# Dr. Friedemann Samrock

###### Senior Research Assistant

Dr. Friedemann Samrock
Geothermal Energy & Geofluids
Institute of Geophysics
NO F 51.1
Sonneggstrasse 5
CH-8092 Zurich Switzerland

Contact
 Phone +41 44 633 6818 Email fsamrock@ethz.ch

 Dominique Ballarin Dolfin Phone +41 44 632 3465 Email ballarin(at)ethz.ch

## Publications

### REFEREED PUBLICATIONS IN JOURNALS

 6 Morschhauser, A., A.V. Grayver, A. Kuvshinov, F. Samrock, and J. Matzka Tippers at island geomagnetic observatories constrain electrical conductivity of oceanic lithosphere and upper mantle, Earth, Planets and Space, 71/17, pp. 1-9, 2019. AbstractGeomagnetic field variations as recorded at geomagnetic observatories are important for global electromagnetic studies. However, this data set is rarely used for studying the local electrical conductivity at depths $<200$ km. The main reasoning being that given a single geomagnetic observatory, one can at most constrain the one-dimensional (1-D) conductivity structure beneath it. At the same time, tippers, magnetic transfer functions resolving these depths, are zero for any 1-D conductivity distribution. We show that the ocean induction effect alleviates these limitations for observatories on islands and develop a method to invert tippers for a 1-D conductivity profile in the presence of three-dimensional conductivity structure due to bathymetry. This allows to recover 1-D upper mantle conductivity profiles at remote oceanic locations where little or no knowledge is available and that would otherwise be difficult to access. We apply the method to Gan in the Indian Ocean and to Tristan da Cunha in the South Atlantic, and the obtained conductivity profiles indicate a normal oceanic mantle and elevated conductivities, respectively, which fits well with their geological settings. 5 Samrock, F., A.V. Grayver, H. Eysteinsson, and M.O. Saar Magnetotelluric image of transcrustal magmatic system beneath the Tulu Moye geothermal prospect in the Ethiopian Rift, Geophysical Research Letters, 2018. AbstractContinental rifting is initiated by a dynamic interplay between tectonic stretching and mantle upwelling. Decompression melting assists continental break-up through lithospheric weakening and enforces upflow of melt to the Earth’s surface. However, the details about melt transport through the brittle crust and storage under narrow rift-aligned magmatic segments remain largely unclear. Here we present a crustal scale electrical conductivity model for a magmatic segment in the Ethiopian Rift, derived from 3-D phase tensor inversion of magnetotelluric data. Our subsurface model shows that melt migrates along pre-existing weak structures and is stored in different concentrations on two major interconnected levels, facilitating the formation of a convective hydrothermal system. The obtained model of a transcrustal magmatic system offers new insights into rifting mechanisms, evolution of magma ascent, and prospective geothermal reservoirs. 4 Kuvshinov, A., J. Matzka, B. Poedjono, F. Samrock, N. Olsen, and S. Pai Probing Earth’s conductivity structure beneath oceans by scalar geomagnetic data: autonomous surface vehicle solution, Earth, Planets and Space, 68 (1)/189, 2016. 3 Bakker, J., A. Kuvshinov, F. Samrock, A. Geraskin, and O. Pankratov Introducing inter-site phase tensors to suppress galvanic distortion in the telluric method, Earth, Planets and Space: EPS, 67/1, pp. 160, 2015. AbstractA common problem when interpreting magnetotelluric (MT) data is that they often are distorted by shallow unresolvable local structures, an effect known as galvanic distortion. We present two transfer functions that are (almost) resistant to galvanic distortion. First, we introduce the electric phase tensor, which is derived from the electric tensor, where the electric tensor relates the horizontal electric fields at a field and base site. The electric phase tensor is only affected by galvanic distortion, if present, at the base site. Second, we introduce the quasi-electric phase tensor, which is derived from the quasi-electric tensor, where the quasi-electric tensor relates the electric field at a field site with the magnetic field at a base site. The quasi-electric tensor is not affected by galvanic distortion. Using a synthetic data-set, we show that the sensitivity of the MT phase tensor, the quasi-electric phase tensor, and the electric phase tensor is comparable for our model under consideration. Furthermore, we demonstrate that stable (quasi-) electric phase tensors can be recovered from a real data-set with the use of existing processing software. In addition, we provide a formalism to propagate the uncertainties from the estimated (quasi-) electric and impedance tensors to their respective phase tensors. The uncertainties of the (quasi-) electric phase tensors are of the same order of magnitude as the uncertainties of the MT phase tensor. From our study, we conclude that the (quasi-) electric phase tensors are an attractive complement to the standard MT responses. 2 Samrock, F., A. Kuvshinov, J. Bakker, A. Jackson, and F. Shimeles 3-D analysis and interpretation of magnetotelluric data from the Aluto-Langano geothermal field, Ethiopia, Geophysical Journal International, 202/3, pp. 1923-1948, 2015. AbstractThe Main Ethiopian Rift Valley encompasses a number of volcanoes, which are known to be actively deforming with reoccurring periods of uplift and setting. One of the regions where temporal changes take place is the Aluto volcanic complex. It hosts a productive geothermal field and the only currently operating geothermal power plant of Ethiopia. We carried out magnetotelluric (MT) measurements in early 2012 in order to identify the source of unrest. Broad-band MT data (0.001-1000 s) have been acquired at 46 sites covering the expanse of the Aluto volcanic complex with an average site spacing of 1 km. Based on this MT data it is possible to map the bulk electrical resistivity of the subsurface down to depths of several kilometres. Resistivity is a crucial geophysical parameter in geothermal exploration as hydrothermal and magmatic reservoirs are typically related to low resistive zones, which can be easily sensed by MT. Thus by mapping the electrical conductivity one can identify and analyse geothermal systems with respect to their temperature, extent and potential for production of energy. 3-D inversions of the observed MT data from Aluto reveal the typical electrical conductivity distribution of a high-enthalpy geothermal system, which is mainly governed by the hydrothermal alteration mineralogy. The recovered 3-D conductivity models provide no evidence for an active deep magmatic system under Aluto. Forward modelling of the tippers rather suggest that occurrence of melt is predominantly at lower crustal depths along an off-axis fault zone a few tens of kilometres west of the central rift axis. The absence of an active magmatic system implies that the deforming source is most likely situated within the shallow hydrothermal system of the Aluto-Langano geothermal field. 1 Samrock, F., and A. Kuvshinov Tippers at island observatories: Can we use them to probe electrical conductivity of the Earth’s crust and upper mantle?, Geophysical Research Letters – AGU Journal, 40/5, pp. 824-828, 2013. Abstract[1] For decades, time series of hourly-mean values of the geomagnetic field measured on a global network of observatories have been routinely used to recover the electrical conductivity distribution in midmantle depths. Nowadays, most observatories provide data in the form of minute-means. This allows for analysis of short-period geomagnetic variations, which, in principle, contain information about geoelectric structures in the crust and upper mantle. However, so far these data have been ignored for induction studies of the Earth due to a theoretical preconception. In this paper, we demonstrate that short-period responses (tippers) at island observatories, being large owing to the ocean effect, are also sensitive to 1-D structures and thus can be used for probing the Earth. This means that a huge amount of data that was not exploited hitherto for induction studies should be reconsidered as a useful source of information about geoelectric structures in oceanic regions where our knowledge is still very limited.

### THESES

 6 Morschhauser, A., A.V. Grayver, A. Kuvshinov, F. Samrock, and J. Matzka Tippers at island geomagnetic observatories constrain electrical conductivity of oceanic lithosphere and upper mantle, Earth, Planets and Space, 71/17, pp. 1-9, 2019. AbstractGeomagnetic field variations as recorded at geomagnetic observatories are important for global electromagnetic studies. However, this data set is rarely used for studying the local electrical conductivity at depths $<200$ km. The main reasoning being that given a single geomagnetic observatory, one can at most constrain the one-dimensional (1-D) conductivity structure beneath it. At the same time, tippers, magnetic transfer functions resolving these depths, are zero for any 1-D conductivity distribution. We show that the ocean induction effect alleviates these limitations for observatories on islands and develop a method to invert tippers for a 1-D conductivity profile in the presence of three-dimensional conductivity structure due to bathymetry. This allows to recover 1-D upper mantle conductivity profiles at remote oceanic locations where little or no knowledge is available and that would otherwise be difficult to access. We apply the method to Gan in the Indian Ocean and to Tristan da Cunha in the South Atlantic, and the obtained conductivity profiles indicate a normal oceanic mantle and elevated conductivities, respectively, which fits well with their geological settings. 5 Samrock, F., A.V. Grayver, H. Eysteinsson, and M.O. Saar Magnetotelluric image of transcrustal magmatic system beneath the Tulu Moye geothermal prospect in the Ethiopian Rift, Geophysical Research Letters, 2018. AbstractContinental rifting is initiated by a dynamic interplay between tectonic stretching and mantle upwelling. Decompression melting assists continental break-up through lithospheric weakening and enforces upflow of melt to the Earth’s surface. However, the details about melt transport through the brittle crust and storage under narrow rift-aligned magmatic segments remain largely unclear. Here we present a crustal scale electrical conductivity model for a magmatic segment in the Ethiopian Rift, derived from 3-D phase tensor inversion of magnetotelluric data. Our subsurface model shows that melt migrates along pre-existing weak structures and is stored in different concentrations on two major interconnected levels, facilitating the formation of a convective hydrothermal system. The obtained model of a transcrustal magmatic system offers new insights into rifting mechanisms, evolution of magma ascent, and prospective geothermal reservoirs. 4 Kuvshinov, A., J. Matzka, B. Poedjono, F. Samrock, N. Olsen, and S. Pai Probing Earth’s conductivity structure beneath oceans by scalar geomagnetic data: autonomous surface vehicle solution, Earth, Planets and Space, 68 (1)/189, 2016. 3 Bakker, J., A. Kuvshinov, F. Samrock, A. Geraskin, and O. Pankratov Introducing inter-site phase tensors to suppress galvanic distortion in the telluric method, Earth, Planets and Space: EPS, 67/1, pp. 160, 2015. AbstractA common problem when interpreting magnetotelluric (MT) data is that they often are distorted by shallow unresolvable local structures, an effect known as galvanic distortion. We present two transfer functions that are (almost) resistant to galvanic distortion. First, we introduce the electric phase tensor, which is derived from the electric tensor, where the electric tensor relates the horizontal electric fields at a field and base site. The electric phase tensor is only affected by galvanic distortion, if present, at the base site. Second, we introduce the quasi-electric phase tensor, which is derived from the quasi-electric tensor, where the quasi-electric tensor relates the electric field at a field site with the magnetic field at a base site. The quasi-electric tensor is not affected by galvanic distortion. Using a synthetic data-set, we show that the sensitivity of the MT phase tensor, the quasi-electric phase tensor, and the electric phase tensor is comparable for our model under consideration. Furthermore, we demonstrate that stable (quasi-) electric phase tensors can be recovered from a real data-set with the use of existing processing software. In addition, we provide a formalism to propagate the uncertainties from the estimated (quasi-) electric and impedance tensors to their respective phase tensors. The uncertainties of the (quasi-) electric phase tensors are of the same order of magnitude as the uncertainties of the MT phase tensor. From our study, we conclude that the (quasi-) electric phase tensors are an attractive complement to the standard MT responses. 2 Samrock, F., A. Kuvshinov, J. Bakker, A. Jackson, and F. Shimeles 3-D analysis and interpretation of magnetotelluric data from the Aluto-Langano geothermal field, Ethiopia, Geophysical Journal International, 202/3, pp. 1923-1948, 2015. AbstractThe Main Ethiopian Rift Valley encompasses a number of volcanoes, which are known to be actively deforming with reoccurring periods of uplift and setting. One of the regions where temporal changes take place is the Aluto volcanic complex. It hosts a productive geothermal field and the only currently operating geothermal power plant of Ethiopia. We carried out magnetotelluric (MT) measurements in early 2012 in order to identify the source of unrest. Broad-band MT data (0.001-1000 s) have been acquired at 46 sites covering the expanse of the Aluto volcanic complex with an average site spacing of 1 km. Based on this MT data it is possible to map the bulk electrical resistivity of the subsurface down to depths of several kilometres. Resistivity is a crucial geophysical parameter in geothermal exploration as hydrothermal and magmatic reservoirs are typically related to low resistive zones, which can be easily sensed by MT. Thus by mapping the electrical conductivity one can identify and analyse geothermal systems with respect to their temperature, extent and potential for production of energy. 3-D inversions of the observed MT data from Aluto reveal the typical electrical conductivity distribution of a high-enthalpy geothermal system, which is mainly governed by the hydrothermal alteration mineralogy. The recovered 3-D conductivity models provide no evidence for an active deep magmatic system under Aluto. Forward modelling of the tippers rather suggest that occurrence of melt is predominantly at lower crustal depths along an off-axis fault zone a few tens of kilometres west of the central rift axis. The absence of an active magmatic system implies that the deforming source is most likely situated within the shallow hydrothermal system of the Aluto-Langano geothermal field. 1 Samrock, F., and A. Kuvshinov Tippers at island observatories: Can we use them to probe electrical conductivity of the Earth’s crust and upper mantle?, Geophysical Research Letters – AGU Journal, 40/5, pp. 824-828, 2013. Abstract[1] For decades, time series of hourly-mean values of the geomagnetic field measured on a global network of observatories have been routinely used to recover the electrical conductivity distribution in midmantle depths. Nowadays, most observatories provide data in the form of minute-means. This allows for analysis of short-period geomagnetic variations, which, in principle, contain information about geoelectric structures in the crust and upper mantle. However, so far these data have been ignored for induction studies of the Earth due to a theoretical preconception. In this paper, we demonstrate that short-period responses (tippers) at island observatories, being large owing to the ocean effect, are also sensitive to 1-D structures and thus can be used for probing the Earth. This means that a huge amount of data that was not exploited hitherto for induction studies should be reconsidered as a useful source of information about geoelectric structures in oceanic regions where our knowledge is still very limited.