Dirk The Superconducting Gravimeter arrives at Queens

Posted on December 11, 2018

By Dr. Alexander Braun, Associate Professor, Department of Geological Sciences and Geological Engineering

A new iGrav superconducting gravimeter by GWR Instruments was installed on a bedrock mounted slab in the basement of Miller Hall earlier this year. The iGrav, serial number 41, has been named “Dirk” after Dirk Nowitzki, one of the greatest basketball players of all time, jersey number 41, and a true expert in handling spherical objects.

The instrument is one of about 30 like it worldwide. It measures subtle changes in the Earth's gravity field. Gravity is expressed in m/s2, or Gal, and the Earth's gravity field averages about 981 Gal. Dirk can detect tiny variations to 0.05 microGal. That’s one part in 20 billion.

It works like this: a spherical niobium test mass, 2 cm in diameter, is housed in a super-cooled, liquid helium-filled dewar. Two current loops create a magnetic field that levitates the sphere against the force of gravity. At 4°K or -269°C, there is virtually no electrical resistance in the coils. The current can theoretically run forever, as long as the temperature stays close to absolute zero. Changes in the Earth’s gravity field cause the sphere to move up or down. A feedback coil registers that motion and applies a current to move the sphere back to its original position. The voltage needed to return the sphere to its normal position is proportional to the change in gravity. Voltage is recorded every second and those data are stored for further processing.

The largest gravity change signals are typically caused by Earth tides. Just as with ocean tides, changing gravity forces as the Moon, Sun, and Earth move relative to one another can deform the Earth by centimetres or even decimetres. Dirk is a great tool for studying Earth tides but it really shines when it comes to detecting other, smaller changes in gravity.

Flooding, hydrology, erosion, snowfall, glacial melting, volcanic activity, atmospheric pressure, sea level changes, and extraction of material from the ground are mass transfer processes that can cause tiny changes in gravity. Even the mass of all the people travelling south towards the equator over the holidays has a tiny impact on the Earth’s gravity. Researchers can zero out the effects of Earth tides in Dirk’s data leaving only gravity change values that can be interpreted for mass change processes, like earthquakes or solid Earth deformation.

Imagine, for example, extracting heavy oil from a reservoir in Alberta or pumping groundwater from an aquifer to serve a pulp mill. In both scenarios, operators need to know how much oil or water has been extracted, and from where, within the given reservoir. Is it oil from the heel or toe of the well? Is it ground water or surface water? Knowing where oil has already been depleted allows operators to reduce the costly process of injecting steam. Also, if oil leaks from a reservoir zone into surrounding aquifers, this process could be detected earlier and environmental hazards can be mitigated. Operators traditionally conduct seismic monitoring or drill monitoring wells to answer those kinds of questions but those methods are expensive and return only localized results. Dirk could add economic and environmental benefits by complementing existing monitoring efforts.

The ultimate scenario for Dirk, though, is to work in concert with another iGrav to answer those questions using gravity gradiometry. It’s a method that amplifies the gravity signals even further, allowing for improved interpretations of reservoir development (Elliott and Braun, Pure & Applied Geophysics, 2017). Rather than more expensive and time-consuming methods for determining what’s going on under ground, an array of iGrav devices, like Dirk, might be used to answer those questions more easily, quickly, and efficiently.

For now, a group of GEOE/L439 (Advanced Applied Geophysics) students have been tasked with developing a live stream of Dirk’s data to be displayed on a monitor in the Miller museum so visitors can see Dirk in action.

Dirk’s acquisition was made possible through funding from the Canada Foundation for Innovation, John Evans Leaders Funds and the Ontario Ministry of Research, Innovation and Science granted to Dr. Alexander Braun of the Geophysics and Geodesy Lab. The long standing support and commitment of the people at GWR Instruments, specifically Richard Warburton and Eric Brinton is greatly appreciated. Dirk was installed with the invaluable help of Ph.D. student Callum Walter, who listens to Dirk 24/7 to sense earthquakes as they happen.

students gathered around Dirk
WAIT IS OVER: Former graduate students Judith Elliott (MSc 2016), Katie Irwin (MASc 2017), Kiyavash Parvar (MASc 2015) and geological engineering student Fouad Faraj meet Dirk for the first time.

seismic waveform
EARTHQUAKE DETECTION: Dirk recorded a magnitude 7.3 earthquake in Venezuela on 21 August, 2018. The first waves arrived on a direct path to Kingston and then, about two hours later, seismic surface waves arrive from the opposite direction. The gravity signal of the Port Hardy earthquake is ~14 microGal.

3D graph
A NEW APPROACH: Heavy oil depletion is indicated by the red and purple colors leading to density loss and gravity change measured from the surface using a grid of iGrav stations in a Steam Assisted Gravity Drainage (SAGD) reservoir in the McMurray formation, AB.

students using Dirk
RESEARCH GROUP: Graduate students Daniela Iribe Gonzales and Shaza Kaoud Abdelaziz at the iGrav controls. Professor Alexander Braun is looking forward to many years of gravity records.

Prof. Braun
PI: Professor Alexander Braun is looking forward to many years of gravity records.