S U M M A R YMagnetotelluric (MT) response function estimates can be severely disturbed by the effects of cultural noise. Methods to isolate and remove these disturbances are typically based on time-series editing, robust statistics, remote reference processing, or some combination of the above. Robust remote reference processing can improve the data quality at a local site, but only if synchronous recordings of at least one additional site are available and if electromagnetic noise between these sites is uncorrelated. If these prerequisites are not met, we suggest an alternative approach for noise removal, based on a combination of frequency domain editing with subsequent single site robust processing. The data pre-selection relies on a thorough visual inspection of a variety of statistical parameters such as spectral power densities, coherences, the distribution of response functions and their errors, etc. Extreme outliers and particularly noisy data segments are excluded from further data processing by setting threshold values for individual parameters. Examples from Namibia and Jordan illustrate that this scheme can improve data quality significantly. However, the examples also suggest that it is not possible to establish generally valid rules for selection as they depend strongly on the local noise conditions. High coherence, for example, can indicate a good signal-to-noise ratio or strongly correlated noise. However, we found that strong polarization of the magnetic field channels and the distribution of response function errors are two important parameters for noise detection.The magnetotelluric (MT) method is based on measuring time variations of orthogonal components of electric and magnetic fields at the surface of the Earth. The MT impedance tensor Z, which generally should be a time invariant quantity is the response of the Earth to electromagnetic induction and carries information about the conductivity distribution of the subsurface. In the frequency domain, the electromagnetic fields are assumed to be linearly related by the impedance tensor Z (e.g. Berdichevsky 1960Berdichevsky , 1964Tikhonov & Berdichevsky 1966):with E being the electric field in mV km −1 , B the magnetic field in nT and Z i j (i, j = x, y) the components of the impedance tensor Z in units of m s −1 . A similar relation can be postulated for the vertical magnetic field (e.g. Schmucker 1970):with T x and T y as the geomagnetic transfer functions. More generally, the relations above can be described by the following expression:Usually, the output channel X is associated with either E x , E y or B z and the input channels Y 1 and Y 2 with B x and B y , respectively (see eqs 1 and 2); Z 1 and Z 2 are response functions of a linear equation system. In general, the estimation procedure for the components of Z as well as T x and T y is based on least-squares (LSQ) methods; the parameter estimate is chosen in order to minimize the misfit between the predicted (right side of eq. 3) and observed output variable (left side of eq. 3) by mini...
The alternating magnetic dynamo field of sea surface waves, a consequence of their Lorentz electric field, has been observed with a pair of simultaneously operated, closely spaced tri-axial magnetometers. Measurements from a magnetometer located in the centre of a tiny, uninhabited island served to compensate measurements from a near-shore magnetometer for magnetic pulsations of ionospheric origin, leaving the water wave dynamo field, effective close to shore only, as the dominant residual magnetic field. Amplitude and frequency of waves and swell were recorded with a vertical accelerometer (wave rider buoy), floating on the sea surface. The wave rider data are in good agreement with those obtained from the magnetometers. Amplitude and phase relations between the three vector components of the magnetic oscillations yield a sea surface wave vector which is consistent with the swell propagation direction usually found in that area. The magnetic field data further demonstrate that the water mass motion close to shore was not confined to a vertical plane (as would be the case for freely propagating gravity waves in the open ocean). The motion rather took place in a plane inclined at about 40° from the horizontal, which is roughly twice the inclination of the island flanks. We conclude that the magnetometer measurements yield a reasonably accurate description of the surface wave water motion within about one wavelength from shore.
The alternating magnetic dynamo field of sea surface waves, a consequence of their Lorentz electric field, has been observed with a pair of simultaneously operated, closely spaced tri-axial magnetometers. Measurements from a magnetometer located in the centre of a tiny, uninhabited island served to compensate measurements from a near-shore magnetometer for magnetic pulsations of ionospheric origin, leaving the ocean wave dynamo field, effective close to shore only, as the dominant residual magnetic field. Amplitude and frequency of waves and swell were recorded with a vertical accelerometer (wave rider buoy) floating nearby on the sea surface. A spectral analysis was performed on ten nighttime intervals of three hours length each, and for every interval, the peak power of the surface waves (obtained from the wave rider) was compared with the peak power of the residual horizontal magnetic field (after the background field had been removed). The results suggest that the dual-sensor magnetic field observations yield, within the limits of statistical significance, a good quantitative description of the amplitude and frequency of sea surface waves and swell.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.