping a range of other mineral systems including gold, silver, copper, lead, zinc and potash, as well as for under-cover geological mapping and groundwater resource estimation.

The survey data are now processed using the National Computational Infrastructure (NCI) facility at the Australian National University to produce GIS-ready interpretation products and GOCADTM objects suitable for 3D modelling.

A number of 3D models are being developed to interpret the near-surface under-cover geology of cratons and mobile zones, the unconformity surfaces between these and the overlying sedimentary basins, and the architecture of those basins. These models are constructed primarily from AEM data using stratigraphic borehole control and show how AEM data can be used to map the cross-over area between surface geological mapping, stratigraphic drilling and seismic reflection mapping. These models can be used by minerals explorers to more confidently explore in areas of shallow to moderate sedimentary basin cover by providing more accurate cover thickness and depth to target information. A 3D model of basement-cover relationships and depth of cover will be developed for the southern Thomson Orogen.”


Supervised Neural Network Targeting and Classification Analysis for Mineral Exploration

Posted by on Tuesday, 29 April, 2014

Karl Kwan (Geotech LTD) presented at Canadian Exploration Geophysical Society meeting (8 April 2014) methodology and examples of using Neural Network Targeting and Classification at mineral exploration.

“Geophysical survey contractors routinely offer multi-parameter data to clients. For example, a helicopter-borne survey may acquire Time-domain electromagnetic (TDEM), magnetic gradiometer and even gamma-ray spectrometer data (i.e., VTEMplus, Geotech LTD). Exploration geophysicists can certainly take advantage some of the readily available multi-disciplinary (geology, geophysics and remote sensing) and multi-parameter (potential field, EM, gamma-ray spectrometry, and others) datasets for mineral exploration. However, the integration and interpretation of these datasets can be time-consuming and even challenging, especially for large-scale datasets covering large areas with diverse geological conditions. The Supervised Neural Network (NN) Targeting and Classification technique for mineral exploration described and demonstrated by Reford, Lipton and Ugalde, 2004, “Predictive Ore Deposit Targeting Using Neural Network Analysis” (http://www.pgw.on.ca/downloads.html), can be a useful and promising tool for the analysis of multi-disciplinary and multi-parameter data.

In this presentation, the properties or responses of the two feed-forward multilayer Neural Networks, Levenberg-Marquardt (NN with LM training) and Fast Classification (FCNN), as implemented in the current version by PGW, are studied in detail. The supervised NN simulations are performed on specially constructed synthetic data. Intended as a tutorial and the NNs treated as black boxes, the objectives of the exercise are twofold, to demonstrate the targeting as well as classification capabilities of the Neural Networks, and at the same time to show one of the known limitations and to suggest a way to get around it. The utility of the NN tool is demonstrated again with real cases from the Republic of Niger.”

Geosoft is delighted to host the online portion of the Canadian Exploration Geophysical Society meeting with keynote speaker Karl Kwan, Geotech LTD.

YouTube:  


High Resolution Airborne Geophysical Survey in Tanzania

Posted by on Sunday, 19 January, 2014

Geological Survey of Tanzania (GST) and Ministry of Energy and Minerals of Tanzania have organized the Workshop “Launching High Resolution Airborne Geophysical Data at Julius Nyerera Convention Center (17 January, 2014, Dar es Salaam, Tanzania). Around 150 delegates have attended the workshop from government and private sectors.

tanz03

Geotech Ltd. and Sander Geophysics have finished the airborne magnetic, gravity, electromagnetic (VTEM), radiometric surveys in 31 districts and presented some results of the surveys.

Permanent Secretary (PS) of Ministry of Energy and Minerals Mr.Eliakim Maswi said – the surveys were carried out under the Sustainable Management of Mineral Resources Project, saying the purpose of the survey was to identify potential zones of mineralization. “The main objective of the project is to improve the socioeconomic impacts of mining for Tanzania and Tanzanians and therefore enhance local and foreign investments”, said PS.

GST Chief Executive Officer , Prof. Abdulkarim Mruma said: “geophysical data acquired through the high resolution airborne geophysical surveys allow fast and accurate delineation of mineralised targets and when augmented with geological and geochemical datasets are highly effective in attracting new exploration ventures.” Prof. Mruma noted that the availability of the modern geo-scientific  data will stimulate investments into mineral and other sectors and will improve the effectiveness of exploration programs.

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Photo: A.Prikhodko


Training Course: Gravity and Magnetic Geophysical Methods in Oil Exploration

Posted by on Wednesday, 23 October, 2013
Location:
Calgary, Alberta
Date:
Wednesday, April 9, 2014

8:30 AM – 4:30 PM

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Course Description
This basic one-day course reviews the fundamental geological and physical concepts behind oil exploration using gravity and magnetic methods. It is presented in plain English with minimal math or technical jargon, and it includes hands-on exploration examples and case studies.

Delineation of regional and local fault networks, which gravity and magnetic data enable, is crucial in both conventional and unconventional exploration and production. In frontier regions, these data help to delineate the raised and subsided crustal blocks and depocenters, as well as the distribution of igneous rocks.

The course reviews all stages of gravity and magnetic survey design, as well as data acquisition and geological interpretation. These steps are put in the context of designing and executing overall exploration programs for both conventional and unconventional targets.

A complete set of course materials and lunch is included in this course.

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Who Should Attend
This course is designed for professional and technical personnel who need to understand the basics of gravity and magnetic methods in order to assess their effectiveness in various exploration circumstances. The course is intended for all staff levels including geological, geophysical, administrative and management personnel.
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Course Outline
  • Geological Meaning of Geophysical Anomalies: Anomaly-Lithology Relationships and Relevant Rock Properties
  • Forms of Rock Bodies Encountered in Oil and Mineral Exploration
  • Gravity Exploration Methods
  • Magnetic Exploration Methods
  • Design of Gravity and Magnetic Surveys for Geologic Targets
  • Processing Methods for Gravity and Magnetic Data to Separate and Enhance Desirable Anomalies
  • Data Display and Anomaly Enhancement
  • Exploration Example: Gravity and Magnetic Studies of Alberta Basement Structure
  • Exploration Example: Gravity and Magnetic Studies in a Frontier Basin Offshore British Columbia
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Instructor
Dr. Henry Lyatsky

Henry Lyatsky is a Calgary-based geophysical and geological consultant who has worked across Canada, northern and western U.S., and internationally in oil and mineral exploration.

He was born in St. Petersburg, Russia and moved to Calgary as a teenager. He holds a B.Sc. in geology and geophysics (1985, University of Calgary), an M.Sc. in geophysics (1988, University of Calgary) and a Ph.D. in geology (1992, University of British Columbia). He is the first or sole author of three books (Springer-Verlag) on the regional geology and geophysics of western Canada, two gravity and magnetic atlases of the Alberta Basin (Alberta Geological Survey) and many papers. He is a member of CSEG, MEG and APEGA.

Henry is a past president of the Mineral Exploration Group, a province-wide mining-industry association in Alberta. To avoid the downtown rat-race and congestion, he works from home, enjoys the free space of the Alberta outdoors, and loves nothing better than in-depth history books and good hikes in the mountains.


“the promise of airborne gravity gradiometry”

Posted by on Wednesday, 10 July, 2013

by DAN ZLOTNIKOV on JUNE 21, 2013 EXPERTISE

A versatile tool, airborne gravity gradiometry is changing the face of mineral exploration.

A relative newcomer to the resource exploration world, gravity gradiometry is already having a major impact. Its potential for producing high-quality data has caused many explorers to take notice and wonder how they might best make use of the technology.

See more at:


Practical Inversion for Geoscientists

Posted by on Wednesday, 10 April, 2013

A short course on the why, how, and what of geophysical inversion

Presented by the B.C. Geophysical Society

This is a 2-day workshop consisting of 1 day of lectures (April 24th) followed by a 1 day hands-on workshop (either April 25th or 26th). Delegates can register for just the lectures or both the lectures and workshop.

Registration available until April 15th at: www.bcgsonline.org

Date: April 24th – 26th 2013

Address: SFU Downtown Campus; Vancouver BC

Registration: Registration is now open!  Please fill out attached pdf form (link below) and click submit.  Registration will be complete once payment is made via Paypal link.  Delegates are asked to choose one workshop day among April 25th or 26th.

Day 1 – Introduction to inversion
– Introduction to inversion in exploration

Reasons for doing inversion
Range of inversion options: magnetics, gravity, EM, IP-resistivity, seismic
Fitting geology and data

– Background on inversion

Unconstrained/constrained
Joint and collaborative
The future of inversion

– Before inversion – forward modeling

forward modeling
petrophysics drives the story
complexity vs. adequacy

– Case studies

Potential fields
EM
IP-resistivity

Day 2 – Workshop with Case studies
Multi-disciplinary teams will use geological, geophysical and geochemical data from two deposits (Babine Lake porphyry Cu-Mo, BC and Minto Cu, YK) to develop exploration targets. Teams will present their evaluations of the data to stimulate group discussion.

Inversion results for the available geophysical surveys will be presented to demonstrate state-of-the-art technology and best practices.
Sponsors for the short course are currently being solicited. Please contact Victoria Sterritt at Victoria.Sterritt@teck.com for details. Proceeds go to the KEGS Foundation.
For those interested in more information about the short course, please email info@bcgsonline.org


USGS: Iowa Meteorite Crater Confirmed

Posted by on Wednesday, 13 March, 2013

Recent airborne geophysical surveys near Decorah, Iowa are providing an unprecedented look at a 470- million-year-old meteorite crater concealed beneath bedrock and sediments.

The aerial surveys, a collaboration of the U.S. Geological Survey with the Iowa and Minnesota Geological Surveys, were conducted in the last 60 days to map geologic structures and assess the mineral and water resources of the region.

“Capturing images of an ancient meteorite impact was a huge bonus,” said Dr. Paul Bedrosian, a USGS geophysicist in Denver who is leading the effort to model the recently acquired geophysical data. “These findings highlight the range of applications that these geophysical methods can address.”

In 2008-09, geologists from the Iowa Department of Natural Resources’ (Iowa DNR) Iowa Geological and Water Survey hypothesized what has become known as the Decorah Impact Structure. The scientists examined water well drill-cuttings and recognized a unique shale unit preserved only beneath and near the city of Decorah. The extent of the shale, which was deposited after the impact by an ancient seaway, defines a “nice circular basin” of 5.5 km width, according to Robert McKay, a geologist at the Iowa Geological Survey.

Bevan French, a scientist the Smithsonian’s National Museum of Natural History, subsequently identified shocked quartz – considered strong evidence of an extra-terrestrial impact – in samples of sub-shale breccia from within the crater.

“The recognition of this buried geological structure was possible because of the collaboration of a local geologist, water well drillers, the USGS STATEMAP program, and the support of the Iowa DNR concerning research on fundamental aspects of Iowa geology,” said McKay.

The recent geophysical surveys include an airborne electromagnetic system, which is sensitive to how well rocks conduct electricity, and airborne gravity gradiometry, which measures subtle changes in rock density. The surveys both confirm the earlier work and provide a new view of the Decorah Impact Structure. Models of the electromagnetic data show a crater filled with electrically conductive shale and the underlying breccia, which is rock composed of broken fragments of rock cemented together by a fine-grained matrix.

“The shale is an ideal target and provides the electrical contrast that allows us to clearly image the geometry and internal structure of the crater,” Bedrosian said.

More analysis of the data will provide additional detail. These data show the impact as a nearly circular region distinct from the surrounding area to a depth of several hundred meters.

“These data, when coupled with physical property measurements on drill core samples, will form the basis for modeling efforts to constrain the impact geometry and energy of the meteorite,” said Dr. Andy Kass, a USGS geophysicist working on the effort.

The Iowa and Minnesota airborne geophysical surveys are targeting an igneous intrusion, known as the Northeast Iowa Igneous Intrusive complex, that may be similar to the Duluth layered igneous complex exposed in the Lake Superior region of northern Minnesota. Known copper, nickel, and platinum group metal resources were deposited during the formation of the Duluth complex. Both of these complexes are associated with a large structural feature known as the Midcontinent Rift, which is exposed in the Lake Superior Region but is covered by younger rocks as it extends to the south through Iowa, Nebraska, Kansas, and Missouri.

This geophysical survey is part of a larger USGS effort to evaluate the concealed mineral resource potential of the greater Midcontinent Rift region that formed about 1.1 billion years ago.

U.S. Department of the Interior, U.S. Geological Survey
Office of Communications and Publishing
12201 Sunrise Valley Dr, MS 119
Reston, VA 20192
Heidi  Koontz 1-click interview

Robert McKay


The new gravity books

Posted by on Sunday, 3 March, 2013

Acquisition and Analysis of Terrestrial Gravity Data

  • Leland Timothy Long, Georgia Institute of Technology
  • Ronald Douglas Kaufmann, Spotlight Geophysical Services

Gravity surveys have a huge range of applications, indicating density variations in the subsurface and identifying man-made structures, local changes of rock type or even deep-seated structures at the crust/mantle boundary. This important one-stop book combines an introductory manual of practical procedures with a full explanation of analysis techniques, enabling students, geophysicists, geologists and engineers to understand the methodology, applications and limitations of a gravity survey. Filled with examples from a wide variety of acquisition problems, the book instructs students in avoiding common mistakes and misconceptions. It explores the increasing near-surface geophysical applications being opened up by improvements in instrumentation and provides more advance-level material as a useful introduction to potential theory. This is a key text for graduate students of geophysics and for professionals using gravity surveys, from civil engineers and archaeologists to oil and mineral prospectors and geophysicists seeking to learn more about the Earth’s deep interior.

Author(s)/Editor(s):Thomas R. LaFehr and Misac N. Nabighian

Fundamentals of Gravity Exploration (Geophysical Monograph Series No. 17) covers a full range of gravity-exploration topics, including first principles, field instrumentation and operations, rock densities and density contrasts, data reduction, methods of interpretation, and geologic examples.  The subject matter includes inversion and an appendix on the Fourier transform.  This book will help students to efficiently gain knowledge and appreciation for the method, and it will provide experienced earth scientists with a valuable addition to their exploration libraries, both for reference and understanding of this important method.


Potential fields for basement investigations

Posted by on Wednesday, 30 January, 2013

Often not given its due in oil and gas geophysics, knowledge of basement geology can be critical to exploiting reservoirs including the unconventional.

by GRAHAM CHANDLER on JANUARY 24, 2013

There is an article in the last Earth Explorer issue:

New Approach to Basement Studies for Oil and Gas Explorers



Details about the history of North America’s strongest discrete gravity anomaly

Posted by on Friday, 2 March, 2012

Darnley Bay Resources announces the release, filing on SEDAR and posting on its own website of the 43-101 Technical Report prepared by Stephen Reford, P.Eng., a Qualified Person for the purposes of National Instrument 43-101 and the Company’s Chief Technical Officer. The Report details the history of North America’s strongest discrete gravity anomaly and its exploration by the Company and others over a period of 17 years.

The Anomaly has been favourably compared by the Geological Survey of Canada (the “GSC”) to other prominent gravity anomalies such as those at the prolific mining camps of Noril’sk and Sudbury Basin. The Darnley Bay Anomaly is stronger than any of these comparatives by a wide margin. The GSC discovered the Anomaly in 1969 .The Company has 100% control of its exploration and potential development subject to certain back-in and other rights of the Inuvialuit Regional Corporation on whose land it occurs.

The Report details the work undertaken by the Company to identify 41 gravity, magnetic and electromagnetic targets widely distributed over the 100 km by 80 km extent of the Anomaly, in addition to larger zones of exploration significance. It recommends an exploration and drilling strategy to fully test the base and precious metal potential of the property. In view of the size and potential significance of the Anomaly, the Company has now decided to undertake a thorough search for joint venture partners to assist in the further exploration of this unique but gigantic occurrence.

http://www.darnleybay.com/DBR_Corporate_Presentation_Feb.2012.pdf


Ground gravity survey in the Ring of Fire

Posted by on Tuesday, 28 February, 2012

MacDonald Mines Exploration Ltd. announces results of a ground gravity survey over the Company’s VMS targets on the Butler Property in the Ring of Fire.

The presence of density anomalies associated with and adjacent to the feeder/stockwork sulphides intersected to date on Butler 3 suggests either:

  • An  increased presence of sulphides or
  • A different rock type of greater density or
  • a combination of both

The measurement of gravity is an effective technique for defining geometry, structure, and a proven tool in mapping intrusions in sedimentary and volcanic terrains. The use of ground gravity surveys, have historical success in the search for volcanogenic massive sulphide (VMS) deposits.  The technique has been attributed to the discovery of the Sunridge Gold Embra Derho deposit in early 2007 (Approx 62.5 m/tonnes) in Eritea.

The gravity data collected over the Butler property by the Ontario Geological Survey (“OGS”) and Geological Survey of Canada (“GSC”) survey confirmed the Company’s interpretation of a large mafic – ultramafic package that exists along the eastern portion of the property (Butler 5).  This insight prompted MacDonald to utilize the technique over the highly prospective Butler 3 zone.

Ground Gravity Survey

At Butler 3, three distinct density anomalies coincident with anomalies from other geophysical techniques have been identified by Abitibi Geophysics.    These density anomalies add credence to the interpretation of a large VMS system in the Zone identified as Butler 3.  This system is associated with the stringer materials in diamond drill hole BP11-Cu06 which intersected 167 metres of 0.39% Cu and 1.13% Zn. Results for Butler 4 and Butler 5 are pending and data collection continues as exploration is ongoing.

Butler 3 – Characteristics of a Volcanic Hosted Massive Sulphide Deposit (VMS)

The Butler 3 targets are quantified by the following geological characteristics:

  1. The volume of alteration is exceptional, indicating that a very large volume of hydrothermal fluid discharged in this region.
  2. Butler 3 is contained in a felsic-dominated sequence.
  3. The alteration zone is Cu-enriched, and the Cu content increases stratigraphically upwards towards the mineralized horizon.  Cu tenor is high (greater than 3%).
  4. Zinc mineralization is less than expected – indicating a higher temperature system (Cu-enrichment).

Geophysically the targets have physical properties commonly identified with massive sulphides.  These are:

  1. The Butler 3 targets are Electrically Conductive – indicative of metallic content.
  2. Elevated magnetic susceptibility – signifying structure and potential sulphides.
  3. Dense – suggesting the presence of sulphides.

In summary Butler 3 exhibits a multitude of exploration vectors and proximity indicators of a VMS system.  These include but are not limited to:

  • Geophysical properties – conductive, dense, magnetic.
  • Mineral zonation – stringer mineralization with elevated Cu values have been intersected over significant lengths.
  • Alteration Indicies – the drilling has identified a large alteration system with sodium depletion.  This is an indication of lateral distance from the core of the discharge zone.
  • Mineral chemistry vectors – these are coincident with the geophysical anomalies (conductance and density)
  • Structural controls – the stringer zone is capped by a key marker horizon comprised of a siliceous relatively impermeable, black felsic unit

Deep Penetration and High Resolution IP Resistivity Survey

Insight Geophysics Inc. has been retained to apply their technology over Butler 3, Butler 4, Butler 5 and the Sanderson properties in the Ring of Fire.  This array is currently being utilized in the Ring of Fire by Noront Resources who have commented on the success of the technique to identify zones of nickel sulphide in their recent press release.

Preliminary results of this survey over Butler 3 have confirmed chargeability anomalies coincident with the gravity results.  Chargeability measures the ability of the sub-surface to temporarily maintain an electrical charge – sulphides are typically chargeable.  The survey is expected to be completed in early March, 2012.


A MATLAB program to invert the gravity anomaly

Posted by on Tuesday, 14 February, 2012

3DINVER.M: A MATLAB program to invert the gravity anomaly over a 3-D horizontal density interface by Parker-Oldenburg’s algorithm.

David Gómez Ortiz and Bhrigu N P Agarwal

A MATLAB source code 3DINVER.M is described to compute 3D geometry of a horizontal density interface from gridded gravity anomaly by Parker-Oldenburg iterative method. This procedure is based on a relationship between the Fourier transform of the gravity anomaly and the sum of the Fourier transform of the interface topography.  Given the mean depth of the density interface and the density contrast between the two media, the three-dimensional geometry of the interface is iteratively calculated. The iterative process is terminated when either the RMS error between two successive approximations is lower than a pre-assigned value- used as convergence criterion, or until a pre-assigned maximum number of iterations is reached. A high-cut filter in the frequency domain has been incorporated to enhance the convergence in the iterative process. The algorithm is capable of handling large data sets requiring direct and inverse Fourier transforms effectively. The inversion of a gravity anomaly over Brittany (France) is presented to compute the Moho depth as a practical example.

Download MATLAB code and data file examples


Butler Project – Exploration Overview with Geophysics

Posted by on Tuesday, 14 February, 2012

MacDonald Mines Exploration Ltd. provides an exploration update in the mineral-rich Ring of Fire region of the James Bay Lowlands.

MacDonald began exploring in the Ring of Fire starting in 2003, shortly after the first volcanogenic massive sulphide (VMS) discovery at McFaulds Lake by Spider Resources and KWG Resources. The McFaulds VMS discovery is characterized by very high-grade intersections of VMS mineralization (McF-04-57 intersected 18.8 meters of 8.02% Copper (Cu), Mc-03-18 averaged 4.83% Zinc (Zn) over 25.75 meters) and at 250 meter-plus depths.

Exploration in the James Bay Lowlands is challenging due to the area being almost entirely a wetland. As a result, there is minimal outcrop, helicopter support is required and exploration time lines are longer. Over $18 million dollars has been spent to date on the Butler property and we have discovered six zones of multi-element mineralization with copper-zinc VMS systems, vandiferous titano-magnetite and magmatic nickel sulphide. This was accomplished through comprehensive exploration programs including geophysics, diamond drill holes and geochemistry.

Magnetic map (TF) of the Butler Property showing the location of identified multi-element, mineralized zones.

The lack of outcrop requires that a significant amount of attention and detail has to be paid to the geophysical techniques used in the area. Historically the programs focused on shallow targets; however down-hole surveys, ground magnetic and gravity surveys have indicated that the potential deposit(s) of interest lies deeper in the structural sequence.

A comprehensive suite of geophysical tools has applied at Butler:

Airborne EM, Magnetics & Gravity

  • 2004 VTEM – 2138.9 line kilometers of data were collected
  • 2008 VTEM – 1325.6 line kilometers of data were collected
  • 2010 AeroTEM IV – 261 line kilometers of data were collected
  • 2011 HeliGeoTEM – 261 line kilometers of data were collected
  • OGS/GSC Airborne Gravity Gradiometer

Surface EM

  • Butler 1 – 13 line kilometers of data were collected
  • Butler 3 – 39.3 line kilometers of data were collected
  • Butler 5 – 12.5 line kilometers of data were collected
  • Butler 3, 5, 6, and 7 – 30.17 line kilometers of data were collected

Surface Magnetics

  • 7 Ground Magnetic surveys completed in 2011 covering 81.1 line kilometers

Down hole EM

  • 64 diamond drill holes probed and modeled

The Company is currently enhancing the geophysical model to include the geochemistry data results. Collectively, all of the above geophysical results have consistently shown that an electrically conductive unit with magnetic properties and high density lies below those prospective zones.  These are the characteristics expected from a sulphide rich zone.

Magnetic map (TF) of the Butler 3 area showing the location of the EM target and magnetic susceptibility anomaly

MacDonald’s upcoming drill program will focus on the Butler property: Prior to drilling, the targets will first be refined using a deep penetrating geophysical method, similar to that used to detect the HudBay – Lalor deposit at Snow Lake, Manitoba.


Visualization of geophysical data: Surfer

Posted by on Thursday, 9 February, 2012

“I have been thinking for a while about writing on visualization of geophysical data. I finally got to it, and I am now pleased  to show you a technique I use often.  This tutorial has shaped up into 2 independent posts: in the first post I will show how to implement the technique with Surfer, in the second one with Matlab (you will need access to a license of Surfer 8.08 or later, and Matlab 2007a or later to replicate the work done in the tutorial).

I will illustrate the technique using gravity data since it is the data I developed it for. In an upcoming series of gravity exploration tutorials I will discuss in depth the acquisition, processing, enhancement, and interpretation of gravity data (see [1] and [4]). For now, suffice it to say that gravity prospecting is useful in areas where rocks with different density are laterally in contact, either stratigraphic or tectonic, producing a measurable local variation of the gravitational field. This was the case for the study area (in the Monti Romani of Southern Tuscany) from my thesis in Geology at the University of Rome [2].

In this part of the Apennine belt, a Paleozoic metamorphic basement (density ~2.7 g/cm3) is overlain by a thick sequence of clastic near-shore units of the Triassic-Oligocene Tuscany Nappe (density ~2.3 g/cm3). The Tuscan Nappe is in turn covered by the Cretaceous-Eocene flish units of the Liguride Complex (density ~2.1 g/cm3).

During the deformation of the Apennines, NE verging compressive thrusts caused doubling of the basement. The tectonic setting was later complicated by tensional block faulting with formation of horst-graben structures generally extend along NW-SE and N-S trends which were further disrupted by later and still active NE-SW normal faulting (see [2], and reference therein, for example [3]).”

Read more…

1] If you would like to learn more about gravity prospecting please check these excellent course notes.

[2] Niccoli, M., 2000:  Gravity, magnetic, and geologic exploration in the Monti Romani of Southern Tuscany, unpublished field and research thesis, Department of Earth Science, University of Rome La Sapienza.

[3] Moretti A., Meletti C., Ottria G. (1990) – Studio stratigrafico e strutturale dei Monti Romani (GR-VT) – 1: dal Paleozoico all’Orogenesi Alpidica. Boll. Soc. Geol. It., 109, 557-581. In Italian.

[4] Typically reduction of the raw data is necessary before any interpretation can be attempted. The result of this process of reduction is a Bouguer anomaly map, which is conceptually equivalent to what we would measure if we stripped away everything above sea level, therefore observing the distribution of rock densities below a regular surface. It is standard practice to also detrend the Bouguer anomaly to separate the influence of basin or crustal scale effects, from local effects, as either one or the other is often the target of the survey. The result of this procedure is typically called Residuals anomaly and often shows subtler details that were not apparent due to the regional gradients. Reduction to rsiduals makes it easier to qualitatively separate mass excesses from mass deficits. For a more detailed review of gravity exploration method check agai nthe notes in [1] and refer to this article on the CSEG Recorder and reference therein.


SEG Continuing Education Courses

Posted by on Sunday, 18 December, 2011

Houston, TX 23-26 January 2012

Seismic Data Acquisition
Seismic Data Processing

Seismic Data Interpretation

Potential Fields/Non-Seismic

Near-Surface Geophysics

Exploration & Production Geophysics