Isostasy

Isostasy is a concept in the Geology invoked to explain why the Earth's structures do not behave in accordance to Gravity, which states that greater mass should have greater attraction. It is expected that there should be a greater gravitational attraction from mountains than from hills, plateaus, and oceans, since mountains are more massive; yet 'gravity' readings do not reflect this. The concept of isostasy suggests that below sea level there is a mass deficit beneath the mountains, and is used to explain the disagreement between theory and data.

Louis Hissink
Louis Hessink is a retired geologist who maintains a science blog called Louis Hissink's Crazy World.

https://www.henrythornton.com/louis-hissink (Archive)

On the topic of isostasy, Louis Hessnik tells us:

Gravity and Isostasy by Louis Hissink

From an article titled Gravity Anomaly Louis Hissnik states:

Davit Pratt
On discrepancies, one writer says:

Geological Society of India
Bouguer Anomalies Over The Continents and Oceans (Archive) in the Journal of the Geological Society of India tells us that the anomalies are greater in the ocean than over the land, which is contrary to gravity theory:

Inverse Mountains
According to the theory of isostasy, there exists inverse anomalous structures beneath the mountain ranges, which mirrors the topography, explaining why the gravity anomalies are negative on the mountains.

https://web.archive.org/web/20130529124307/http://earthsci.org/education/teacher/basicgeol/earthq/earthq.html



Bouguer Anomalies - Alps of Germany
https://www.leibniz-liag.de/en/research/methods/gravimetry-magnetics/bouguer-anomalies.html (Archive)

The above shows that the anomalies are negative in the Alps of Germany.

An Alternative Explanation
One alternative explanation as to why the gravity readings appear to be opposite from expected may be related to the the gravity measuring device being used and the nature of what the Gravimeter device is actually detecting. In the article Gravimetry the gravimeter is described by contemporary sources to be a seismometer which does not detect gravity directly, but is instead detecting the alleged density variations caused by gravity in the subseismic band. It is suggested in the article that rather than gravity-induced density variations, the device is measuring background subseismic activity from within the Earth, which is why the highest gravity anomalies are correlated with the seismic zones. If this were true, it should be a natural expectation that mountains and continental masses would act to dampen the subseismic waves which emanate up from the Earth, producing negative readings in mountainous areas as compared to other areas.