<p>Free-air gravity anomaly measurements from <a href="https://www.nasa.gov/mission_pages/icebridge/instruments/index.html" rel="nofollow noreferrer">Operation Icebridge</a> aircraft are described in <a href="https://www.sciencedirect.com/science/article/pii/S0012821X20305914?via%3Dihub" rel="nofollow noreferrer">A fault-bounded palaeo-lake basin preserved beneath the Greenland Ice Sheet</a> (open access).</p>
<p><strong>Question:</strong> These appear to be at least smooth to 1 mGal if not less. Standard gravity is nearly one million <s>Gal</s> mGal, so these are part-per-million type acceleration measurements made inside an airplane. How does one do this? <em>How are mGal-smooth free-air gravity anomaly measurements made exactly on a presumably bumpy airplane ride?</em></p>
<p>A heck of a lot of averaging or damping and GPS and/or lidar/radar reconstruction of average height?</p>
<hr />
<blockquote>
<p>Fig. 7. Gravity and magnetic anomalies... (b) Gravity anomaly modelling. The open circles mark free-air gravity anomaly measurements along the profile (<strong>the gap at ∼50 km reflects a turn in the flight line, where gravity data are unavailable</strong>), and the coloured lines denote the calculated gravity effect associated with the interfaces in our crustal model (panel c). The gravity effect of the topography contains a signal from both the ice surface and bed interfaces.</p>
</blockquote>
<p><a href="https://i.sstatic.net/SlCtk.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/SlCtk.png" alt="Fig. 7. Gravity and magnetic anomalies... from A fault-bounded palaeo-lake basin preserved beneath the Greenland Ice Sheet" /></a></p>
<p>Popular press linking to this paper:</p>
<ul>
<li><a href="https://www.sciencealert.com/ancient-lake-discovered-under-greenland-may-be-millions-of-years-old-scientists-say" rel="nofollow noreferrer">Ancient Lake Discovered Under Greenland May Be Millions of Years Old, Scientists Say</a></li>
<li><a href="https://www.livescience.com/ancient-lake-under-greenland-ice.html" rel="nofollow noreferrer">Primeval Greenland lake found buried beneath a mile-thick slab of ice</a></li>
</ul>

CC BY-SA 4.0
2020-11-11T23:17:14.773Z
There may be additional/different tags that apply.
uhoh
CC BY-SA 4.0
2020-11-12T00:26:37.293Z
Query: are your mathematical factors correct? Milli is one thousandth, thus wouldn't 1 mGal be one part per thousand, not one part per million of 1 Gal?
Fred
CC BY-SA 4.0
2020-11-12T00:36:56.427Z
@Fred I think the math is fine but the "m" was dropped. I've made an edit to show the change, how does it look now? Your point about altitude is important. The horizontal scale is 250 km long, 25 km per tick, so the resolved features in the anomaly are several km in wavelength. While multipole terms will drop with distance faster than $1/r^2$  there will be some sensitivity at 3.3 km altitude.
uhoh
CC BY-SA 4.0
2020-11-12T00:41:49.657Z
Much clearer than before
Fred
CC BY-SA 4.0
2020-11-12T09:41:00.107Z
The paper cites Tinto et al. (2019) as source of the gravity data: https://doi.org/10.5067/R1RQ6NRIJV89 The 'user guide' tab has some details about data acquisition and processing that might be worth reading (but are too technical for me).
Jean-Marie Prival
CC BY-SA 4.0
2020-11-12T11:26:29.893Z
@Jean-MariePrival yes that's really helpful. "3.2 Derivation Techniques and Algorithms
The gravity signal is extracted from an inertially based system in which a small mass is suspended within a magnetic field. Tiny variations in the acceleration of the gravimeter produce small electrical signals in the sensor as the mass moves within the magnetic field." The magnetic force opposes the gravitational force. If gravity changes the mass will move to a height where the magnetic field is lower. That displacement can be measured and with a known gradient the gravity change determined Very nice! :-)
uhoh
CC BY-SA 4.0
2020-11-12T12:33:10.733Z
@uhoh Glad you could make something out of this! :)
Jean-Marie Prival
CC BY-SA 4.0
2020-11-12T13:05:34.680Z
@Jean-MariePrival that's as far as I can go with it, but I think it will be helpful to someone writing an answer. Thanks!
uhoh
<gravity><ice-sheets><topography><greenland>
How are mGal-smooth free-air gravity anomaly measurements made exactly? (tiny variations of acceleration on a bumpy airplane ride)
uhoh
# How are mGal-smooth free-air gravity anomaly measurements made exactly? (tiny variations of acceleration on a bumpy airplane ride)

(Asked by: username_0 on 2020-11-11)

Free-air gravity anomaly measurements from [Operation Icebridge](https://www.nasa.gov/mission_pages/icebridge/instruments/index.html) aircraft are described in [A fault-bounded palaeo-lake basin preserved beneath the Greenland Ice Sheet](https://www.sciencedirect.com/science/article/pii/S0012821X20305914?via%3Dihub) (open access).

**Question:** These appear to be at least smooth to 1 mGal if not less. Standard gravity is nearly one million ~~Gal~~ mGal, so these are part-per-million type acceleration measurements made inside an airplane. How does one do this? *How are mGal-smooth free-air gravity anomaly measurements made exactly on a presumably bumpy airplane ride?*

A heck of a lot of averaging or damping and GPS and/or lidar/radar reconstruction of average height?

---

> Fig. 7. Gravity and magnetic anomalies... (b) Gravity anomaly modelling. The open circles mark free-air gravity anomaly measurements along the profile (**the gap at ∼50 km reflects a turn in the flight line, where gravity data are unavailable**), and the coloured lines denote the calculated gravity effect associated with the interfaces in our crustal model (panel c). The gravity effect of the topography contains a signal from both the ice surface and bed interfaces.

[![Fig. 7. Gravity and magnetic anomalies... from A fault-bounded palaeo-lake basin preserved beneath the Greenland Ice Sheet](https://i.sstatic.net/SlCtk.png)](https://i.sstatic.net/SlCtk.png)

Popular press linking to this paper:

* [Ancient Lake Discovered Under Greenland May Be Millions of Years Old, Scientists Say](https://www.sciencealert.com/ancient-lake-discovered-under-greenland-may-be-millions-of-years-old-scientists-say)
* [Primeval Greenland lake found buried beneath a mile-thick slab of ice](https://www.livescience.com/ancient-lake-under-greenland-ice.html)

**Tags:** gravity, ice-sheets, topography, greenland

> **Comments:**
> **username_0**: There may be additional/different tags that apply.  
> **username_1**: Query: are your mathematical factors correct? Milli is one thousandth, thus wouldn't 1 mGal be one part per thousand, not one part per million of 1 Gal?  
> **username_0**: @username_1 I think the math is fine but the "m" was dropped. I've made an edit to show the change, how does it look now? Your point about altitude is important. The horizontal scale is 250 km long, 25 km per tick, so the resolved features in the anomaly are several km in wavelength. While multipole terms will drop with distance faster than $1/r^2$  there will be some sensitivity at 3.3 km altitude.  
> **username_1**: Much clearer than before  
> **username_2**: The paper cites Tinto et al. (2019) as source of the gravity data: https://doi.org/10.5067/R1RQ6NRIJV89 The 'user guide' tab has some details about data acquisition and processing that might be worth reading (but are too technical for me).  
> **username_0**: @username_2 yes that's really helpful. "3.2 Derivation Techniques and Algorithms The gravity signal is extracted from an inertially based system in which a small mass is suspended within a magnetic field. Tiny variations in the acceleration of the gravimeter produce small electrical signals in the sensor as the mass moves within the magnetic field." The magnetic force opposes the gravitational force. If gravity changes the mass will move to a height where the magnetic field is lower. That displacement can be measured and with a known gradient the gravity change determined Very nice! :-)  
> **username_2**: @username_0 Glad you could make something out of this! :)  
> **username_0**: @username_2 that's as far as I can go with it, but I think it will be helpful to someone writing an answer. Thanks!  

---

## 0 answers

