Sign Convention for gravity anomalies while inverting actual datasets

Potential Fields
1

Hello again,

I have reviewed the sign convention information for the forward gravity operator in SimPEG. I have a few doubts I was hoping someone could solve.

  1. If the sign convention for the forward operator differs, how are we supposed to use the inversion codes for actual data, where the positive density anomalies create gravity highs?

  2. In Dr Astic’s 2021 paper, as well as the PGI tutorial (Joint PGI of Gravity + Magnetic on an Octree mesh using full petrophysical information — SimPEG 0.19.0 documentation), the gravity anomaly is a gravity low, and the inverted density model consists of a negative density contrast, which follows the general convention, but goes against the positive-Z up convention given as an explanation in this topic (Simulation: Gravity Anomaly Data on Tensor Mesh - #2 by thibaut.astic). How should one achieve the results in Dr Astic’s paper and the example in the above link (as a positive density contrast makes sense for the Kimberlite pipe anomaly, given the convention used in SimPEG)?

Please let me know; I appreciate any help that I can get.

Thanks and regards
Akshay

2

You might be overthinking this a little bit.
1)

  • Historically gravity measurements have been associated with a z positive downward orientation, thus positive gravity correspond to positive density.

  • All of SimPEG uses a right handed coordinate system with z positive upwards. This makes positive values of gravity associated with negative density contrasts. (It’s much easier to have the entire package follow one convention than to arbitrarily switch conventions depending on the method).

  • To switch between the historical convention and the SimPEG convention, just negate your data.

  1. I believe Thibaut has done this exact operation, and states in his paper that his convention was gravity positive downwards (done to align with historical precedent). If you look at the examples then all of the plotted data has been negated.
3

Hello Joseph,

Thank you for the quick response; I shall do so immediately. I definitely missed the negation of the data for plotting, which is a blunder on my part. Apologies.

Many thanks
Akshay