’til I found you

You  may have heard from the news about the  discovery of a nearly submerged continental fragment that sank after breaking away from Australia 60-80 million years ago. This vast landmass which is attached to New Zealand has been named Zealandia, and with a total area of approximately 4,920,000 km2 it is currently the world’s largest microcontinent. It was considered because of its distinct geological features which met all the criteria applied to Earth’s seven other continents—elevation above the surrounding area, distinctive geology, a well-defined area and a crust much thicker than that found on the ocean floor.

The team composed of 11 researches discovered Zealandia using upgraded satellite-based elevation and gravity maps. The same technology was used to to get much detailed images of Mars’ hidden interior, just like a doctor uses X-ray to see the inside of a patient. This technique also known as Gravity Gradiometry, is the study and measurement of variations in the acceleration due to gravity. The gravity gradient is the spatial rate of change of gravitational acceleration.

Image result for gravity maps

Mars gravity map showing Tharsis volcanoes and surrounding flexure.  https://www.nasa.gov/feature/goddard/2016/mars-gravity-map/

Gravity gradiometry is used by oil and mineral prospectors to measure the density of the subsurface, effectively the rate of change of rock properties. From this information it is possible to build a picture of subsurface anomalies which can then be used to more accurately target oil, gas and mineral deposits. It is also used to image water column density, when locating submerged objects, or determining water depth.

Gravity measurements are a reflection of the earth’s gravitational attraction, its centripetal force, tidal acceleration due to the sun, moon, and planets, and other applied forces. Gravity gradiometers measure the spatial derivatives of the gravity vector. The most frequently used and intuitive component is the vertical gravity gradient, Gzz, which represents the rate of change of vertical gravity (gz) with height (z). It can be deduced by differencing the value of gravity at two points separated by a small vertical distance, l, and dividing by this distance.

G_{{zz}}={\partial g_{z} \over \partial z}\approx {g_{z}\left(z+{\tfrac  {l}{2}}\right)-g_{z}\left(z-{\tfrac  {l}{2}}\right) \over l}

The two gravity measurements are provided by accelerometers which are matched and aligned to a high level of accuracy.

Gravity gradiometry has predominately been used to image subsurface geology to aid hydrocarbon and mineral exploration. Over 2.5 million line km has now been surveyed using the technique. The surveys highlight gravity anomalies that can be related to geological features such as  Salt diapirs, Fault systems, Reef structures, Kimberlite pipes, etc. Other applications include tunnel and bunker detection and the recent GOCE mission that aims to improve the knowledge of ocean circulation.