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...

Superior detective work: the promise of airborne gravity gradiometry

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.

Just like the commonplace airborne gravity survey, an airborne gravity gradiometry survey responds to the gravitational pull of large masses on an accelerometer. Here’s how it works: Fly over a particularly dense object, such as a rich underground ore deposit, and you register a spike. Traditional gravimeters measure the force exerted on them from one direction only, usually straight down. If a survey does not fly directly over an anomaly but slightly to one side, the odds it will detect that anomaly decrease sharply. Gravity gradiometers, on the other hand, measure forces from the sides as well, greatly improving the ability to detect objects.

Dan DiFrancesco, Business Development Manager for Gravity at Lockheed Martin, U.S.A., explains that the technology originated with a US military project in the 1970s: a navigation tool for Trident nuclear submarines. When the Cold War ended, the project was declassified and gradiometers became available for commercial use.

There are two gradiometer designs in use today, says DiFrancesco. One is the Full-Tensor Gradiometer (FTG), which is the original instrument, developed by Bell Aerospace and now being made by Lockheed Martin following its acquisition of that company in 1996. The other is the FALCON®, a partial-tensor instrument jointly developed by Lockheed Martin and BHP Billiton which was designed to aid in the latter’s search for diamond-bearing kimberlite pipes – small targets not easily found with a traditional airborne scalar gravity survey.

DiFrancesco notes that FALCON® measures two tensors with high fidelity whereas FTG measures all six with a lower fidelity. Though the two instruments arrive at the solution by different means, they produce very similar results.

Two of the world’s three leading gravity gradiometry providers, Bell Geospace and ARKeX, use the FTG, while Fugro Airborne Surveys holds exclusive rights to fly FALCON® surveys.

In addition to the three commercial providers, a number of new airborne gravity gradiometry solutions are under development, but are not yet commercially available.

Practical trade-offs

The Zeppelin airship used by Bell Geospace for a gravity gradiometry survey commissioned by De Beers. The resulting data allowed De Beers to find anomalies less than 100 metres across.


“In oil and gas exploration, gravity gradiometry has proved valuable for sub-basalt and sub-salt definitions, pinnacle reefs, and dykes."
- Dan DiFrancesco

Chris van Galder, AGG technical advisor for Fugro Airborne Surveys, says BHP was so pleased with the results produced by the FALCON® that it fast-tracked completion of a second instrument. Fugro, which took control of BHP’s gradiometers in 2008, has gone a step further: in 2012 it began offering commercial HeliFALCON® surveys, the helicopter platform allowing it to fly slower, take more measurements per kilometre, and produce higher-resolution data.

Airborne gradiometry data isn’t cheap. Alan Reid, a potential field geophysics expert who has served as client consultant, puts the price of a large airborne gravity gradiometry survey at roughly $200 per line-kilometre (though current market conditions have seen this figure decrease slightly). The high price reflects the high cost of the instrument itself.

“You spend several million dollars to buy one, work out how much you can charge per kilometre, and soon you decide you want to eat up a lot of kilometres,” Reid says.

Flying slower means booking more time on a rare machine (only 11 gradiometers exist, notes DiFrancesco), and that in turn means higher costs per kilometre.

Still, when the budget permits, gravity gradiometry delivers. Bell Geospace CEO and President Scott Hammond recalls a survey De Beers commissioned because it needed extremely high-resolution data to find small kimberlite pipes. Bell used a Zeppelin airship and the resulting data allowed De Beers to find anomalies less than 100 metres across. However, practical constraints intervened: The Zeppelin was caught in a dust devil and both it and the gradiometer suffered significant damage. Therefore, Hammond doesn’t expect to see widespread use of Zeppelins for surveys any time soon.

DiFrancesco believes more robust and resilient designs could prevent such problems. “There are hybrid airships coming on-stream that I think will make a difference.”

Anything and everything

A versatile tool, gravity gradiometry can locate anything with a density gradient.

“In oil and gas exploration, gravity gradiometry has proved valuable for sub-basalt and sub-salt definitions, pinnacle reefs, and dykes,” says DiFrancesco. “It has also been used in mining applications, to find iron oxide-copper-gold type deposits, banded iron, and kimberlites, as well as porphyry copper and placer gold-type activity. There have been a lot of case studies showing the value of this technology.”

The Fugro HeliFALCON system ready for deployment. The HeliFALCON system is credited with the discovery of new kimberlites in the Ekati diamond field.

[Click to enlarge]

Data visualization comparing fixed-wing versus helicopter survey data. Courtesy of Fugro Airborne Surveys.

But the gradiometer’s sensitivity can be a curse as well as a boon. As van Galder notes, “the good thing about a gradiometer is that it can detect anything; the bad thing is it detects everything.” This means geologic “noise” (van Galder points out that real-world rocks don’t have perfect, uniform densities), not to mention the noise produced by the plane’s acceleration.

Reid says the “astonishingly crisp” results produced by airborne gravity gradiometry are all the more impressive given that “the accelerometers are being run at their absolute limit. “You’re putting these accelerometers into something that's being buffeted around. What the pilots are doing is just a shade short of impossible. Flying sensitive accelerometers in a light aircraft is an incredibly difficult thing to do.”

Hammond puts the sensitivity into context:

“We're looking at one part in 10^11th in the force of gravity. It's such a small signal that it's hard to see. When you're trying to do that on an airplane that's flying through all kinds of turbulence, the noise is many orders of magnitude higher than the signal you're looking for. Getting rid of that noise is critical.”

Data processing algorithms are just one part of the puzzle. Bell has been using turbine-retrofitted Second World War DC-3 planes, which Hammond says are large, slow, and able to handle much more turbulence.

Marco Antonio Braga is exploration manager for iron at Vale, which has flown some 100,000 line-km of airborne gravity gradiometry over the past 10 years. He says it’s important to know how factors on the ground affect the quality of data. “We fly surveys at 5 a.m., stop at 8 a.m., then come back at 4 to 5 p.m. and fly again,” he says, adding that if the conditions aren’t right (e.g., strong winds or heat), they won’t fly since they know the data won’t be good enough.

Education and experience

An ARKeX aircraft on survey in Kenya. The Full Tensor Gravity Gradiometry technology utilised by ARKeX  has helped oil and gas exploration companies accelerate their search for oil in East Africa.

Aerial pictures taken from an ARKeX aircraft over Greenland.

Neil Dyer, chief technology officer for ARKeX, says experience and specialization are equally important. ARKeX specializes in hydrocarbons, which enables it to focus on its customers’ end-goals,

“We pay a lot of attention to the design of our ratio between the spacing of the survey lines, the tie lines, and the processes we use to level the data and minimize the filtering that needs to be applied in the post-processing sequences,” says Dyer.

DiFrancesco points to customer education as a crucial component of successful gravity gradiometry use: “I strongly believe that the end-users – the oil and gas and mining companies – need a deep understanding of what the capability provides.”

Dyer agrees. He notes that just a few years ago, ARKeX was rejecting more than half of all inquiries they received after feasibility modelling indicated gravity gradiometry was not suitable in certain cases. ARKeX and its peers have been trying to educate potential customers about what gravity gradiometry can and cannot do, and these efforts are beginning to bear fruit. “There are now repeat customers for these services,” notes DiFrancesco, “and more and more people are using this technology in conjunction with, or in advance of, seismic surveys. In short, there’s more acceptance now.”

Right tool for the job

Still, despite the promise it holds, gravity gradiometry should not be seen as a cure-all. As Dyer points out, “there is perhaps a feeling that gravity gradient will solve all problems. Rather, the technology should be seen as expanding the proportion of situations in which a gravitational measurement is applicable.”

“We're still looking for density contrasts; if there isn't a structure with a density contrast, we're not going to help,” says Dyer.  “That’s where we like to get involved, in identifying that situation before a lot of money is spent.”

Further reading

Technical papers on gravity gradiometry, and other resources, are available on Gradiometry.com.


View all articles »