Gravity of the Situation: Gravity Surveys and Geological Data Mining
It is one of the considerations of geological data mining, that one of the geophysical inputs often needed is gravity data. These surveys are often ground gravity surveys. It is on the geological data miner's wish list to have gravity surveys as one of the inputs into the data mining process to find minerals. Not all countries have this gravity data, not all countries release this data to the public.
For planetary scale geological data mining for mineral resource, it would be a definite wish list item that Elon Musk, or NASA, or another bright aerospace company, would solve the issue of reducing noise in gravity mapping with respect to satellite based systems. Such satellite-based gravity surveys, would produce planet wide surveys useful for mineral exploration. The challenge of measuring gravity from space for mineral exploration would seem to be something Elon Musk should be very concerned with here on Earth to fulfil his need for nickel for Tesla electric vehicles, and on Mars for future colonization. One would more likely have a successful colony on Mars, if it is put beside mineral resources (...there is also the hyperspectral satellite approach, a topic for another article).
Scientists are measuring gravity from space at present over the earth. The NASA's Gravity Recovery and Climate Experiment (GRACE) project does measure gravity of the earth from above our planet as a satellite(s) based system. GRACE twin satellites measure gravity, but this twin satellite system measures gravity at too coarse and noisy a gravity survey level for the purpose of finding economic minerals via geological data mining. GRACE satellites are tasked to measure large changes in water levels in the earth's crust, and relative changes in ice thicknesses and mass. That is their design purpose.
Curious about what a GRACE scientist would think about the potential use of Elon Musk SpaceX internet communication satellites zipping around our planet as a gravity measuring tool, I emailed a GRACE NASA scientist. SpaceX Internet satellites communicate with lasers amongst each other as a cohort, and thus should be reasonable accurate for measuring changes of distances between satellites and the earth. It should be like a bunch of geodesy instruments talking to each other in a 3D space around the earth, moving at orbital speeds, and by variation of movement measuring gravity in high detail. Or so I speculate.
In an email conversation with Felix W. Landerer, GRACE Follow-On Project Scientist, Jet Propulsion Laboratory, who works on the GRACE, I asked Felix if Elon Musk's SpaceX communication satellites could be repurposed, or side purposed, to also gravity map this planet. Hey... for the record, I am not a geophysics person, just a geological data miner, so this is a discovery process for me as well with respect to finding more sources of geological data.
Felix kindly replied to this query in January of this year:
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Jan 19, 2021, 12:37 PM
Hi James:
Thanks for your message; it’s a good suggestion, and in fact there are people looking at constellations of small-sats to do gravity field estimation as we discuss future measurement systems. It’s an interesting proposition for several reasons (cost, redundancy etc.). One challenge remains that there are very tight requirements on knowing each satellite relative position, orientation etc to do precise (enough) satellite-to-satellite range tracking. Also, the satellite altitude puts constraints on what can be resolved on or below the surface of the Earth. Beating down noise by adding more measurements helps to some degree, a resolution of hundreds of meters is still a (big) stretch though.
Thanks again for your comments, glad to see that people out there are thinking along these lines!
Regards,
Felix
Felix W. Landerer, GRACE Follow-On Project Scientist, Jet Propulsion Laboratory / California Institute of Technology, 3X Sea Level and Ice Group
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Left with Felix's insightful opinion to mull over, it was back to considering gravity ground surveying, using qualified geophysicist(s) gravity surveying on the ground to expand data minable inputs for geological data mining. But it seems to me that gravity surveys from satellites can be done according to spline regressions versus orbital paths, and in all likelyhood in some quiet corner, such surveys are carried out in science.
Ground surveys are a much slower process, as you must "get there on the ground" to do the survey physically, the equipment is heavy, it apparently is one of the most demanding and difficult manufacturing tasks in physics instrument building to build extremely accurate relative gravity spring type gravity meters. The present manufacturing process requirements for the gravity plate assembly is a penultimate level skill in manufacturing process ill suited to mass production, and as such, manufacturing time of these devices appears to be one of the main limiters, followed by finding a supply of trained survey geophysicists.
Once the gravity meter instrument is built, there is the consideration that one needs available qualified geophysicists to carry out these surveys. These ground base gravity survey can take quite a long time to carry out, and are expensive. Not being familiar with gravity point surveys, I looked up a figure from 1995, which ran in price from approximately $50.00 and $170.00 per station (including processing and interpretation), depending on survey conditions and requirements. In 2021 price adjusted terms for inflation, that means surveying for gravity point readings on the ground will vary in price from $86.90 to $295.47 US per reading by station. How does this relate to country level gravity surveys?
Take for instance, Canada, with respect to gravity surveying on the ground. Resurveying some of the Canadian national dense gravity survey data would be an appropriate QA/QC step: there are gravity point data from 61 years ago in the Federal gravity dataset for Canada. Old survey data is generally inaccurate in terms of location as gravity survey data points (...and in sensitivity of the actual reading) and eliminating these gravity survey points from analysis for geological data mining is a cleaning step that is needed during data cleaning before data mining. But this step coarsens the gravity survey grid as one removes gravity points. This coarsening of the grid increases risk of missing deposits and makes it harder for the data mining algorithm to pick up on deposits. So that creates a dilemma, only solved by resurveying older data points.
A strong hint here: Canada should be gravity resurveyed again, using modern gravity instruments, to remove those old gravity points, and replace them with an up to date survey of values.
If all the Federal gravity data for Canada point data was resurveyed for modern accuracy of gravity readings (see below image of all Canadian national gravity point data by station), we would be looking at 44 to 149 million US Dollars to resurvey all Canada to modern standards, ignoring travelling costs and lodging. If resurveying the existing dense gravity national grid only for those older points that are pre 1983, which is generally speaking, more inaccurate by location data, we could reduce this issue of cost of resurveying down to 7.1 to 24.1 million US Dollars in resurvey cost, again ignoring travel and accommodation costs.
All of Canada's National Gravity Readings, GIS image from Eureka Maps Inc.
However, with respect to Canada, and Australia, these commonwealth countries are in good shape with respect to relatively accurate, existing dense grid national ground gravity surveys of geophysical data, compared to, say, the USA, or Europe. The USA, in particular, has a national level gravity surveys far too coarse for input into geological data mining. One of the key needs one can see already for the USA in their upcoming plan to expand exploration for mines for the green economy is the need to densify their national gravity grid for mineral exploration at a national survey level.
To survey the USA at the density of Canada, Canada having an existing density of gravity measurements which is adequate as one of the inputs for geological data mining purposes, one would be looking at a cost of between 43.1 to 145.7 million US dollars, again ignoring travel costs and lodging costs. However, this is a minor cost in comparison to potential discovery of new mines in the hundreds of billions of dollars range for the USA. It is clear this benefit already exists in Canada and Australia at present, as Canada and Australia have carried out high density national gravity surveys at the needed accuracy level, and the value is there to be seen by a skilled geological data miner. The USA doesn't have a dense gravity survey done at a national level, to match up with America aspirations to support American companies first, such as US based mining companies, for wide area explorations that reveal economic resources. A USA based dense gravity grid survey will be needed for exploration to succeed in supplying metals for upcoming infrastructure builds.
Other countries are keeping high definition gravity surveys non-public. As an opinion, this is a pointless exercise of keeping data in a state where no use is leveraged from possession of that data. It is common to see national surveys in other countries being "held useless" and offline from public view for ongoing geological science research, and the business of exploration for minerals. Generally, there may be one or two academic papers by country generated by this nonsense of sitting on data, reduced down to discussion and research between a chosen few. This approach produces no further potential in terms of betterment of national economies. We hope that changes in the future.
As a non-geophysicist who data mines geological data, for me it seems as an outsider looking into the science of geophysicists, that good gravity data boils down to higher accuracy of gravity measurements above deposits, better accuracy of location, elimination of near coastline tidal effects, accuracy of geoid measurement corrections (in processing the data, these corrections are made), reduction of thermal noise effects in gravity instrument reading to reduce noise, regular calibration of the relative gravity meter with an absolute gravity meter to ensure accuracy (see below absolute gravity meter, FG5 Absolute Gravity Meter), and increasing the density of national level surveys. And quite possibly, making cheaper, lighter, smaller, but still accurate gravity meters, using MEMs technology. More on that later.
Ground gravity readings do deliver the accuracy in detecting mineral deposits needed for geological data mining purposes, when measured by relative gravity meters, such as the CG-6 Autograv, calibrated against an absolute gravity meter, or, as is typically done, using closed loop surveys that in a repeated sense deliver the accuracy of local surveys against a closed loop, in similar fashion to how optical surveys for measurement of altitude above sea level are carried out, in repeated fashion, often to deliver over time an accurate estimate of error in survey at a more local level. But, overall, such systems of gravity need to have a network of absolute gravity measures to set surveys to an absolute benchmark.
But many relative gravity meters are limited by instrument price, and weight for other market available relative gravity meters are around 8 kg in weight; a hernia in the 'weight' for a field geophysicist lugging these instruments around in a field.
On average, one should carry at most 13.6 Kg of weight in the woods while hiking. However, REI Co-op recommends only at most 15 pounds weight (6.8 Kg) for hiking. The relative gravity instrument seen on the image below, which is top of the market in field accuracy for relative gravity meters, weighs 5.2 kg. Carrying this meter, that leaves very little weight left over to carry other things in the field comfortably, such as water, food, emergency supplies, and if the geophysicist spots mineralized rocks during his survey, rock samples picked up in the field often weigh kilograms. Otherwise, it is carrying these relative gravity instruments in helicopters, ATVs, or in regular field vehicles if deposits intersect existing roads. That option of using vehicles in many areas for these ground gravity surveys is not always an option, in dense forest or rough ground.
And dropping one of these instruments in the field, when it is outside a protective case, well, let's just say, insurance coverage expense must be tough, as we discuss the price of these instruments next.
Image Courtesy of Scintex, Relative Gravity Meter, CG-6 Autograv
Relative gravity meters are expensive, costing above 110K in US dollars for the CG-6 Autograv, which take several months to build on order. These meters can be rented at much less expense, but it is obvious there will be an upcoming wait in line situation for these instruments as upcoming mine exploration activity is fully triggered in response to demand for more copper metals, and nickel, for electrical vehicles and the expansion, and improvement of electrical grids.
As noted, all these relative gravity meters need to be tied on a regular schedule to absolute gravity meters for calibration, using absolute gravity meters, such as the one below (see below image), to ensure accuracy of readings taken by the relative gravity meters versus a final benchmark network that adjusts the entire mass of readings of local closed loop gravity surveys.. Calibration of instruments is another cost. There is, however, an answer coming with respect to cheaper instruments.
FG5 Absolute Gravity Meter (see left)
Cheaper, lighter, smaller, easier to build relative gravity meters more suitable for mass production, have popped up recently in industrial research as an answer to more rapid surveys with more instruments in the field.
MEMs technology, which is suitable for mass gravity instrument production, can reduce wait times for new relative gravity instruments, reduce price, and potentially reduce instrument packaging size. Micro-electromechanical systems (MEMs) is a process technology used to create tiny integrated devices or systems that combine mechanical and electrical components. They are fabricated using integrated circuit (IC) batch processing techniques and can range in size from a few micrometers to millimeters.
One such patented relative gravity MEMs device by Chinese researchers, has recently been created by a research group, that performance wise, appears to in the range of accuracy useful for exploration for minerals, and potentially may be far more portable, and cheaper to build en masse: a few hundreds of US dollars price tag, rather than an above 110K plus US dollar price tag of the CG-6 Autograv. This MEM device was recently built as a prototype by this Chinese research group.
The central MEM mechanism for this prototype device, is a silicon gravity plate which uses a spring system to measure movement - deflection - of a plate in earth's gravity. The central spring plate moves downward with more gravity, and vice versa for less gravity. The Chinese design appears to be approximately close to the sensitivity of the CG-5 Autograv (a Scintex relative gravity instrument that preceded the new CG-6 Autograv). The Chinese researchers measured the CG-5 as being as low as 2 uGal in sensitivity, whereas the Chinese MEM device at present has a claimed sensitivity of 8 uGals. That is down to almost nothing in terms of mechanical and electronics tweaks needed for the Chinese instrument to have the accuracy of the Scintex GC-5 to 6 series relative gravity instruments, for a fraction of price.
The Chinese designed the gravity "spring" plate to be be of very compact design as a MEMs gravity plate (see below view on Chinese gravity plate design as an illustration, dimensions added). Note: normally this gravity plate is oriented vertically, but for illustration purposes, we are showing it in isometric projection.
A high-sensitivity MEMS gravimeter with a large dynamic range, Shihao Tang, Huafeng Liu, Shitao Yan, Xiaochao Xu, Wenjie Wu, Ji Fan, Jinquan Liu, Chenyuan Hu and Liangcheng, Tu, 2019, PGMF and School of Physics, Huazhong University of Science and Technology, 430074 Wuhan, PR, China, Illustration of Chinese MEMs Gravity Plate by Eureka Maps Inc.
If qualified geophysicists could apply these MEMs devices in ground surveys, with the device designed with a suitable shell that vibrationally and thermally isolates the gravity plate yet is shaped more for the field geophysicist to carry at a weight less than 5 kg, or be lofted on a large survey drone down a 100 km survey line for rapid survey purposes, then the field exploration industry would gain the ability to more rapidly scale to wide area exploration gravity surveys. Devices would be cheaper, there would be more availability of relative gravity instruments, geophysical flights carrying these lighter instruments would be cheaper as now one can put these instruments on large drones rather than plane or helicopter systems. Resolution of surveys would tend to go up.
Cheaper and more available gravity survey devices would better fulfill the upcoming exploration need to find metal mines. More metal mines, especially in a North American or Australia context, would allow upgrade of our aged electrical grids- grids that are often more than a century old and prone to failure. More metal mines for nickel and copper would allow the coming EV market.
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