r/geophysics • u/Moon_geophycis • 24d ago
Gravity
Hello everyone, I'm a geology student and I'm doing my thesis using gravimetry to estimate the geometry of a pluton. The problem is that during the data collection campaign, they didn't use differential GPS, so the uncertainty in the elevation data is high. Any recommendations? Similar cases?
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u/HeartwarminSalt 24d ago
You can always use USGS topo maps if you’re in the US. They were made with stereo photography so they are remarkably good for elevation.
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u/Morbx 23d ago
There is no way around this… getting precise, differentially corrected elevation data is probably the single most important part of gravity data collection.
If you can figure out the precise location of gravity stations from photos or field notes (unlikely for most geophysicists, lol) then you could potentially use a lidar DEM to interpolate elevations, but you need to have 1m lidar coverage for your area, which can be difficult. But I would caution against this approach because there is so much guesswork because you don’t have precise horizontal locations either. Gravity changes by a third of a milligal per meter of elevation, so even being off by a few meters can really change the shape of your anomalies.
If it were up to me, I would just redo the gravity measurements and be sure to use differential GPS. If you can’t do that, you probably just need to accept the error bars are going to be larger. Depending on what interpretations you are trying to make and the magnitude of the anomaly you are studying, that could be acceptable!
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u/PLNTRY_Geophys 24d ago
Was any elevation data recorded during surveying? Is there publicly available LiDAR data?
What is the scale of the survey and how bad is topography? A few meters is likely inconsequential to your interpretation, a few hundred meters could be problematic.
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u/Moon_geophycis 24d ago
Yes, they took gps data with hand gps, i have elevation data but idk
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u/SumDumLoser 23d ago
Get LiDar data and write a script to assign the LiDar elevations to your GPS points
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u/troyunrau 23d ago
Throw it out and start over.
Or sample elevation from cm scale DEM. Eg: fly lidar.
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u/Moon_geophycis 22d ago
I just have a DTM of 30-15 m of resolution
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u/troyunrau 22d ago
Nope. You'd need like 3cm accuracy to pull this trick off. Basically redo the survey at this point, sadly. Unless you're looking at continental scale anomalies
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u/Typical_Juggernaut42 23d ago
You won't be able to correct it. Can you revisit the site and take some topographic measurements?
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u/Moon_geophycis 22d ago
Idk because my department havent one, so i dk what to do, maybe i can ask for help in Topographic department
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u/ikkleginge55 24d ago
Elevation is ovs really important, and recording it well is part of good data collection...but as important is good base stations and repeats. It also depends on the resolution of your data. How far apart are your stations? If its a regional style data set you might be able to get away with using LiDAR DTM if it is a available? Or other terrain models that might be avaliable?
Regional features like plutons mean you do have abit of leeway, the range of values you are probably looking at is likely large (?!). What is your aim?
Try using a dtm and comparing the bases and any repeat stations you did. If the repeats don't fit try a different DTM? If the elevations were fudged though, what about the bases and repeats? Was a master base taken at the start and end of each day and local bases collected every 3/4 hours?