scholarly journals Joint Inversion of Daily and Long‐Period Geomagnetic Transfer Functions Reveals Lateral Variations in Mantle Water Content

2020 ◽  
Vol 47 (10) ◽  
Author(s):  
F. D. Munch ◽  
A. V. Grayver ◽  
M. Guzavina ◽  
A. V. Kuvshinov ◽  
A. Khan
Keyword(s):  
Water Content ◽  
Transfer Functions ◽  
Joint Inversion ◽  
Long Period ◽  
Lateral Variations

Responses of a Tall Building in Los Angeles, California, as Inferred from Local and Distant Earthquakes

2016 ◽  
Vol 32 (3) ◽  
pp. 1821-1843 ◽  
Author(s):  
Mehmet Çelebi ◽  
Hasan S. Ulusoy ◽  
Nori Nakata
Keyword(s):  
Los Angeles ◽  
Transfer Functions ◽  
Tall Buildings ◽  
Tohoku Earthquake ◽  
The United States ◽  
The 2011 Tohoku Earthquake ◽  
Low Frequencies ◽  
Long Period ◽  
Period Effects ◽  
Story Building

The increasing inventory of tall buildings in the United States and elsewhere may be subjected to motions generated by near and far seismic sources that cause long-period effects. Multiple sets of records that exhibited such effects were retrieved from tall buildings in Tokyo and Osaka ∼350 km and 770 km, respectively, from the epicenter of the 2011 Tohoku earthquake. In California, very few tall buildings have been instrumented. An instrumented 52-story building in downtown Los Angeles recorded seven local and distant earthquakes. Spectral and system identification methods exhibit significant low frequencies of interest (∼0.17 Hz, 0.56 Hz, and 1.05 Hz). These frequencies compare well with those computed by transfer functions; however, small variations are observed between the significant low frequencies for each of the seven earthquakes. The torsional and translational frequencies are very close and are coupled. Beating effect is observed in at least two of the seven earthquake data.

Implications of the Born approximation for the magnetotelluric problem in three‐dimensional environments

Geophysics ◽  
1992 ◽  
Vol 57 (4) ◽  
pp. 587-602 ◽  
Author(s):  
Carlos Torres‐Verdín ◽  
Francis X. Bostick
Keyword(s):  
Magnetic Field ◽  
Electric Field ◽  
Born Approximation ◽  
Transfer Functions ◽  
Pass Filter ◽  
Near Surface ◽  
Low Pass ◽  
Surface Electric Field ◽  
Subsurface Resistivity ◽  
Lateral Variations

A first‐order Born approximation is obtained for the integral equations governing the surface magnetotelluric response over a three‐dimensional earth. Although accurate only in cases of low resistivity contrasts, the resulting expressions: (1) exhibit a linear relationship between a spatial perturbation in subsurface resistivity and the ensuing perturbation on the surface field response, and, more importantly, (2) allow arbitrary degrees of complexity in the geometrical characteristics of the subsurface. The linear system solutions derived from the Born approximation are studied by examining the properties of their associated kernels. These kernels may be thought of as a suite of horizontal magnetotelluric “wavelets” weighting the subsurface resistivity distribution at different depth levels. Analytical expressions for the wavelets are obtained in the wavenumber domain, thus generating a suite of magnetotelluric “transfer functions.” Expressions for the latter are particularized to the cases of one‐ and two‐dimensional geolectric media yielding results consistent with the characteristics of the magnetotelluric fields known to hold in these low‐order environments. Inspection of the electric transfer functions reveals severe sensitivity to near‐surface lateral variations of resistivity, which persists even at deep sensing frequencies. This near‐surface sensitivity is the result of an additive term in the electric field transfer functions, the static component, acting as a spatial highpass filter of the lateral variations of surface resistivity. A second additive component in the electric transfer functions, the induction component, functions as a spatial lowpass filter of the lateral variations in subsurface resistivity, and is primarily responsible for the inductive part of the surface electric field response. A common problem in magnetotelluric interpretation, the electric static effect can be reduced by inverting the role of the static component, i.e., by spatially low‐pass filtering the surface electric field. The suggested low‐pass filter for such an operation is one for which the cutoff wavenumber increases with frequency and is therefore insensitive to the response from the induction component. Low‐pass filtering of the surface electric field is best implemented in the field if the electric dipoles are deployed end‐to‐end continuously along a survey path. The magnetic field transfer functions, on the other hand, exhibit a single induction term with band‐pass filter properties which may actually lead to some amount of local distortion on the measured surface magnetic field. We propose to reduce this distortion by referring all electric field measurements to the primary magnetic field within the survey area. The primary magnetic field components, in turn, can be estimated by the spatial average of the magnetic measurements acquired at an array of magnetic stations. The suggested procedures for both the acquisition and processing of natural electric and magnetic field data encompass altogether a novel adaptation of the magnetotelluric method.

Lateral variations in D″ thickness from long-period shear wave data

1994 ◽  
Vol 99 (B6) ◽  
pp. 11575-11590 ◽  
Author(s):  
J.-M. Kendall ◽  
P. M. Shearer
Keyword(s):  
Shear Wave ◽  
Long Period ◽  
Wave Data ◽  
Lateral Variations

Geomagnetic depth sounding and magnetotelluric results from a seismically active region northeast of Quebec City

1977 ◽  
Vol 14 (2) ◽  
pp. 256-267 ◽  
Author(s):  
Y. Honkura ◽  
R. D. Kurtz ◽  
E. R. Niblett
Keyword(s):  
Active Region ◽  
Transfer Functions ◽  
Impedance Tensor ◽  
Quebec City ◽  
Electric Currents ◽  
Magnetotelluric Data ◽  
Geomagnetic Induction ◽  
Tensor Methods ◽  
Depth Sounding ◽  
Lateral Variations

Observations of geomagnetic and telluric field variations have been made at four stations near La Malbaie, Quebec, in a seismically active region northeast of Quebec City. The data were analysed by transfer function and impedance tensor methods to study the nature of geomagnetic induction in the region and its structural implications. Transfer functions and telluric fields were influenced by several factors including the nearby St. Lawrence River, lateral variations in conductivity caused by geological complexities, and conduction of electric currents probably originating through induction in the ocean.It is demonstrated that it is possible to distinguish between induction by the vertical geomagnetic field and induction by the horizontal components. Anomalous vertical fields caused by vertical field induction are observed in the area and the electric currents associated with them have a flow pattern which is similar to that of currents induced by the horizontal field. Further, the Z-induced currents changed impedance tensor elements by as much as 20% though the effect was usually smaller than this. Nonetheless, the magnetotelluric data confirmed the presence of a conductivity anomaly in the Laurentides Park region of Quebec.

Transfer functions for the seismic research observatory seismograph system

1978 ◽  
Vol 68 (2) ◽  
pp. 501-512 ◽  
Author(s):  
Douglas W. McCowan ◽  
Richard T. Lacoss
Keyword(s):  
Polynomial Approximation ◽  
Group Delay ◽  
Transfer Functions ◽  
Phase Response ◽  
Impulse Responses ◽  
Long Period ◽  
Short Period ◽  
Group Delays

Abstract Transfer functions are given for all basic elements of the Seismic Research Observatory (SRO) seismograph system. These include the seismometer, response shaping filters, and anti-alias filters. With these transfer functions the amplitude and phase responses can be calculated at any frequency by the evaluation of a ratio of polynomials in s = iω. Amplitude and phase response plots and impulse responses are given for the most important overall transfer functions. In addition, long- and short-period group delays have been calculated by numerical differentation of phase responses. In the long-period case a simple polynomial approximation to the group delay is given as an aid in interpretation of data. The transfer functions are exact and follow from the integral design of the system.

scholarly journals 3-D joint inversion of the magnetotelluric phase tensor and vertical magnetic transfer functions

2015 ◽  
Vol 203 (2) ◽  
pp. 1128-1148 ◽  
Author(s):  
Kristina Tietze ◽  
Oliver Ritter ◽  
Gary D. Egbert
Keyword(s):  
Transfer Functions ◽  
Joint Inversion ◽  
Phase Tensor ◽  
Magnetic Transfer
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