The resistivity structure of the North Alex Mud Volcano as derived from the interpretation of CSEM data

EGU2010-9841 Active mud volcanoes, where changing salinities of pore fluids, large temperature gradients and occurrences of free gas are frequently observed, should potentially exhibit significant variability in their internal resistivity structure. This is due to the fact that the bulk resistivity is mainly determined by the porosity of sediments and the electrical resistivity of the pore filling contained therein. The resistivity variations may be derived from controlled source electromagnetic (CSEM) measurements. CSEM systems consist of an electric dipole transmitter producing a time varying source field and electric dipole receivers, which measure the earth´s response to this signal. For a RWE Dea funded investigation of fluid and gas leakages at the North Alex Mud Volcano (NAMV) - a comparatively small target with an area of about 1km2 - we have developed a new high resolution CSEM system. The system consists of several autonomous electric dipole receivers and a lightweight electric dipole transmitter, which can be mounted on a small remotely operated underwater vehicle (ROV). The use of a ROV allows for a precise placement of the transmitter, which is a necessary prerequisite for the investigation of such a small target. Furthermore, electromagnetic signals may be transmitted from different directions with respect to the stationary receivers, allowing for a 3D-style tomographic experiment. In this experiment, ten receivers were deployed over the surface of NAMV at a total of 16 receiver locations. During three successful dives with a Cherokee ROV (Ghent University, Belgium), the transmitter was deployed at a total of 80 locations. Here we present first quantitative results consisting of apparent resistivity estimations from the CSEM time domain data for each transmitter-receiver pair. The apparent resistivity map shows that the NAMV indeed has a heterogeneous resistivity structure with apparent resistivities varying by at least a factor of two: low apparent resistivities (~ 0.8Ωm) are found towards the center of the MV, whereas higher apparent resistivities (~ 1.6Ωm) prevail away from the center. In a second step, we interpret the time-domain data based on 1D inversions. Good data fits can be achieved by models containing 2-3 layers. Generally, the models indicate low resistivities at the surface, which can be associated with penetrating salt water and/or high temperatures. Toward greater depths, increasing resistivities presumably are due to a combination of compaction of sediments (i.e. reduced pore space), an increased presence of fresh water and possible occurrences of free gas. For some 1D models, the increase in resistivity exceeds a factor of 10 or more and layer interfaces are indicated down to depths of up to 70m. The derived resistivity variations observed at the NAMV will be interpreted in conjunction with temperature (Feseker, this session), fluid flow (Brückmann et al., this session) and seismic data (Bialas et al., this session) acquired. Temperature variations measured in the upper few meters are related to fluid flow, where high temperatures are indicative of upwelling fluids of low salinity and low temperature of either a downward flow of saline fluids or no flow activity. This type of surface measurement constitutes an integrative fluid flow gauge, which we can resolve vertically with our resistivity models. Seismic data yield a background structure to our resistivity model. New analysis of seismic data shows that seismic activity may also be linked to fluid flow activity, which we aim to match with resistivity variations and oscillations, which were observed in the electric and magnetic fields (Lefeldt et al., this session)

Five years of marine research using EM methods at the IFM-GEOMAR

Even though first experiments for the characterization of the seafloor using marine electromagnetic (EM) methods were already carried out in the mid 1960’s, they have only played a minute role in marine academic investigation for several decades. Only in the past decade, the strongly increasing interest of oil companies for alternative investigation methods for marine oil and gas exploration brought the use of EM methods into the focus of attention. Traditional founders of marine EM methods (Scripps, U of Toronto, U of Southampton) are now accompanied by newly established commercial (e.g. Exxon, AOA Geophysics, OHM surveys, EMGS, Statoil) as well as academic groups. The marine EM group at the IFM GEOMAR, which was established in 2006, initially focused on the development and testing of EM receivers (RX) for magnetotelluric (MT) measurements. Successful measurements were taken during a cruise to the Costa Rican trench (2007/08, see Worzewski, this session). However, these measurements revealed some problems with this first generation of instruments (e.g. stability of stations on the ocean-floor). A subsequent, much improved generation of MT receivers developed in 2008 was successfully deployed during cruises to the Alboran Sea (2009) and the Cyprus Arc (2010) and is currently used in investigations of the Walvis Ridge (Namibia, 2011) and the New Zealand Subduction Zone (2011). For a RWE Dea funded project at the North Alex Mud Volcano (NAMV), a second line of development at the IFM-GEOMAR focused on development of controlled source electromagnetic (CSEM) equipment. For this first project, safety concerns (slop stability) as well as the comparatively small size of the investigated target ([ca.] 1km2) required a new approach to allow for a secure, high resolution CSEM experiment. For this type of experiment, the existing MT receivers were extended to include a high frequency CSEM mode (10kHz) for the electric fields. Additionally, a lightweight electric dipole transmitter (TX), which can be mounted on a small remotely operated underwater vehicle (ROV) was developed. In a 3D-style tomographic experiment (Nov. 2008), ten receivers were deployed over the surface of NAMV at a total of 16 receiver locations and in three successful dives with a Cherokee ROV (Ghent University, Belgium), the transmitter was deployed at a total of 80 locations. Since both RXs and TX were stationary during measurements, a small dipole moment of 200Am (20A current, 10m dipole length) was sufficient to collect transient data up to RX-TX distances of more than 1km. Generally, navigational inaccuracy limits the accuracy and thus also the resolution of CSEM measurements, which is mainly due to the constantly moving sources used in most commercial systems. The good quality of data recorded during the initial experiment at the NAMV raises the question, if this issue may for some types of CSEM experiments may be remedied by using stationary transmitters instead of flying sources. During the upcoming experiment in New Zealand (April 2011), we will find some answers to this question with our new CSEM transmitter system, which has a higher dipole moment ([ca.] 1kAm) and the capability to perform the navigation between TX and the RXs directly on the ocean floor

Using pressure and seismological broadband ocean data model shear wave velocities in the North Atlantic

EGU2010-10518 Seafloor compliance is the transfer function between pressure and vertical displacement at the seafloor Infragravity waves in the oceanic layer have long periods in the range of 30 – 500 s and obey a simple frequencywavenumber relation. Seafloor compliance from infragravity waves can be analyzed with single station recordings to determinate sub-seafloor shear wave velocities. Previous studies in the Pacific Ocean have demonstrated that reliable near-surface shear wave profiles can be derived from infragravity wave compliance. However, these studies indicate that, beside the water depth the compliance measurements are limited by instrument sensitivity, calibration uncertainties and possibly other effects. In this work seafloor compliance and infragravity waves are observed at two different locations in the Atlantic Ocean: the Logatchev hydrothermal field at the Mid Atlantic Ridge and the Azores (Sao Miguel Island). The data was acquired with the broadband ocean compliance station developed at the University of Hamburg as well as ocean station from the German instrument pool for amphibian seismology (DEPAS) equipped with broadband seismometers and pressure sensors. Vertical velocity and pressure data were used to calculate power spectral densities and normalized compliance along two profiles (one in each location). Power spectral densities show a dominant peak at low frequencies (0.01-0.035Hz) limited by the expected cut-off frequency, which is dependent on the water depth at each station. The peak has been interpreted as a strong infragravity wave with values between 10-14 and 10-11 (m/s2)2/Hz and 104 and 106 (Pa2)2/Hz for acceleration and pressure respectively. The results show compliance values between 10-10 and 10-8 1/Pa and its estimations take into account the coherence between seismic and pressure signals in order to confirm that the seismic signals in the infragravity waves are caused by pressure sources. Shear wave velocity models, with depth resolution from 200 to 2500 m for the deep water stations, were derived from compliance. Preliminary results indicate shear wave velocity increasing from 200 to 3500 m/s

2. Wochenbericht MSM20/2

FS „Maria S. Merian“, MSM 20-2 17.1.2012 Walvis Bay – 16.2.2012 Recife 2. Wochenbericht (23.1. bis 29.1.

3. Wochenbericht MSM20/2

FS „Maria S. Merian“, MSM 20-­2 17.1.2012 Walvis Bay – 16.2.2012 Recife 3. Wochenbericht (30.1. bis 5.2.

Using Empirical Mode Decomposition (EMD) for the processing of marine MT data

Magnetotelluric (MT) method determines a frequency dependent impedance tensor using the spectra of associated time-varying horizontal electric and magnetic fields measured at the Earth’s surface. In this abstract, we present a dynamic time series analysis method dealing the non-stationary MT data to infer the impedance tensor. Most current methods to determine the spectra use Fourier transform based procedure and, therefore, assume that the signals are stationary over the record length. We introduce a new method for dealing with non-stationarity of the MT time series based upon empirical mode decomposition (EMD) method, a dynamic time series analysis method. Using EMD complicated data sets can be decomposed into a finite and small number of "intrinsic mode functions" (IMFs), which are mono-component signals and allow the calculation of physical meaningful instantaneous frequencies. EMD has no bias due to non-stationary of geomagnetic time series, since the IMFs are based entirely on signal characteristics and not on any given set of base functions such as sines and cosines in the Fourier transform or wavelets in the Wavelet transform. We use the EMD method to decompose MT data into IMFs and calculate the instantaneous frequencies and spectra to determine the impedance tensor. The method is tested in synthetic and real marine MT data sets, the obtained estimate results are reliable compared to frequently-used BIRRP processing method. Furthermore, new method has the possibility of noise visualization and filtering, which is especially important in marine applications, where noise free time segments maybe short

Joint inversion scheme with an adaptive coupling strategy - applications on synthetic and real data sets

Joint inversion strategies for geophysical data have become increasingly popular since they allow to combine complementary information from different data sets in an efficient way. However, for joint inversion algorithms that use methods that are sensitive to different parameters it is important that they are not restricted to specific survey arrays and subsurface conditions. Hence, joint inversion schemes are needed that 1) adequately balance data from the different methods and 2) use links between the parameter models that are suited for a wide range of applications. Here, we combine MT, seismic tomography and gravity data in a non-linear joint inversion that accounts for these critical issues. Data from the different methods are inverted separately and are joined through constrains accounting for parameter relationships. An advantage of performing the inversions separately (and not together in one matrix) is that no relative weighting between the data sets is required. To avoid that the convergence behavior of the inversions is profoundly disturbed by the coupling, the strengths of the associated constraints are re-adjusted at each iteration. As criteria to control the adaption of the coupling strengths we used a general version of the well-known discrepancy principle. Adaption of the coupling strengths makes the joint inversion scheme also applicable to subsurface conditions, for which the assumed relationships are only a rough first order approximation. So, the coupling between the different parameter models is automatically reduced if for some structures the true rock property behaviors differ significantly from the assumed relationships (e.g. the atypical density-velocity behavior of salt). We have tested our scheme first on different synthetic 2-D models for which the assumed parameter relationships are everywhere valid. We observe that the adaption of the coupling strengths makes the convergence of the inversions very robust and that the final results are close to the true models. In a next step the scheme has been applied on models for which the assumed parameter relationships are invalid for some structures. For these structures deviations from the relationships are present in the final results; however, for the remaining structures the relative behaviors of the physical parameters are still approximately described by the assumed relationship. Finally, we applied our joint inversion scheme on seismic, MT and gravity data collected offshore the Faroe Islands, where basalt intrusions are present

Magnetotelluric image of the fluid cycle in the Costa Rican subduction zone

Fluids entering the subduction zone are a key factor in the subduction process. They determine the onset of melting, weakening and changes in the dynamics and thermal structure of subduction zones and trigger earthquakes when being released from the subducting plate in a series of metamorphic processes. However, the amount of water carried into the subduction zone and its distribution are not well constrained by existing data and are subject of vigorous current research in SFB574 (Volatiles and Fluids in Subduction Zones: Climate Feedback and Trigger Mechanisms for Natural Disasters). Electromagnetic methods like magnetotellurics have been used widely to recognize fluid release and melt production through enhanced electrical conductivities. Here we present an image of the hydration and dehydration cycle down to 120 km depth in one setting derived by an onshore-offshore transect of magnetotelluric soundings in Costa Rica. An electrically conductive zone in the incoming plate outer rise is associated with sea water penetrating down extensional faults and cracks into the upper mantle possibly causing serpentinization. Along the downward subducting plate distinct conductive anomalies identify fluids from dehydration of sediments, crust and mantle. A conductivity anomaly at a depth of approx. 12 km and at a distance of 65 km from the trench is associated with a first major dehydration reaction of minerally-bound water. This is of importance in the context of mid-slope fluid seeps which are thought to significantly contribute to the recycling of minerally-bound water. The position of the conductivity anomaly correlates with geochemical and seismic evidence stating that mid-slope fluids are originated at >=12 km depth before rising up through deep faults to the seeps. The conductivity anomaly is therefore associated with a fluid accumulation feeding the mid-slope seeps. Another fluid accumulation is revealed by a conductivity anomaly at 20-30 km depth and a distance of approximately 30 km seaward from the volcanic arc. This lower crustal fluid accumulation could likely be caused by trapping of fluids released due to de-serpentinization processes or due to other mineral dehydration processes. While we are at the moment not able to attribute one specific process causing the anomaly based on electromagnetic data alone, this feature is however of fundamental importance. A comparison with other electromagnetic studies from subduction zones around the world reveal that such a conductivity anomaly is a global feature suggesting the presence of a global fluid sink. Based on very simplified assumptions we are able derive rough estimates for the amount of water being stored in the overriding plate. Relating seismic evidence as well as petrological results collected in the multi-disciplinary study on the Costa Rican subduction zone we introduce budget estimations for the water cycle in the subduction zone

Cyclic volcanism at convergent margins: linked to aarth orbital parameters or climate changes?

EGU2010-13373 The frequency of volcanic activity varies on a wide rangeof spatial and temporal scales, from <1 yr. periodicities in single volcanic systems to periodicities of 106 yrs. in global volcanism. The causes of these periodicities are poorly understood although the long-term global variations are likely linked to plate-tectonic processes. Here we present evidence for temporal changes in eruption frequencies at an intermediate time scale (104 yrs.) using the Pleistocene to recent records of widespread tephras of sub-Plinian to Plinian, and occasionally co-ignimbrite origin, along the Pacific Ring of Fire, which accounts for about half of the global length of 44,000 km of active subduction. Eruptions at arc volcanoes tend to be highly explosive and the well-preserved tephra records from the ocean floor can be assumed to be representative of how eruption frequencies varied with time. Volcanic activity along the Pacific Ring of Fire evolved through alternating phases of high and low frequency; although there is modulation by local and regional geologic conditions, these variations have a statistically significant periodicity of 43 ka that overlaps with the temporal variation in the obliquity of the Earth’s rotation axis, an orbital parameter that also exerts a strong control on global climate changes. This may suggest that the frequency of volcanic activity is controlled by effects of global climate changes. However, the strongest physical effects of climate change occur at 100 ka periods which are not seen in the volcanic record. We therefore propose that the frequency of volcanic activity is directly influenced by minute changes in the tidal forces induced by the varying obliquity resulting in long-period gravitational disturbances acting on the upper mantle

Towards 3D joint inversion of full tensor gravity, magnetotelluric and seismic refraction data

EGU2010-4184-2 Joint inversion of different datasets is emerging as an important tool to enhance resolution and decrease inversion artifacts in structurally complex areas. Performing the inversion in 3D allows us to investigate such complex structures but requires computationally efficient forward modeling and inversion methods. Furthermore we should be able to flexibly change inversion parameters, coupling approaches and forward modeling schemes in order to find a suitable approach for the given target. We present a 3D joint inversion framework for scalar and full tensor gravity, magnetotelluric and seismic data that allows us to investigate different approaches. It consists of two memory efficient gradient based optimization techniques, L-BFGS and NLCG, and optimized parallel forward solvers for the different datasets. In addition it provides the necessary flexibility in terms of model parametrization and coupling method by completely separating the inversion parameters and geometry from the parametrization of the individual method. This separation allows us to easily switch between completely different types of parameterizations and use structural coupling as well as coupling based on parameter relationships for the joint inversion. First tests on synthetic data with a fixed parameter relationship coupling show promising results and demonstrate that 3D joint inversion is becoming feasible for realistic size models