# Presentation: 154710

## Analysis of USGS One-Second Data

- Bill  Worthington, Jeffrey Love, and Edward Sauter
- U. S. Geological Survey

- Introduction
- In 2007 and 2008 the USGS deployed their newest data acquisition system at 13 geomagnetic observatories.  The new system used a new sampling technique to produce one-second data that should have a noise level of 1 – 10 pT RMS.  The magnetic field is sampled at 100 Hz, with an analog low-pass filter at 50 Hz.  One-second averages are computed using a Gaussian filter using 91 data values centered on the second.
- Analysis of this data are presented that show estimates of the noise level from the observatories where the new system is employed.

- Single Magnetometer Analysis Technique
- At most of the observatories there is only a single magnetometer in operation.  At these locations we have computed the first difference over intervals of 5 minutes.  The standard deviation was computed to determine the RMS error for these intervals.  The results were then plotted as a function of time of day.  A dynamic spectrogram was also computed and plotted to evaluate the frequency content of the data.  An example is shown below using data from Honolulu.  In this example the noise level is about 10 pT at its lowest value.

- Dual Magnetometer Analysis Technique
- At some of our observatories there is a second magnetometer in operation that allows us to compare the two data sets.  At Boulder we have two identical systems operating on site.  At College and Honolulu there are systems operated by other agencies that collect one-second data.
- In these cases the data is first checked to make sure the data are synchronized in time.  We then compute the differences between the data from the two systems.  From this we can compute the standard deviation of the difference to obtain an estimate of the RMS error.  It was necessary to determine the difference with F, computed from H, E, and Z, to account for differences in the orientation of the different sensors.
- The example below shows data from the two systems at Boulder.  The top graph shows computed F for both systems, the middle graph shows the difference between the two, and the bottom graph shows the RMS error computed over two minute intervals.

|   | RMS Noise (pT) |  |
| --- | --- | --- |
|   | Single | Two |
| Obsy | Mag. | Mags. |
| BOU | 9 | 18 |
| BSL | 9 |   |
| BRW | 10 |   |
| CMO | 12 | 20 |
| FRD | 18 |   |
| FRN | 18 |   |
| GUA | 15 |   |
| HON | 10 |   |
| NEW | 10 |   |
| SHU | 25 |   |
| SIT | 22 |   |
| SJG | 17 |   |
| TUC | 11 |   |

- In the table to the right, a summary of the noise tests for all observatories tested is shown.  Results of both noise analysis techniques are shown when available.  Seven of our observatories show a RMS noise level of about 10 pT using the single magnetometer technique.
- At both Boulder and College Narod magnetometers were compared and the RMS noise level is about 20 pT.

- Discussion
- Our current results show that most of our observatories have noise levels of about 10 pT RMS.  Unfortunately we are not seeing noise levels significantly lower than 10 pT.  We feel that there are several reasons why this is so:
- The fluxgate scale values (nT/Volt) used by our data acquisition software may not be precise enough.  Currently they are measured to 1%, we need to increase the precision to 0.1%.
- The noise analysis techniques may not be rigorous enough.  We may need to examine the spectrum to examine the noise as pT per square root Hz at a specific frequency.
- It may be necessary to re-examine the acquisition technique.
- The spectrogram shown at the far left has shown that most of the observatories have no external frequency lines in the data.  It did help in identifying noise produced by our temperature controller at Shumagin.  In the graphs below, the set on the left shows about 200 pT of noise with a frequency line at about 60 mHz.  The right set of graphs shows data a few days later when the heat was turned off.  The noise level decreased by an order of magnitude and the 60 mHz frequency line is gone.