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When are gravity anomalies positive and when are they negative?

matematika wrote:When are gravity anomalies positive and when are they negative?

Consider the meaning of a gravity anomaly being positive or negative in the simplest sense. Now think of several different geologic features and determine whether their anomaly would be positive or negative. Make sure to account for corrections - typically, the most common interpretable anomaly is the Bouguer anomaly.

Unome wrote:

matematika wrote:When are gravity anomalies positive and when are they negative?

Consider the meaning of a gravity anomaly being positive or negative in the simplest sense. Now think of several different geologic features and determine whether their anomaly would be positive or negative. Make sure to account for corrections - typically, the most common interpretable anomaly is the Bouguer anomaly.

I know the meaning of a gravity anomaly, but how do I relate that to landforms? I know that negative gravity anomalies relate to thick crusts, but then why aren't mountain ranges negative gravity anomalies?

matematika wrote:

Unome wrote:

matematika wrote:When are gravity anomalies positive and when are they negative?

Consider the meaning of a gravity anomaly being positive or negative in the simplest sense. Now think of several different geologic features and determine whether their anomaly would be positive or negative. Make sure to account for corrections - typically, the most common interpretable anomaly is the Bouguer anomaly.

I know the meaning of a gravity anomaly, but how do I relate that to landforms? I know that negative gravity anomalies relate to thick crusts, but then why aren't mountain ranges negative gravity anomalies?

Gravity anomalies aren't so much affected by the thickness of crust as by the amount of mass (because, y'know, GMm/r^2 type of stuff). Yes, if you just add mass on top of an existing landform, making a thicker area of the same density, you should have more gravity, but that's not what happens with a mountain - isostasy means the mountain also has a deep extent into the mantle of less dense crustal material.

But honestly, I just spent a lot of time answering these questions based off a chart I found relating free air and bouger anomalies over different features, and kind of mindlessly copying, so idk.

Thanks! What is the difference between free-air and Bouguer anomalies? How would you use a chart to generalize the sign of the anomaly at a certain geological feature? Free-air and Bouguer anomalies tend to be exact opposites of each other in the charts I've seen...which one to look at?

matematika wrote:Thanks! What is the difference between free-air and Bouguer anomalies? How would you use a chart to generalize the sign of the anomaly at a certain geological feature? Free-air and Bouguer anomalies tend to be exact opposites of each other in the charts I've seen...which one to look at?

The wikipedia page on gravity anomalies (or maybe on Bouguer anomalies, I can't remember which) gives a good overview of how the corrections work. Generally the Bouguer anomaly is the one you would want to look at.

I've looked there, I have a general idea of Bouguer vs. free-air, but some quick examples please? I can't find them anywhere.

1. examples of negative and positive gravity anomalies;
2. is there any list of scientists? Every single new practice test I take, they're like 10 new ones. I can't seem to find a complete list.

matematika wrote:Thanks! What is the difference between free-air and Bouguer anomalies? How would you use a chart to generalize the sign of the anomaly at a certain geological feature? Free-air and Bouguer anomalies tend to be exact opposites of each other in the charts I've seen...which one to look at?

For the first question, Bouguer correction takes into account the attraction by terrain while the free-air correction does not

matematika wrote:2. is there any list of scientists? Every single new practice test I take, they're like 10 new ones. I can't seem to find a complete list.

Make a list and add to it every time you take a test.