3-D Magnetotelluric Image of Offshore Magmatism at the Walvis Ridge and Rift Basin

Highlights • We report on marine 3D Magnetotelluric study on Walvis Ridge • Derived 3D electrical resistivity model shows a large scale resistive zone, which we link to crustal extension due to local uplift. It might indicate the location where the hot-spot impinged on the crust prior to rifting • Smaller scale resistive region is attributed to magma ascent during rifting • Rift basin is identified by low resistivity region The Namibian continental margin marks the starting point of the Tristan da Cunha hotspot trail, the Walvis Ridge. This section of the volcanic southwestern African margin is therefore ideal to study the interaction of hotspot volcanism and rifting, which occurred in the late Jurassic/early Cretaceous. Offshore magnetotelluric data image electromagnetically the landfall of Walvis Ridge. Two large-scale high resistivity anomalies in the 3-D resistivity model indicate old magmatic intrusions related to hot-spot volcanism and rifting. The large-scale resistivity anomalies correlate with seismically identified lower crustal high velocity anomalies attributed to magmatic underplating along 2-D offshore seismic profiles. One of the high resistivity anomalies (above 500 Ωm) has three arms of approximately 100 km width and 300 km to 400 km length at 120 degree angles in the lower crust. One of the arms stretches underneath Walvis Ridge. The shape is suggestive of crustal extension due to local uplift. It might indicate the location where the hot-spot impinged on the crust prior to rifting. A second, smaller anomaly of 50 km width underneath the continent ocean boundary may be attributed to magma ascent during rifting. We attribute a low resistivity anomaly east of the continent ocean boundary and south of Walvis Ridge to the presence of a rift basin that formed prior to the rifting

Reducing motion noise in marine magnetotelluric measurements by means of tilt records

The analysis of marine magnetotelluric data is often complicated by disturbing signals that are caused by small-scale periodic movements of the instrument. The motion-induced noise leads to a bias and/or severe scattering in the derived magnetotelluric transfer functions. Both the motion itself and its effects on the magnetic and telluric time-series are investigated in this study using an 80 d magnetotelluric data set that includes dynamic tilt records measured in the Pacific Ocean off Costa Rica. We apply a standard motion removal technique as well as a newly developed method to correct for motion-induced noise. The resulting magnetotelluric transfer functions are of significantly better quality than the uncorrected ones. Furthermore, the study of the properties of motion noise leads to conclusions about the optimal processing approach even in case of data sets where an explicit correction for that noise is not possible

ein Vergleich magnetotellurischer Prozessingmethoden auf der Grundlage Kleinster Quadrate

Title and contents 1 Introduction: On comparableness of magnetotelluric processing methods 5 1\. Fundamentals: The single site method 9 1.1 The impedance equation and its solution 9 1.2 On cascade decimation, Fourier coefficients, spectra, and instruments' responses 14 1.3 Noise 21 1.3.1 Statistic noise in output channels 21 1.3.2 Statistic noise in input channels 26 1.3.3 Correlated noise 27 2\. Evading noise: The Remote Reference technique 31 2.1 New transfer functions due to a second site 31 2.2 Application to data in need 34 2.3 Requirements to a remote site 40 3\. Inspecting the evil: Larsen's Signal-Noise Separation 50 3.1 Description of the method 51 3.1.1 Discerning signal and noise 51 3.1.2 A transfer function for correlated noise 54 3.1.3 Some general consequences 57 3.2 Comparison with Remote Reference 59 3.2.1 The debate and open questions 60 3.2.2 Answers from a consistent least-square point of view 61 4\. Rescuing the Separation approach: Oettinger's extension 71 4.1 A remote reference approach stabilizes the Separation tensor 71 4.2 Supporting independency of input variables 78 4.3 Information about correlated-noise sources 90 Conclusions 97 Acknowledgements 99 Bibliography 101Two magnetotelluric processing methods, the remote reference technique (RR) and the signal-noise separation (SNS) after Larsen and after Oettinger, have been compared. The authors of the SNS claim to have improved the RR due to a more detailed conditional equation for the desired transfer functions, i.e. there are taken into account additional transfer functions for correlated noise. "Correlated noise" is a term for artificial electromagnetic signals emitted by near-by sources (in the present case: by electric railways in Poland and Berlin run by direct current) that breach the far-field condition, which is essential in magnetotellurics. Such noise causes big problems in transfer functions and makes it necessary to include data of a reference station situated beyond the reach of the artificial signal into the processing, as it is done in both RR and SNS. The comparison has taken place under equal conditions, i.e. except for the conditional equations themselves the entire mathematic environment of the processing has been kept the same. Thereby the suspicion could be confirmed that both methods yield virtually the same results for the magnetotelluric transfer functions.In der vorliegenden Arbeit wurden zwei magnetotellurische Prozessingmethoden, die Remote-Referenz-Technik (RR) und die Signal-Noise-Separation (SNS) nach Larsen und nach Oettinger, miteinander verglichen. Die Autoren der SNS erheben den Anspruch die RR verbessert zu haben, indem sie eine genauere Bestimmungsgleichung für die gesuchten Übertragungsfunktionen benutzen: sie berücksichtigen zusätzliche Übertragungsfunktionen für korreliertes Rauschen. "Korreliertes Rauschen" ist ein Ausdruck für künstliche elektromagnetische Signale, die von Quellen in der Nähe ausgehen (im vorliegenden Fall von mit Gleichstrom betriebenen Eisenbahnen in Polen und Berlin) und damit die Fernfeldnäherung verletzen, die in der Magnetotellurik essentiell ist. Diese Art von Rauschen verursacht große Probleme in den Übertragungsfunktionen und macht eine Einbindung von Daten einer Referenzstation, die sich außerhalb der Reichweite des künstlichen Signals befindet, sehr ratsam. Sowohl die RR als auch die SNS tun dies. Der Vergleich hat unter gleichen Bedingungen stattgefunden, d.h. außer den Bestimmungsgleichungen selbst gibt es keine Unterschiede in der gesamten mathematische Prozessingumgebung. Damit konnte die Vermutung bestätigt werden, dass beide Methoden für die magnetotellurischen Übertragungsfunktionen praktisch dieselben Ergebnisse liefern

Crustal Structure of the nascent Fonualei Rift and Spreading Center, Lau basin, derived from Magnetotelluric Measurements

The greater area of the Lau Basin consists of a mosaic of microplates with various spreading centers and arc rifts. Repeated crustal recycling and addition of volatiles and metals in this tectonic setting leads to fertilization of the crust. This assimilation of mineral deposits is associated with building blocks of continental crust, and is thus an important setting to study crustal growth on Earth. During research cruise SO267 in 2018/2019, multi method geophysical and geological data were acquired on the newly formed Niuafo’ou microplate in the Lau Basin to study the links of rifting, magmatism and hydrothermal circulation in the early evolution of back arc spreading. The geophysical data encompass refraction- and reflection seismic, seismological, backscatter, gravity, magnetic and magnetotelluric data. Here we present a first electrical conductivity model derived from 3D inversion of marine magnetotelluric (MT) data. Electrical conductivity is a key proxy for imaging hydrothermal circulation and magmatic processes, since both processes effect the bulk electrical conductivity strongly. Our model exhibits three larger scale crustal conductivity anomalies. Based on the electrical conductivity model alone, we cannot discern whether the anomalies are caused by hydrothermal activity through pathways created by tectonic spreading or by melt accumulation caused by magmatic processes. However, a spatial correlation of our major conductivity anomaly with a seismicity cluster and a comparison with seismic velocity and backscatter data allows us to hypothesizeinfer about the geodynamic processes acting in the region. Our study documents the benefit of integrating different geophysical methods to understand rift arc evolution