Welcome to the History of the Earth, which has now evolved into a general podcast covering all things geological. I’m your host, geologist Dick Gibson.
Today I’m going to talk a bit about one of my specialties, interpretation of gravity data. Specifically, gravity data derived from a satellite. Measurements of the earth’s gravity field are essentially measurements of the attraction of the earth on a spring – the more the spring extends, the stronger the pull of gravity, and the stronger pull of gravity occurs where denser materials are present beneath that spring. We can actually measure those attractions with such precision that we can identify areas where there are varying distributions of rocks of different density – or more correctly, we can identify locations of density contrast, where there is a change from one density to another. A classic example is a salt dome. Salt, the mineral halite, has a density of around 2.15 grams per cubic centimeter, while common rocks like shale and sandstone have densities of anywhere from 2.4 to 2.6 grams per cubic centimeter, within an even larger range. So when a low-density, buoyant salt dome rises up through shales and sandstones, it creates a pretty significant density contrast, and a salt dome often produces a strikingly intense, circular gravity low, representing the low-density salt versus the surrounding denser rocks.
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| Satellite gravity map of western India, from Technical University of Delft. The gravity low discussed in the podcast is circled. |
In the middle 1990s incredibly accurate radar altimeters were developed and deployed on satellites. A radar altimeter is basically a range-finder, an extremely accurate tool for measuring distance. The radar signal goes out and bounces back, and the time it takes for the trip is proportional to the distance the radar beam traveled. So you can visualize a sensitive radar altimeter on a satellite as something that can give incredibly accurate measurements of the height of the land – topography. The satellite-borne radar altimeters had centimeter-scale accuracy.
But it can do more. Over the oceans, the radar altimeter measures the distance from the satellite to the surface of the ocean. That’s cool, but so what? Ocean surfaces are really very irregular, with waves, currents, and so on to make any measurements at the level of centimeters irrelevant, right? Right. But if you make the measurements dozens, hundreds, thousands of times, you effectively average out things like waves and currents. You get an average measurement of the height of the surface of the ocean. OK, really it’s the distance from the satellite – whose elevation is precisely known – to the ocean surface, but it’s OK to think of that as the sea’s height.
Again, so what? Well, the average height of the sea surface on a perfectly uniform sphere, the earth, would be a uniform surface, and it actually has a name, the geoid. But the earth is anything but uniform. And in fact we can use the satellite radar altimeter measurements to make maps that are essentially representations of the attraction of gravity – and the accuracy is high enough that we can actually see geological features.
Imagine the sharp slope on the sub-sea edge of a continent – the position where the water gets abruptly deeper. This happens around all the continents. The dramatic contrast in density between water and any rock, any rock at all, it huge, from 1.0 to more than 2 grams per cubic centimeter, so the under-water topography, called bathymetry, is by far the strongest component of the gravity maps that are made from satellite radar measurements. But where the water bottom is relatively flat, the variations in the radar measurements, which translate to gravity values, really do represent geology.
Yes, these data are lower quality than the gravity data that come from gravity meters sitting out there on the land surface. But they are essentially free – all this comes from data acquired from satellites paid for by tax money, so they are in the public domain. And they are often much better data sets than anything that might have been acquired from a ship or aircraft, just because there are not many such data sets.
These satellite-derived gravity maps are really useful for strategic planning for oil companies. Most of the consulting work I did from 1997 to 2002 was interpretation of such data. I did projects that covered all the coastal areas of Africa and India, the east coast of South America, the Maritime Provinces of Canada, the Gulf of Mexico, the South China Sea, and all the waters offshore Indonesia. The information is pretty amazing, if you can figure out how to read the data, and that was my job.
Some of it is pretty unsurprising. For example, off the coast of Bangladesh, in the Bay of Bengal arm of the Indian Ocean, there’s a really wide, flat continental shelf. The Ganges River has carved a submarine channel across that submarine shelf, and no surprise – the channel, which in terms of density is a narrow canyon of water surrounded by rock and sediment that are much denser, shows up in the satellite gravity map as a long, sinuous gravity low. Offshore southwest Africa, there are features in the gravity map that represent huge igneous intrusives that might be wonderful places to explore for minerals, if they were not 150 kilometers offshore and under 200 meters of water. But figuring out where they are located can guide our understanding of the structural geology and tectonics that may help with exploration onshore.
On the west coast of India there is a nearly circular gravity low. You should think of that as low-density rock, but compared to what? It turns out it is really a block of granitic crust, rifted away from the Indian Subcontinent, but it shows up as a low because even at a density of about 2.7 grams per cc, it’s really low compared to the oceanic crust, basalt at maybe 3.3 grams per cc, that surrounds it. This block is actually a high-standing bit of continent that tried to rift away from India – but never really managed to separate. And guess what – today, the largest oil field in India, the Bombay Field, sits right on top of that gravity low. So knowing that, we can look for similar, perhaps less obvious places where there might be additional oil or gas fields. That was the nature of the work I did in the late 1990s for oil companies interested in evaluating the strategic potential of offshore India. It wasn’t really a question of “where do we drill,” – it was more a question of “should we be interested in this region or not.”
Because the satellite data are essentially free, and a geological interpreter like me is relatively cheap in oil company terms, there was a lot of analysis of these data sets back about 15 years ago. The data are still useful and oil companies routinely use them as they plan their exploration programs.
Thanks for listening. I appreciate your interest and support.
—Richard I. Gibson
