Proceedings of the 24Th Schmucker-Weidelt-Kolloquium

24. Schmucker-Weidelt-Kolloquium Neustadt an der Weinstraße, 19.–23. September 2011 The Permanent Magnetotelluric Remote Reference Station D. Eydam & G. Muñoz from GFZ Potsdam Summary Magnetotellurics is a so called passive method where natural electromagnetic variations are used as signal excitation. The convenience of passive methods works at the expense of signal - to - noise ratios which are generally poor due to the absence of control of signal strengths. The Remote - Reference - Technique is an effective way to improve magnetotelluric data quality by referencing the local fields to simultaneously recorded and undisturbed fields at a remote reference site. In areas where noise level is high remote reference processing is required but finding and maintaining a reference site during a campaign is expensive and time consuming. Therefore a permanent and self - sustaining reference station simplifies MT measurements which are carried out in a radius where source fields are still coherent, which depends on the frequency. For high frequencies up to 1000 Hz this coherency radius can amount to over 800 km, which was proven by referencing data from the Black Forest to Rugen¨ (Schill et al., 2011). Locating appropriate reference sites across Germany was not an easy task. We found high quality data in the urban forest of Wittstock where the permanent reference station was installed end of 2011. Reference data is being permanently recorded with sampling rates up to several kilohertz since 2010. Meantime the Wittstock station operated repeatedly as reference and magnetotelluric data from northern and eastern Germany could significantly be improved. Access to the reference site data is open to the MT community. 1 - Introduction The magnetotelluric (MT) method uses amplitude - and phase - relations of quasi homo- geneous electromagnetic (EM) fields at the earth’s surface to obtain resistivity informa- tion of the subsoil. Inhomogeneous fields of nearby current sources overlie intrinsically proper MT - signals and disturb passive recordings. If any noise is present in one field component, it is amplified by calculating auto - power spectra which causes impedances to be biased. Considering that the uncontaminated natural part of the induced field is coherent over many kilometres whereas noise is generally random and incoherent, bias effects can be removed by substitution of local magnetic variations with simultaneously recorded, undisturbed magnetic variations at a remote reference site (for more detail 45 Eydam et al., The Permanent Magnetotelluric Remote Reference Station Rügen Wittstocker Heide Bombodrom Havelberg Westerwald Eifel Oberpfalz Fränkische Alb Allgäu Figure 1: Test-areas across Germany: Rugen,¨ northern Brandenburg, northern Bavaria, Eifel, West- erwald and Allgäu. see, ex. Gamble et al., 1979). A remote reference site is thus located far away from current sources such as settle- ments, gas and power lines, electric fences etc. Among the variety of noise sources, noise influence is different since it can disturb just some definitive frequencies or a broad frequency range such as ground leakage currents arising from electric fences. Promising test sites are preferably located above well - conducting subsoil considering that disturbing fields propagate over shorter distances here. Geographic and geologic maps were used to decide about testing areas. Long term recording requires year - round accessibility which disqualifies sites at agricultural crop land, high altitudes or coastal areas. We concentrated our search on woodlands where we focused at clearances to ensure future power supply by photovoltaics. 2 - Locating appropriate remote reference sites in Germany Data quality of about 25 locations across Germany was tested (fig.1). Data was commonly recorded for at least three days with sampling rates between 500 to 6000 Hz. Geographic coordinates of the test - sites are listed in the appendix. A site was qualified as a reference if the time series of the electromagnetic field com- ponents particularly the magnetic ones do not contain any obvious noise signal and if the local transfer functions are reasonable smooth and continuous especially in the MT dead band, the natural excitation minimum between 10 Hz and 10 s. 2.1 – Allgäu Site Mol near Kempten was used as a fairly good reference for MT measurements in the southern Black Forest. However magnetic data is quite disturbed (fig.3) and important annual snowfalls disqualify the site for long - term recordings. 46 T T Par the permanent remote reference (upper right corner) are due to s (left) for exemplarily chosen sites Par -permanent remote reference -permanent WH3 WH3 Eydam et al. , The Permanent Magnetotelluric Remote Reference Station T T T T Rügen andRügen Ummanz islands cy data at the expense of high frequency recording. At site ¨ ugen and Brandenburg. Shown are magnetic transfer function he area (right). Corrupt high frequency data points at site T T T T Northern Brandenburg - Bombodrom region 47 T T T T station has been set up. Figure 2: Overview about data quality for tested areas: R enabling induction coil chopper which improves low frequen and qualitive best magnetotelluric transfer functions of t nd tipper values (right). Note Parkstein - Oberpfalz chopper. Eydam et al. , The Permanent Magnetotelluric Remote Reference Station Havelberg ten, Havelberg and Parkstein. Shown are impedances (left) a longer than one second is due to disabling the induction coil 48 Kempten that at the Parkstein - site the quality decrease for periods Figure 3: Overview about data quality for tested areas: Kemp Eydam et al., The Permanent Magnetotelluric Remote Reference Station 2.2 – Rugen¨ & Ummanz islands Islands should be preferable for an undisturbed recording of natural electromagnetic fields as the sea’s conductivity is low so that tidal effects are negligible. Inspired by results from Schäfer (2010) who found good data quality at northern Rugen,¨ we tested four sites at Rugen¨ and Ummanz (fig. 4). Par Leb Uma - 001 Uma Gor Leb - 007 Gor - 003 Par - 002 Figure 4: Test - sites at Rugen¨ and Ummanz islands. The quality of magnetic data is commonly good but deteriorates southward since pop- ulation density increases which is demonstrated by a distinct 50 Hz signal and its har- monics at site Gor (fig. 2). At northern Rugen¨ as well as at Ummanz transfer functions are smooth even in the MT dead band and time series are without any obvious noise; regarding that disturbed long periods at Ummanz (Uma) are probably just due to a nearby, mobile electric fence. Notable are the small apparent resistivities which may shield nearby noise signals ef- fectively (fig. 2). The congruent curvature up to periods of 100 s indicates horizontal layering of the subsoil which is confirmed by the vanishing geomagnetic transfer functions. Site Par operated repeatedly as reference but is situated on agricultural crop land which is not year - round accessible. Since both islands are in intensive agricultural use, alternative sites for Par and Uma are difficult to find. Leb lies next to Par but at the other side of the indentation and data quality is still good, however time series of the magnetic fields show a minor 50 Hz signal. The influence of the recently built Baltic Sea - gas line has not been identified yet. 2.3 – northern Brandenburg Eastern Germany is poorly settled and power and gas lines are less dense in northern Brandenburg and Mecklenburg - Vorpommern. The polish DC - railways are an important noise source in easternmost Germany so that this region was expected to be inappropriate as reference area. 2.3.1 – Havelberg Tests were carried out in the woodland area east of Havelberg. Data quality of all five 49 Eydam et al., The Permanent Magnetotelluric Remote Reference Station WH1-4 TP1-4 Bombodrom AB Figure 5: Test - sites in northern Brandenburg - Bombodrom and Wittstock region. Site AB is located near the highway A24, sites WH1-4 are in the urban forest of Wittstock and sites TP1-4 adjoin to the former military drill ground Bombodrom (red line). Red circle: chosen permanent reference site. sites is poor and data scatters especially in the MT dead band representing a more re- gional than local noise character (fig. 3). Major gas and power lines are more than 5 km away, but could still affect the quality of long period data. Middle to high frequencies 2 are influenced by a distinct 16 3 Hz signal of the nearby ICE - railway. 2.3.2 – the Bombodrom - region Bombodrom is a former military drill ground in the Kyritz - Ruppiner Heide. The ter- rain is an open and priory wild area of 144 km2 but is severely mined and maintains the judicial status of a military safety zone. A petition for permission of test measurements was filed at the Bundeswehr service centre in Potsdam but has not been answered. Therefore we sidestepped to adjacent woodlands as well as to an extended forestry area north of Wittstock (fig.5). Data quality differs: Distinct local noise sources are present along the highway (site AB) and adjacent to the Bombodrom (sites TP1-4 ) where time series show 50 Hz signals with temporally varying amplitudes (fig. 2). Noise sources might be underground cables (sites AB, TP2 ), electric fences (site TP1 ) and minor gas pipelines which were not included in the maps we used (sites TP3-4 ). Data recorded at all four sites in the urban forest north of Wittstock are quite undisturbed; transfer functions are reasonable and smooth (fig. 2) and time series are without any distinct noise. However spectra show minor 16 Hz and 50 Hz signals which is slightly more pronounced in the Zyx - component (see fig. 8). Similar to northern Ru-¨ gen, impedances curve shapes are congruent up to periods of 100 s indicating horizontal layering of upper earth levels which concurs with the vanishing magnetic transfer function. Apparent resistivities are comparatively low and may shield noise signals.

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