The people, science and technology behind discovery

Subscribe to the Magazine

Get new articles sent directly to
your mailbox.


Join the network

Twitter Updates Group Forum

Earth Explorer is an online source of news, expertise and applied knowledge for resource explorers and earth scientists. Sponsored by Geosoft.


News & Views

News Archive

December 7, 2016

Geosoft introduces IP and resistivity inversion in VOXI Earth Modelling

Geosoft has added induced polarization and resistivity data inversion to its VOXI Earth Modelling 3D inversion software service. Geoscientists are now able to create detailed 3D models of conductivity and chargeability from IP and resistivity survey data with VOXI. The resulting models can assist in interpreting and targeting regions for mineral and environmental applications...

December 5, 2016

Mexico's Deepwater Round Exceeds Expectations

Some of the world's biggest oil companies showed up Dec. 5 and agreed to invest on the Mexican side of the Gulf of Mexico, proving that deepwater exploration still has a pulse despite challenging market conditions...

November 30, 2016

Magnetic inversion results for Ngamiland available for download

A regional-scale geophysical inversion of magnetic field data in the Ngamiland region of northwestern Botswana is now available for download from the Botswana Geoscience Portal, a partnership initiative of the Botswana Geoscience Institute, industry sponsors and Geosoft...

November 30, 2016

LIGO Resumes Search for Gravitational Waves

After a series of upgrades, the twin detectors of LIGO, the Laser Interferometer Gravitational-wave Observatory, have turned back on and resumed their search for ripples in the fabric of space and time known as gravitational waves. LIGO transitioned from their engineering test runs to full science observations at 8 a.m. Pacific Standard Time on November 30...

November 9, 2016

International Volcano Scientists Unite

For the first time, the United States will host the international Volcano Observatory Best Practices workshop, previously held only in Italy. The workshop will take place this month in Vancouver, Washington. It is designed specifically for volcano observatories around the world and their staff to exchange ideas and best practices with each other...

October 4, 2016

USGS Assesses Mineral Potential for Sagebrush Habitats in Six Western States

USGS has completed a comprehensive assessment and inventory of potential mineral resources covering approximately 10 million acres of Federal and adjacent lands in Idaho, Montana, Nevada, Oregon, Utah and Wyoming...

October 3, 2016

Uganda Targets Up to U.S.$100 Million for Mineral Exploration

Uganda is well endowed with mineral resources and, like many naturally-gifted African countries, is becoming keen on ensuring that these resources play a transformative role in its long-term structural transformation dream - the Vision 2040...

September 9, 2016

Small-Scale Fishers Get A Big Boost With First-Of-Its Kind Impact Investment Fund

Conservation organization Rare announces the Meloy Fund for Small-Scale Fisheries at Our Ocean Conference. The Global Environment Facility, one of the largest funders of conservation worldwide, will be investing $6 million into the fund...

September 1, 2016

Scientists take to the skies to track West African pollution

Scientists operating research aircraft over West Africa have detected organic materials in the atmosphere over a number of urban areas, contributing to concerns of the rise in pollution across the region...

August 17, 2016

New IGS Xplore prospectivity maps for Botswana

International Geoscience Services have released a series of base metal prospectivity maps for the Ngamiland District of northwestern Botswana using free geodata available on the recently-launched Botswana Geoscience Portal, hosted by Geosoft. The maps identify favorable areas for copper, zinc and lead mineralization using geological, geochemical and geophysical datasets downloaded directly from the portal.

August 11, 2016

NexGen Makes New High Grade Discovery

NexGen Energy reported the discovery of a new high grade zone of mineralization 4.7 km northeast of the Arrow Deposit as part of an on-going summer drilling program on its 100% owned, Rook I property, Athabasca Basin, Saskatchewan...

August 10, 2016

Rampion UXO Disposal to Take Place This Week

E.ON has confirmed that the two unexploded devices, detected along the Rampion offshore cable route will be safely disposed this week following the consultation with the Marine Management Organisation...

August 9, 2016

Diamonds In The Rough: E&Ps Find New Reserves In Mature Basins

The oil industry’s history demonstrates clearly that new plays and prospects have long been found in mature basins that were thought to be well on the way to being squeezed dry. Through the acquisition of new data, developing new concepts and coming up with fresh interpretations, long-producing basins around the world from the North Sea to Malaysia have continued to reveal new riches...

August 8, 2016

Northern Shield Identifies High Quality VTEM Targets at Séquoi

Northern Shield Resources announced the results of the interpretation and modelling of the VTEM survey from the Séquoi Property in the Labrador Trough of Quebec . Séquoi is owned 100% by Northern Shield and is being explored for Noril'sk style Ni-Cu-PGE massive sulphides. After geophysical modelling and interpretation of the VTEM data from Séquoi, six VTEM anomalies of significant interest have been identified...

August 3, 2016

Rio Tinto tailors big data drive to copper

Rio Tinto will put the weight of an exploration big data push and its newly-formed Growth and Innovation group behind its desire to identify a "tier 1" copper asset. Speaking at the annual Diggers & Dealers conference in Kalgoorlie, Growth and Innovation group executive Stephen McIntosh said Australia was "overdue for a tier 1" mineral discovery of any type...

August 1, 2016

Tetra Tech Awarded $200 Million Navy CLEAN Contract

Tetra Tech announced that it has been awarded a $200 million, single-award contract by Naval Facilities Engineering Command (NAVFAC) Atlantic. Through the Comprehensive Long-term Environmental Action Navy (CLEAN) contract, Tetra Tech will provide environmental engineering support services to installations within the NAVFAC Atlantic Area of Responsibility...

May 3, 2016

NATO Science for Peace and Security (SPS), MODUM Partners announce "Young Scientist Summer School on Sea Dumped Chemical Weapons"

This international project cooperates closely with CHEMSEA (Search and Assessment of Chemical Weapons) Project for and sharing and knowledge transfer...

April 12, 2016

Monday mad rush for gold stocks

Renewed optimism about the outlook for gold saw investors pile back into gold stocks, pushing many stock to 52-week highs in heavy volumes...

April 11, 2016

Medgold Resources: Continues to Expand the Boticas Gold Project, Portugal; Proposes $200,000 Private Placement

Medgold Resources is pleased to announce new assay results from contiguous rock-chip sampling from the Limarinho South zone at its Boticas gold project in Portugal, which include a highlight of 6.0m @ 5.7 g/t Au...

April 8, 2016

De-carbonizing our energy sector

Nuclear energy currently provides around 11 percent of the world's electricity. China, the European Union, the United States, India, Russia, South Korea, and other nations’ have major existing fleets...

April 1, 2016

Follow-Up Drilling Results Indicate Wide Gold Zones at Hendricks Gold Discovery

Gascoyne Resources Limited announced that it has received the final assay results from the 10,000 metre aircore exploration drilling programme...

March 26, 2016

The Oil Market Is Finally Hitting Its Breaking Point

After a significant reduction in investments over the past two years, oil companies can no longer overcome the production declines from legacy wells...

March 15, 2016

N-Sea Expands into the French Offshore Wind Industry

Subsea IMR provider, N-Sea, has signed a letter of intent with CERES Recherches & Expertise Sous-Marine and TechSub Industrie Environement, to provide subsea survey, installation and remediation services to the French offshore wind industry...

March 9, 2016

PDAC 2016 Convention Exceeds 22,000 Attendees

Optimism and opportunity abounded at the PDAC 2016 Convention of The Prospectors & Developers Association of Canada in spite of recent industry challenges...

March 3, 2016

6 Alpha Launches EOD Contracting Division

6 Alpha Associates, a specialist risk consultancy practice, with expertise in the assessment and management of unexploded ordnance, has launched a dedicated explosive ordnance disposal division...

Gravity inversion helps solve pluton emplacement debate

by Virginia Heffernan on December 1, 2016 applied

[Click to enlarge]

Figure 1: Geological map of Southwest England. The surface outcrops of plutons associated with the Cornubian batholith have been labelled. Map axis are labelled with British National Grid coordinates. This map was produced using Edina’s geology Digimap service.

[Click to enlarge]

Figure 2: Gravity model which was produced using an intermediate density contrast. The model is colour shaded with the same key as the depth map on Figure 3. British National Grid coordinates are plotted along the model axis.

[Click to enlarge]

Figure 3: (a.) Observed and (b.) calculated Bouguer gravity anomaly grids, and (c.) a map of the depth to the top of the granite for the gravity model shown by Figure 2. The granite depth map is contoured with the absolute misfit between the observed and calculated anomaly grids at a contour interval of 1 mGal. British National Grid coordinates are plotted along the map axis. Dashed black lines have been included on the depth map to outline shallow and deep sections of the gravity models which were interpreted to represent separately intruded sheets of granite.

[Click to enlarge]

Figure 4: (a.) Plot of modelled granite thickness and (b.) a schematic of pluton emplacement based on the geometry of the gravity models. A dashed black line has been included on the plot to mark the maximum granite sheet thickness which is supported by power-law expressions that describe the relationship between the width and thickness of granitic intrusions.

A longstanding debate about whether granitic magmas in England ascended through the crust as diapirs or along networks of dykes to form tabular structures has taken a step closer to resolution.

A gravity inversion performed to model the 3D structure of the St Austell pluton, one of six major plutons associated with the Cornubian batholith of Southwest England (Figure 1), suggests the pluton is comprised of at least two separately intruded sheets of granite.  

“This finding is consistent with the hypothesis that the pluton was emplaced by dykes and contributes to the debate about whether dyking, or the long-held view of diapirism, is the dominant granitic magma ascent mechanism,” says Adam Goddard, who recently graduated with a BSc in geophysics from the University of Durham.

Drawn to gravity work as an undergraduate, Goddard chose to study the St. Austell pluton for his final year thesis because the pluton is associated with a large Bouguer gravity anomaly (approximately 50 mGal) and he was able to tap into a rich body of research that he could integrate with his own findings.

Goddard performed the inversion using data from 2,702 gravity stations from the British Geological Survey (BGS)’s gravity databank and a separate gravity survey designed to supplement the BGS data. First, he reduced the data from both sources to Bouguer gravity anomaly data by applying Drift, Latitude, Free-Air, Bouguer and Terrain corrections using Geosoft Oasis montaj. Then he gridded the data using a minimum curvature algorithm.

Goddard used density contrasts of 110-180 kgm-3 to model a range of possible pluton thicknesses. He produced a model for each density contrast by performing a structural inversion on a low density sheet representing the roof of the pluton (Figure 2; Figure 3). By varying the depth of the base of the model at each density contrast, he found the thickness of granite that produced the most realistic model (i.e. that had small mean and residual misfit errors and an upper surface geometry that fit the St Austell pluton’s outcrop and depth constraints from boreholes.)

“I used Oasis montaj to grid my gravity data and GM-SYS 3D to make my 3D models,” says Goddard, who is currently pursuing a master’s degree at the University of Leeds. “Thanks to Geosoft’s excellent help guides and the clarity of the user interface in Oasis montaj, I mastered the software over a short period of several weeks.”

Using an Oasis montaj research subscription, Goddard modelled pluton thicknesses ranging from 11 to 25 km (Figure 4a), but concluded that intermediate density contrasts (140-150 kgm-3) produced the most accurate results. The resulting 14.5-16.5 km thickness estimate suggests the pluton cannot be underlain by a basal thrust at depths of 11-15 km as earlier theories proposed. The estimate also backs up recent geothermal studies that have shown that the Cornubian plutons must be 16 km thick to satisfy heatflow data.

All the models included a shallow east-west trending section of granite that outcropped at the surface and a broader, northeast-southwest trending sheet at depth. Goddard concluded that these shallow and deep sections of the models could represent two separately intruded sheets of granite, although gravity data would also support a more complex structure including at least three sheets of granite.

Goddard aspires to a career in geophysical exploration when he completes his MSc but is keeping an open mind about working in a different sector of geoscience or pursuing a PhD.


View all articles »