# Easter Island: How to start a remote observatory from scratch

X. Lalanne, A. Peltier, A. Chulliat

► To cite this version:

X. Lalanne, A. Peltier, A. Chulliat. Easter Island: How to start a remote observatory from scratch.
IAGA workshop 2010, Sep 2010, Changchun, China. hal-03926491

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# Easter Island: How to start a remote observatory from scratch

<figure><img src="image_3.png" /></figure>

B. Heumez, X. Lalanne, A. Peltier, A. Chulliat

Institut de Physique du Globe de Paris, France

<figure><img src="image_4.png" /><figcaption>Fig.1. Location of Easter Island and its closest INTERMAGNET observatories</figcaption></figure>

## 1. Introduction

Easter Island, for its geographic location, represents an important site for magnetic measurement. Situated at 3510Km from the coast of Chile, Easter Island is one of the world's most isolated inhabited islands.

A geomagnetic observatory producing vector data sampled every second has been implemented in August 2008, absolute measurements started in spring 2009.

The observatory is part of a collaboration project between the Institut de Physique du Globe de Paris (IPGP) and the Dirección Meteorológica de Chile (DMC).

The observatory is designed to fulfill INTERMAGNET operational standards and data quality requirements.

Here we review the particularities of the site, list the problems encountered since the implementation and assess the quality of the data.

<figure><img src="image_5.png" /><figcaption>Fig.2. Location of the magnetic sensors in the southern part of the island.</figcaption></figure>

## 2. Particularities of the site location

The Observatory was set as close as possible to the Meteorological station in the Island, where observers trained for meteorological measurement were available.

Situated between the landing field and a quarry, there are significant daily perturbations caused by landing planes (once or twice a day causing perturbations up to a few minutes) and lorry traffic from and to a nearby stone quarry (causing disturbances of a few nT for a few seconds), involving later on a specific data treatment.

<figure><img src="image_6.png" /><figcaption>Fig.4. View of the absolute (front) and variations (back) buildings.</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>Fig.3. Aerial view of the magnetic observatory showing the meteorological station (a), the stone quarry (b) and the geomagnetic station (c).</figcaption></figure>

At the end of 2009, the absolute data quality and reliability of the Easter Island observatory declined significantly. In April 2010, a visit took place to the Island to meet with the observatory personnel, assess the condition of the facilities and equipments, and address data quality.

Absolute measurement training was launched. It appeared that the previous measurements were compromised by irregular procedures and improper data entry in the provided interface. Absolute and data entry trainings were performed daily for 9 days to 5 observers.

In order to facilitate the work of the observer, the theodolite was equipped with a cover. This reduces the duration of the measurements (allowing measurement between the landing and taking-off of the morning plane) as the theodolite is left on the pillar. The removal and leveling of the theodolite is not necessary for every measures.

<figure><img src="image_8.png" /><figcaption>Fig.5. The theodolite has been equipped with a plexi-glass cover and the DF pillar is measured using a fixed sensor on the wall inside the measurement pavilion. This improving the measurement conditions and reducing the duration of the measurements.</figcaption></figure>

## 4. Discussion

The Easter Island Observatory is now producing quality data for the geomagnetic community.

Observers are stationed in Easter Island for a limited period, from 3 to 5 years. Therefore, there is a need for constant and regular support, training and encouragement to the local observers to provide reliable data and measures to ensure that INTERMAGNET requirements are followed.

The construction of a second runway strip is in project, the plans are not known to this date and it is yet unclear how it will affect the geomagnetic observatory.

The last training session appears to have a positive effect on the frequency and quality of the absolute measurements.

## 3. Quality of the data

Variation data are available from 2008 and absolute measurements started in spring 2009.

### a. Variation data:

There were large data cutout in 2009 due to a malfunction of the radio system between the sensors and the datalogger and, later on, cables were damaged by rodents, those issues were resolved before the end of 2009. As shown in table.1, the continuity of the variation data improved considerably from 2009 to 2010. Fig.6. shows the local perturbations caused by landing airplanes (a) and lorry traffic (b) happening almost every day. They have a typical signature that cannot be mixed with real Geomagnetic events and have to be removed by hand using a magnetometer edition software developed by IPGP, leaving a few gaps every day in one-second data.

<table><caption>Table.1. Percentage of missing values in 2009 and 2010.</caption><thead><tr><th>Time range</th><th>H</th><th>E</th><th>Z</th><th>F</th></tr></thead><tbody><tr><td>2009</td><td>17.00%</td><td>17.00%</td><td>17.00%</td><td>25.68%</td></tr><tr><td>2010(January to August)</td><td>0.44%</td><td>0.44%</td><td>0.44%</td><td>0.34%</td></tr></tbody></table>

### b. Baseline:

Fig.7. shows a large improvement in the frequency and quality of the measurements over time and specifically since the last training in April 2010. The following tables attest of the data quality:

Root mean square differences between measured and adopted baseline values in 2010:

Sd(Do): 12° Sd(Ho): 0.94nT

Sd(Zo): 1.38nT Sd(Fo): 0.52nT

<table><caption>Table.2. showing the variation of the geomagnetic components of 2009 and 2010 using IGRF model and the yearly means obtained from the data recorded at IPM.</caption><thead><tr><th rowspan="2">Yearly means</th><th colspan="3">Obtained with IGRF model</th><th colspan="3">Obtained with measured values at IPM</th></tr><tr><th>2009</th><th>2010</th><th>Variation</th><th>2009</th><th>2010</th><th>Variation</th></tr></thead><tbody><tr><td>Total Intensity (F):</td><td>32481.5nT</td><td>32421.2nT</td><td>-60.3nT</td><td>32823.57</td><td>32808.94</td><td>-14.6nT</td></tr><tr><td>Declination (D):</td><td>15.360°</td><td>15.342°</td><td>-0.018°</td><td>15.904°</td><td>15.873°</td><td>-0.027°</td></tr><tr><td>Inclination (I):</td><td>-36.014°</td><td>-36.013°</td><td>0.001°</td><td>323.226°</td><td>323265°</td><td>0.038°</td></tr><tr><td>Northward (X):</td><td>25334.9nT</td><td>25290.5</td><td>-44.4nT</td><td>25285.4nT</td><td>25290.8nT</td><td>5.3nT</td></tr><tr><td>Eastward (Y):</td><td>6959.4nT</td><td>6938.4nT</td><td>-21nT</td><td>7205.0nT</td><td>7191.7nT</td><td>-13.3nT</td></tr><tr><td>Downward (Z):</td><td>-19098.7nT</td><td>-19062.7nT</td><td>36nT</td><td>-19649.9nT</td><td>-19623.4nT</td><td>26.4nT</td></tr><tr><td>Horizontal (H):</td><td>26273.4nT</td><td>26225.0nT</td><td>-48.4nT</td><td>26291.9nT</td><td>26293.4nT</td><td>1.5nT</td></tr></tbody></table>

## 5. Conclusion

Due to its geographic location, the value of IPM data for the scientific community data is important. After more than two years of variation data and several months of absolute measurements, the magnetic data quality of IPM is now up to the INTERMAGNET standards, the observatory will be proposed as an INTERMAGNET observatory this year. IPM with its unique geographical location will be very useful in the future, for internal as well as external field studies.

## References

* Chulliat, A., X. Lalanne, L. R. Gaya-Piqué, F. Truong and J. Savary, *The new Easter Island magnetic observatory*, In: *Proceedings of the XIIIth IAGA Workshop on geomagnetic observatory instruments, data acquisition and processing*, J. J. Love (ed.), U.S. Geological Survey Open-File Report 2009-1226, 271 p., 2009.

* Aerial pictures generated through Google Earth (http://www.google.com/earth/index.html)

* International Geomagnetic Reference Field (IGRF) obtained from the World Data Center for Geomagnetism, Kyoto (http://wdc.kugi.kyoto-u.ac.jp/index.html)

## Acknowledgements

The authors would like to acknowledge the local staff of the Dirección Meteorológica de Chile in Easter Island for their cooperation.