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

Sponsored by Geosoft.

July 21, 2014

HPC's Role in Energy Exploration

How high-performance computing helps companies track down hidden fields of oil and natural gas. The easy oil and gas has already been extracted, and sophisticated discovery and extraction techniques are needed to extend production...

July 20, 2014

ENI signs deal to expand oil, gas exploration in Congo

Italian oil and gas group Eni said it signed a deal to explore for hydrocarbons in the Republic of Congo's coastal basin, expanding its foothold in sub-Saharan Africa's No. 4 oil producer...

July 20, 2014

Cameco: Light At The End Of The Japanese Tunnel?

Declining Japanese bond spreads bode well for the supply/demand dynamics of the uranium market and uranium stocks such as Cameco...

July 20, 2014

The tiger roars as the dragon sleeps

India's coal demand could hit 787 million t this year, according to coal and power minister, Piyush Goyal, with imports required to make up a more-than 200 million shortfall in domestic production...

July 14, 2014

Geosoft previews new Voxel Assisted Layered Earth Modelling technology for imaging base of salt

Geosoft previewed new Voxel Assisted Layered Earth Modelling (VALEM) technology at the EAGE 2014 in Amsterdam. Delivered within Geosoft’s GM-SYS 3D gravity and magnetic modelling environment, VALEM is a cloud-based gravity inversion service that improves imaging of base of salt and sub-salt regions...

June 30, 2014

Digital Tools for Mines Help Maintain a Competitive Edge

Many mining executives are convinced there’s really no alternative: either join the industry’s quickening interest in digital technology or risk losing ground...

June 24, 2014

Hottest lava eruption linked to growth of first continents

A collaborative research team has discovered an important link between the eruption of Earth's hottest lavas, the location of some of the largest ore deposits and the emergence of the first land masses on the planet - the continents - more than 2500 million years ago...

June 16, 2014

Drilling activity near Chibougamau: More than just Monster Lake

Quebec's historical Chibougamau mining district, located within the prolific Abitibi Greenstone Belt, is attracting renewed attention. Against a background of lackluster gold prices and generally low levels of exploration activity worldwide, one of the most recent market-dominating announcements came from the Monster Lake joint venture among IAMGOLD Corp. (50%), TomaGold Corp. (45%), and Quinto Real (5%)...

April 2, 2014

TGDG Mini-Symposium on Mexico Exploration

On April 9, the TGDG is presenting a mini-symposium on Mexico Exploration: Finding the Treasure of the Sierra Madre and more. A portion of this event is being broadcast online free of charge...

Even with the dramatic advances in computation speed, the demands of the petroleum exploration industry to find more oil faster has grown exponentially. Geosoft’s Gerry Connard explains the advantages of using Fast Fourier Transforms (FFT) for gravity and magnetic modelling.

**What research is the approach based on?**

Robert Parker’s seminal paper published in 1972, *The Rapid* *Calculation of Potential Anomalies*, has led to a revolution in using
FFT for gravity and magnetic modeling using ‘gridded’ data. Since
then, a number of additional papers, including some by Parker
himself, have extended and improved Parker’s original work.
Richard Blakely’s 1995 book summarizes most of this development
and provides some additional, related FFT algorithms for FFT-based
gravity and magnetic calculations (such as Blakely’s ‘Earth Filters’).

**What are the advantages of the FFT-based methods?**

Parker describes in the introduction of his 1972 paper that the computation time for a model grid containing N points is proportional to N ln(N). For equivalent space-domain calculations on the same type of model, the computation time is proportional to N2. This difference is not so significant for small models. For example, to calculate the response of a grid with 100 points, the space-domain approach would require approximately 20 times as many calculations as the FFT approach. However, as the number of grid point increase, the ratio of the number of calculations in the space-domain approach compared to the number of calculations in the FFT-based approach increases rapidly. For a grid with 1000 points, the ratio is 144. For a grid with one million points, the ratio is 72,382. So if the FFT-based approach took one minute of compute time, the space-domain approach would take 50 days.

**Are there disadvantages of the Parker method?**

There are pitfalls in the FFT-based calculation approach, including but not limited to the problems of edge effects, periodicity, and the fact that the original Parker algorithm is based on the convergence of a Taylor-series approximation. With two decades of practical experience using the FFT-based approach, most of these problems have been solved or at least minimised. For example the “Earth Filter” algorithm does not need the Taylor-series approximation.

**What are your future predictions?**

The FFT approach continues to evolve. Two papers by Parker in 1995 and 1996 extend his method to compute the results on an uneven surface. This allows more flexible models that more closely resemble a real earth model which are more appropriate in many exploration scenarios. In 2009, a new paper published by Caratori Tontini and others describes how to use 3D FFTs for gravity and magnetic calculations that will likely lead to significant improvements in FFT-based methods. Although raw computing power continues to grow according to Moore’s Law, our computing needs are growing even faster. Explorers today are taking advantage of a wealth of detailed exploration data to build larger and more complex models with grid sizes often exceeding one million points. FFT-based methods will play an important role in tools for the utilization of gravity and magnetic data in large, detailed models.

In spite of the disadvantages of the computational speed of space-domain techniques, they will also continue to be important tools in the interpretation tool kit. Space-domain techniques are inherently more flexible than FFT-based techniques and more easily utilised for inversion schemes. One powerful approach is to use a hybrid technique that combines both FFT-based and space-domain techniques in a modeling tool. Cloud computing (such as using large banks of parallel processors at a remote site) is also likely to be part of our exploration computing future. This approach is already being used by companies like Microsoft and Google to bring amazing computing power to desktops around the world.

*Gerry Connard is the Petroleum Industry Market* *Manager with Geosoft. Connard is a geophysicist* *with over 30 years’ experience in the exploration* *industry. He founded NGA, Inc. and developed* *gravity and magnetic profiling and modelling* *software – GM-SYS and GM-SYS 3D.*