Establishing a New Absolute Gravity Base Station Using a Relative Gravity Meter in Lincoln, NE

Kris Guthrie Author

04/04/2021 Added

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Presentation on the process of establishing a new absolute gravity base station in Lincoln, NE

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[00:00:00.000]Hello, I'm Kris Guthrie, a 2nd-year Master's Student My presentation is about Establishing a New Absolute Gravity Base Station Using a Relative Gravity Meter in Lincoln, NE.
[00:00:13.718]Before I really get going, I would like to quickly thank the Department of Earth and Atmospheric Sciences at UNL,
[00:00:20.050]the Geological Society of America, and the Nebraska Geological Society for supporting my research and education.
[00:00:27.307]Many thanks go to the United States Postal Service and Sheri Kellogg for letting me go to the Hastings Post Office to collect gravity data, and everyone in the Fillmore County Clerk's Office for allowing me to collect data in the Fillmore County Courthouse.
[00:00:43.299]First, I will introduce my project and state the motivation behind it. Then, I will describe the possible locations for the gravity base station at UNL's City campus, and the absolute gravity base stations available for transferring the measurements.
[00:00:57.493]I will then briefly describe the absolute gravity values I have, and summarize the research and outline the next steps.
[00:01:09.730]There are two types of gravity meters: relative and absolute. While I must admit that I've never seen one in person, I am reliably informed that absolute gravity meters are large, clunky and inconvenient at best for gravity surveys. In contrast, relative gravity meters are smaller and more portable.
[00:01:27.707]Here is a picture of the CG-5 gravimeter that was used for my project. The downside of relative gravity meters is that they need to be tied to an absolute gravity base station in order to get meaningful data. These absolute gravity base stations should be located nearby and need to be publically accessible.
[00:01:46.738]The nearest official gravity base station is in Omaha, however it is on a military base. This makes access to it problematic to say the least.
[00:01:55.458]There is an unofficial absolute gravity base station in Lincoln. It was established by NOAA for one of their airborne surveys in 2013. It is located on the tarmac of the Lincoln airport and is difficult to gain access to.
[00:02:11.709]My advisor, Dr. Filina, was given a permit to access it once in 2017. Unfortunately, the Lincoln Airport Authorities have not responded to any access requests since then.
[00:02:23.722]In addition to the issue of access, the location of the base station was marked by NOAA with a painted X in 2013. When accessed in 2017, the cross was not evident on the tarmac as the paint was extremely faded. The photograph on the right might show some of the paint, but it is not evident on the tarmac itself.
[00:02:44.008]This station was used to tie measurements from two surveys conducted by the UNL Geophysics Team and there was a large mismatch of approximately 40 mGal between the survey values and the official USGS published values.
[00:02:56.493]The next two nearest official gravity base stations are located in the US Post Office in Hastings and the Fillmore County Courthouse in Geneva. These are the stations used in this study.
[00:03:07.723]The objective of this study is to establish a reliable gravity base station for people in the Lincoln area to use in their gravity surveys that doesn't need special permission to access or a long drive to get to.
[00:03:20.530]Four potential locations for the new gravity base station were identified on the UNL campus.
[00:03:26.138]Two were inside of Bessey Hall: one in a corner of the Geology Library and the other one in the Eastern Stairwell on the basement level.
[00:03:33.283]The other two locations were outside: one in front of the Bell Tower and the other in front of the State Museum. I have taken regular measurements at all of these locations since October 2019 to measure the noise level at each location.
[00:03:47.041]Each loop of measurements at all four possible locations started and ended in Dr. Filina's office.
[00:03:53.172]Measurements were taken in order to analyze the repeatability.
[00:03:56.775]Due to Covid-19 shutting down Bessey Hall in March of 2020, two of the four potential locations were eliminated due to inaccessibility.
[00:04:06.631]Here are the two remaining possible outside locations for the new absolute gravity station. Since October 11th, 2019, 54 measurements have been taken at these locations.
[00:04:17.764]The standard deviation for the Bell Tower is 38 µGals, and the standard deviation for the State Museum is 36 µGals. The location has not been finalized at this time, in part due to how close the standard deviations are.
[00:04:31.679]Initial research plans were to travel to five gravity base stations within a 2-day round trip, but UNL's Covid-19 travel restrictions eliminated all of the base stations outside of Nebraska.
[00:04:43.190]Fortunately, there are two within a day's drive and in Nebraska. They are located in the U.S. Post Office in Hastings, NE and in the Fillmore County Courthouse in Geneva, NE.
[00:04:52.634]Both of these stations were established in 1968. They originally were marked with a USAF Gravity Disk but the disks were removed with replacement of flooring. As of March 16th, 2021, I have made five trips to these gravity base stations.
[00:05:10.291]There is a difference of about 40 mGal between the gravity values tied at the KLNK marker and the values tied at Hastings and Geneva. This is the same difference found in the two previous surveys.
[00:05:22.485]Using measurements taken at the KLNK marker in 2017, I estimated its gravity value by tying the readings to the Hastings and Geneva stations. This revealed that the gravity value was higher by the same amount as in the surveys.
[00:05:37.106]Upon reaching out to NOAA, we learned that they returned to the KLNK marker in 2019 with a different gravity value. This newer value is consistent with my estimate, so the mismatch is in the absolute gravity value from 2013.
[00:05:53.178]Here are the transferred absolute gravity values with their standard deviations from the Hastings, and Geneva base stations to the UNL State Museum and the Bell Tower.
[00:06:04.056]There is a difference of 129 to 157 µGal between the gravity values tied to Hastings and Geneva. This is much smaller than the previous mismatch with the KLNK marker but is still too large for a gravity base station.
[00:06:22.419]It is likely that this mismatch is related to a difference in floor heights between now and when the stations were established. Unfortunately, we do not know which station is more reliable at this time.
[00:06:36.074]In summary, based on the two gravity surveys and the comparisons between the two established gravity base stations in Hastings and Geneva and the NOAA value from 2019, the KLNK marker's 2013 gravity value appears to have an error around 40 mGal.
[00:06:52.859]Here are the reported values from NOAA's 2013 and 2019 surveys and the estimated value. The best location for the new base station on the UNL campus has been narrowed down to the Bell Tower and the UNL State Museum.
[00:07:06.477]After five trips to the official base stations, here are the absolute gravity values. All of the four values have standard deviations below 50 µGal.
[00:07:15.600]Unfortunately, the smallest difference between the sets of values tied to the Hastings and Geneva base stations is around two-and-a-half times higher than the largest standard deviation. This uncertainty between base stations is too large for an absolute gravity base station.
[00:07:31.271]Since the absolute gravity values are not finalized at this time, I will continue regular measurements at the potential locations, and it will be finalized later based on the lowest noise level.
[00:07:44.111]The next step for this project is to meet with a representative from NOAA to determine the best way forward for this project.
[00:07:51.431]These are our references.
[00:07:53.373]Thank you for your time.

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Establishing a New Absolute Gravity Base Station Using a Relative Gravity Meter in Lincoln, NE

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