Monitoring Crustal Movements and Sea Level inLanzarote

L. García-Cañada, M.J. SevillaInstituto de Astronomía y Geodesia (CSIC-UCM),Facultad de Ciencias Matemáticas, Universidad Complutense de Madrid, 28040 Madrid, Spain.laura _garcia@mat.ucm.es

Abstract. The Institute of Astronomy and Geodesyis measuring sea level variations in Lanzarote Islandby two automated tide gauges of precision since1993. In order to obtain "real" sea level variations apermanent GPS station has been installed near thetide gauges. The goal of this GPS station is tomeasurement vertical crustal movements in order toobtain absolute sea level variations removing thesemovements from tide gauge data.

A vertical tie with high accuracy between thereference point of the pillar where the GPS antennais installed and the tide gauges bench marks isabsolutely necessary. We have carried out thisaltimetric link yearly since year 2000. Methods ofrepeated geometric and trigonometric levelling ofvery short tracts have been used due to the greatlevel difference existent.

The control of the local stability of the pillarwhere the GPS antenna has been established, iscarried out by a micro-geodetic control networkbuilding around it. This network of 13 benchmarkshas been regularly observed by classic and GPSgeodetic techniques in years 2000, 2001, 2002,2003 and 2004. The data of these campaigns havebeen processed by different types of adjustments inthe same reference system. The precision of themeasurements and the reliability of the networkshave been calculated.

In this work the results obtained, the evaluationof the altimetric links and levelling campaigns, andthe comparison of the levels of differentmeasurements are presented.

Keywords. Sea level, crustal movements, geodeticnetworks and GPS.

1 Introduction

Changes in sea level are usually recorder by tidegauges, but their data corrupted by vertical land

movements and they measure relative sea level.These vertical land movements at tide gauges haveto be measure in order to obtain absolute sea leveland separate crustal movements and sea levelvariations (Zerbini et al., 1996).

The aim of this work is monitoring the areawhere the tide gauges are installed using GPS andlevelling techniques in arder to provide absolute sealevel instead "land relative" sea level. That is whycontinuous geodetic positioning oftide gauge benchmarks (TGBMs) is required (IOC, 2000).

This allow to remove vertical motions of the landwhere tide gauge is attached from the sea level dataand studying the correlation between relative sealevel and crustal vertical movements.

2 Tide gauges and GPS sites

Lanzarote is the most Northeast island of theCanarian Archipelago, it is 100 km off the Africancoast and 1000 km off Iberian Peninsula. "Jameosdel Agua" is a place in the north coast where thelnstitute of Astronomy and Geodesy has ageodynamic laboratory (Vieira, 1994).

There, the Corona volcano lava tube intersectswith the ocean forming two lakes. It is an idealplace for very sensitive instruments to be hostedand a good area to sea level researches becausethere are not perturbations like winds and waves.

Two automated tide gauges of precision (Druckpressure sensors) are located in the two lakes thatmeasure raw sea level in a relative reference systemsince 1993. Really near each tide gauge there is analtimetric bench mark.

A perrnanent GPS station has been installed todetect vertical velocity at tide gauges. It is locatedon the top of "La Casa de los Volcanes" building,which is a good place for a CGPS because it has aclear view of sky in all directions, area is free ofinterference sources and it is near tide gauges(Figure 1).

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Fig. I Antenna ofthe CGPS Lanzarote station.

The instrumentation installed is a dual frequencycode and phase measuring Ashtech Z-Surveyorreceiver and Dome Margolin choke ringASH701945B_M antenna. It follows the guidelinesofthe IGS.

In addition tide gauges and a perrnanent GPSstation there are other sensors to measure water andair temperature, pressure, humidity and gravity.

3 Geodesic control

Top of a building is not the best place to have aCGPS, that is why a micro geodetic control networkhas been designed and installed around it in order tomonitor the stability of the monument and thebuilding.

The network has 13 bench marks and thedistance between these points is from 65 to 300 m(Figure 2). It has been regularly observed everyyear by classical geodetic techniques and GPS sinceyear 2000.

But if a somewhat remote CGPS station ismoving by an unknown amount relative to its tidegauge, then the absolute position of the tide gaugeis being determined only as frequently as thelevelling tie is being made (Figure 3).

The methodology to measure in the campaigns isdifferent depending on the topography ofthe land.

Classical precise spirit levelling is use whenheight difference between bench marks is not sogreat. But when it is more important, trigonometriclevelling with reciprocal and simultaneous verticalangles and geometric distance measure is used.

-13_435 -13.432 -13.429 -13.428·13431 -13.0)-13.434 -13.433

Fig. 2 Micro geodetic control network installed aroundCGPS (point number O).

GPSRec:eivu

Rod

oree

Giluge

Fig.3 Schematic oflevelling tie between TGBM and CGPS.

In trigonometric method (Figure 4) the heightdifference between two points, A and B, iscalculated using (Sevilla and Romero, 1991):

(

Z¡ -Z2) ú.I2·sin --2- .sin2f.h=hi2-hi¡=(R+hi¡) ( ) (1)

Z2-Z¡ ú.Icos ---+-

2 2

where z¡ y Z2 are vertical angles measured, hi¡ y hi2are instruments heights, R is terrestrial mean radioand ea is the angle that forrn the vertical of points Aand B in the centre ofthe Earth.

The angle ea is calculated by:

Lú.I=-

R(2)

where L is the distance on the cord of ellipsoid,

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(3)

Sh is obtained by iteration method, using aapproximated L in the first iteration.

MIHmetricrod

Bench markA

Zenith

Millimetricroe

Zenith

Retractíonare

~ Totalstaticn

ench mark 8Theodolite

t.;distance

Fig. 4 Schematic of reciprocal and simultaneoustrigonometric levelling.

The height difference using geometric levellingis calculated by:

(4)

where LA and LB are the readings in the rods locatedin the points A and B respectively (Figure 5).

Rod

Rod

BenchmarkA

Fig. 5 Schematic of spirit levelling.

4 Levellingresults

CGPS-tide gaugetie

The trigonometric methodology has been used inthe levelling tie between TGMBs and CGPS since2000. lt is carried out in four tracts using threeauxiliary points in year 2000. In 2001 someadditional bench marks had installed in order tohave a better monitoring in the area in case anydeforrnation were detected.

The results of altimetric profile in 5-yearcampaigns (Figure 6) show there is not significantdifferences in consecutive years, les s than 3mm;this suggest that the tide gauge did not experienceany uplift or subsidence relative to CGPS.

o Campaign 2000

BCampaign 2001

30.0 • Campaign 2002

25.0

I20.0....:¡:

~ 15.0:¡:

10.0

Fig. 6 Altimetric profiles GPS-MAR I in 2000-2004campaigns, with precision trigonometric levelling technique.

5 Control network results

The micro-geodesic control network (Figure 2) hasbeen measurement yearly since 2000 too.Reciprocal and simultaneous trigonometriclevelling is use to tie CGPS with network benchmarks when there is visibility. Spirit levelling iscarried out in the surveys for the external ring.

The least-square adjustment ofthe campaign datahas been carried out with the method of variationcoordinates using four different models:l. Free network and equal weights (M 1).2. CGPS fixed and equal weights (M2).3. Free network and weights inversely

proportional to the distance (M3).

162

6

og 4N

"~ 2(!)

1:: O+tI",~II'T'=-;9 (j)

" "-(!) -2 (9(¡)::>:c -4«

N C"l

---+--- Adjusted M1 - Observed

-6___ Adjusted M2 - Observed

Adjusted M3 - Observed6

.. "•.. Adjusted M4 - Observedg 4N

"~ 2(!)

1:: O9"~ -2VI::>:c -4«

-6

6

Ng 4N

"~ 2(!)

1:: O+tIfo-~~.-tIIII~1t-T ••C""'~ •• .a~""",~:--",h9~ -2VI::>:c -4«

-6

6Mg 4N

"~ 2(!)

1:: OHllfo-~--=-'"9"~ -2VI::>:c -4«

•-6

6 ••..g 4N

"~ 2(!)

1:: Otilfo-~"r.lIIIIi~~":f "'-~~~-----'\':;---7Ji"9"s -2VI::>

~ -4

•

-6

Fig. 7 Adjusted network results with four models in the fivesurveys.

4. CGPS fixed and weights inversely proportionalto the distance (M4).

In Figure 7 we can see the difference betweenthe difference height observed and adjusted with thefour models in the 5 years and there is nosignificant difference in the results using onemethod or another.

If the results in the 5 years with one model arecompared (Figure 8), a large subsidence at points 6(13 mm) and 13 (10 mm) has been detected in theperiod 2000-200 l. The reason was that these benchmarks had been moved by a man-effect and theyhad to be repaired and the problem was sol ved since2001.

12·

".Es ..§.

:fi 6·Uc:~ 4·.¡¡;.c"2·ti)

~ .

Fig. 8 Subsidences (in mm) detected in years 200 1, 2002,2003 and 2004 in the control network points refer to 2000campaign.

The results of the repeated precise levellingsurveys showed no significant changes in heightwithin the control network over the period from2000 to 2004. This confirms the stability of themonument and building on which the antenna GPSis located relative to the area.

6 GPS time series

CGPS data has been processed in a regionalnetwork of permanent stations formed by four lGSstations, MASI, MADR. RABT and SFER, fourEUREF stations, ALAC, CASC, CEUT and LPAL,and the Lanzarote station (LACV) (Figure 9).

The network is processed with Bemese v.4.2software using IGS final precise orbits and Quasilonosphere Free (QIF) strategy for the ambiguityresolution (Hugentobler et al., 2001). Elevation cutoff angle of 10°, elevation dependent observation

163

weighting and the dry-Niell mapping function(Niell, 1996) has been used.

34045

345 350 355 5

40

kmo 500 - S---

MDR•( Ase ..• A~ e

SFER .r· ...".--• 7 EUT".RABT•...

1

LPAL L'¡\..V• MA 1A

•

-

35

30

25340 345 300 355

Fig.9 Map ofthe CGPS regional network.

The Saastamoinen global tropospheric delaymodel is used to generate a priori values for thetropospheric delay but total troposphere zenithdelay has been estimated for all the stations everyhour. The ionosphere effect is eliminatedintroducing the ionosphere free linear combinationof the observables.

~~1Sopa -0.331 rmYy9Bf

1150 12501200GPSweek

Fig. 10 LACV weekly time series.

45

40

35

30

The ocean tide loading effect has been taken intoaccount according to the GOTOO.2 model.Amplitudes and phases of this model have beenobtained from the H.G. Scherneck site(http://www.oso.chalmers.se/~Ioading/).

Sessions for 24 hours have been defined and foreach session the network has been adjusted inCartesian threedimensional coordinates in theITRF2000 reference frame. The IGS stationsMAS 1, MADR and SFER have been consideredfiducial stations constrain their lTRF2000coordinates.

Daily coordinates and normal equations arecalculated in this way and they are combined byADDNEQ subroutine (Hugentobler et al., 2001) toobtain weekly solution.

Although time span is too short and there aresome outlier period, preliminary results of theweekly time series (Figure 10) show there is novertical rate and horizontal velocity in the sameorder of other stations in the same tectonic plate.

7 Conclusions

1300

After measure sea level by tide gauges in Jameosdel Agua (Lanzarote Island) since 1993 we advertedthat geodetic techniques for monitoring rates ofvertical land movements were needed if we want toobtain sea level change without land movements attide gauges.

That is why a CGPS stations has been installednear tide gauges, a micro-geodesic control networkhas been established around it and it is surveyedregularly; levelling tie between CGPS and TGBM iscarried out every year since 2000.

The geodesic control network results over aperiod of four years show the monument of the GPSantenna and the building where the CGPS is, arestable and there is not any deformation between thepillar in the top of the building and the land is thisarea around it.

Due to the great height difference, the tie CGPS-TGBMs is measure using precise trigonometriclevelling techniques. The results of these campaignsconfirm that the CGPS do not show any verticaldeformation respect to TGBMs.

This ensures that the relative sea level measuredby the tide gauges is not affected by landmovements in the area, so it is representative of thesurrounding area.

About time series of CGPS, preliminary resultsbased on 2.5 years time span, suggest that the

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vertical component does not present any rate, butobservations spanning a longer time period will benecessary to get more reliable values.

Acknowledgement

By the support coming from research projectREN2001-2271, "Geodesic networks, spacetechniques and gravimetry applied to the study ofthe structure and dynamic ofthe crust in Canary" byScience and Technology Spanish Ministry.

References

Hugentobler, U., S. Schaer and P. Fridez (Edts.), 200!.Bemese GPS Software Version 4.2. AstronomicalInstitute. University ofBerne.

roc, 2000. Manual on sea-level measurement andinterpretation. Volume 3 Reappraisals andRecommendations as of the year 2000. IntergovemmentalOceanographic Commission. Manuals and Guides No. 14.roc, Paris, 52pp.

Niell, A.E., 1996. Global mapping functions for theatmosphere delay at radio wavelengths. Journal ofGeophysical Research, Vol. 101, No. B2, pp. 3227-3246.

Sevilla M. J. and P. Romero, 1991. Ground deformationcontrol by statistical analysis of a Geodetic network in thecaldera of Teide. Journal of Volcanology and GeothermalResearch, Vol. 47 pp. 65-74. Elsevier Sc. Pub.Amsterdam.

Vieira, R., 1994. La estación geodinámica de Lanzarote.Serie Casa de los Volcanes. Monografia nº 3, pp. 31-40.

Zerbini, S., H.P. Plag, T. Baker, M. Becker, H. Billiris, B.Bürki, H.G. Kahle, 1. Marson, L. Pezzoli, B. Richter, C.Romagnoli, M. Sztobryn, P. Tomasi, M. Tsimplis, G.Veis, and G. Verrone, 1996. Sea level in theMediterranean: a first step towards separating crustalmovements and absolute sea level variations. Global andPlanetary Change, 14, pp. 1-48.

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