Reduction to the pole at low latitudes by Wiener filtering

Geophysics ◽  
1989 ◽  
Vol 54 (12) ◽  
pp. 1607-1613 ◽  
Author(s):  
R. O. Hansen ◽  
R. S. Pawlowski
Keyword(s):  
Frequency Domain ◽  
Synthetic Data ◽  
Wiener Filtering ◽  
Magnetic Data ◽  
Noise Power ◽  
Low Latitude ◽  
High Quality ◽  
Computational Load ◽  
Low Latitudes ◽  
Reduction To The Pole

Using simple estimates of the signal and noise power from gridded magnetic data, we design regulated frequency‐domain operators for reduction to the pole at low magnetic latitudes. These operators suppress the artifacts along the direction of the magnetic declination associated with the conventional reduction‐to‐the‐pole procedure, with negligible increase in computational load. The new procedure is applied to produce high‐quality reductions to the pole for noisy low‐latitude synthetic data and for magnetic data from the Dixon Seamount.

2.5‐D inversion of frequency‐domain electromagnetic data generated by a grounded‐wire source

Geophysics ◽  
2002 ◽  
Vol 67 (6) ◽  
pp. 1753-1768 ◽  
Author(s):  
Yuji Mitsuhata ◽  
Toshihiro Uchida ◽  
Hiroshi Amano
Keyword(s):  
Frequency Domain ◽  
Regularization Parameter ◽  
Synthetic Data ◽  
Magnetic Data ◽  
Statistical Criterion ◽  
Model Parameters ◽  
Inversion Algorithm ◽  
Gas Field ◽  
Data Set ◽  
Transient Electromagnetic

Interpretation of controlled‐source electromagnetic (CSEM) data is usually based on 1‐D inversions, whereas data of direct current (dc) resistivity and magnetotelluric (MT) measurements are commonly interpreted by 2‐D inversions. We have developed an algorithm to invert frequency‐Domain vertical magnetic data generated by a grounded‐wire source for a 2‐D model of the earth—a so‐called 2.5‐D inversion. To stabilize the inversion, we adopt a smoothness constraint for the model parameters and adjust the regularization parameter objectively using a statistical criterion. A test using synthetic data from a realistic model reveals the insufficiency of only one source to recover an acceptable result. In contrast, the joint use of data generated by a left‐side source and a right‐side source dramatically improves the inversion result. We applied our inversion algorithm to a field data set, which was transformed from long‐offset transient electromagnetic (LOTEM) data acquired in a Japanese oil and gas field. As demonstrated by the synthetic data set, the inversion of the joint data set automatically converged and provided a better resultant model than that of the data generated by each source. In addition, our 2.5‐D inversion accounted for the reversals in the LOTEM measurements, which is impossible using 1‐D inversions. The shallow parts (above about 1 km depth) of the final model obtained by our 2.5‐D inversion agree well with those of a 2‐D inversion of MT data.

Using an equivalent source with positivity for low-latitude reduction to the pole without striation

Geophysics ◽  
2014 ◽  
Vol 79 (6) ◽  
pp. J81-J90 ◽  
Author(s):  
Yaoguo Li ◽  
Misac Nabighian ◽  
Douglas W. Oldenburg
Keyword(s):  
Large Scale ◽  
Computational Cost ◽  
Equatorial Region ◽  
Magnetic Data ◽  
Data Sets ◽  
Low Latitude ◽  
Equivalent Source ◽  
Large Scale Data ◽  
Reduction To The Pole ◽  
Equivalent Sources

We present a reformulation of reduction to the pole (RTP) of magnetic data at low latitudes and the equator using equivalent sources. The proposed method addresses both the theoretical difficulty of low-latitude instability and the practical issue of computational cost. We prove that a positive equivalent source exists when the magnetic data are produced by normal induced magnetization, and we show that the positivity is sufficient to overcome the low-latitude instability in the space domain. We further apply a regularization term directly to the recovered RTP field to improve the solution. The use of equivalent source also naturally enables the processing of data acquired on uneven surface. The result is a practical algorithm that is effective at the equatorial region and can process large-scale data sets with uneven observation heights.

Differential reduction‐to‐the‐pole of regional magnetic anomalies

Geophysics ◽  
1988 ◽  
Vol 53 (12) ◽  
pp. 1592-1600 ◽  
Author(s):  
J. Arkani‐Hamed
Keyword(s):  
Frequency Domain ◽  
Regional Scale ◽  
Magnetic Anomalies ◽  
Positive Anomaly ◽  
Low Latitude ◽  
Mean Values ◽  
The North ◽  
Differential Reduction ◽  
Reduction To The Pole ◽  
Spectral Frequency

I present a differential reduction‐to‐the‐pole technique that reduces regional scale magnetic anomalies to the geomagnetic pole, while taking into account the variations in the direction of the geomagnetic field and that of the magnetization of the crust over the region. The technique is developed in the spectral frequency domain as an inverse problem solved iteratively using a perturbation method. I regard the variations in the directions as finite perturbations about the mean values of the directions over the region and evaluate the nonlinear coupling terms due to these perturbations in the space domain at each iteration before transforming them into the spectral frequency domain. The technique is applied to the magnetic anomalies of three semiinfinite prisms, which are inductively magnetized and located at high, mid, and low latitudes in a region where the inclination of the ambient field changes from 10 to 90 degrees and its declination changes from −30 to +30 degrees. The differential reduction to the pole shifts the positive anomaly of the low‐latitude prism toward the north more than it shifts that of the mid‐latitude prism, which in turn is shifted northward more than that of the high‐latitude prism. The reduction also suppresses the negative lobes to the north of the mid and low‐latitude prisms and moves the positive anomalies to points directly over the prisms. I also apply the technique to the marine magnetic anomalies off the east coast of Canada. The positive magnetic anomalies in the southern part of the area are displaced northward by about 30 km, whereas those in the northern part are not moved significantly.

scholarly journals Enhancing the quality of interpretation magnetic data at low latitudes

2017 ◽  
Vol 1 (T4) ◽  
pp. 105-114
Author(s):  
Hai Hong Nguyen ◽  
Nhan Thanh Nhan ◽  
Liet Van Dang ◽  
Thu Ngoc Nguyen
Keyword(s):  
Magnetic Anomaly ◽  
Hilbert Transform ◽  
Amplitude Spectrum ◽  
Data Interpretation ◽  
Analytic Signal ◽  
Magnetic Data ◽  
Low Latitudes ◽  
Reduction To The Pole ◽  
Signal Method

Magnetic anomalies are antisymmetrical and often skewed to the location of the sources, because both of the magnetization and ambient field are not directed vertically, so it’s difficult to interpret. For reducing the magnetic anomaly to a symmetrical one – this located on the source of the anomaly – people often use the reduction to the pole (RTP) where the magnetization and ambient field are both directed vertically. However, at low latitudes (an absolute inclination less than 16o30’), the amplitude spectrum of the RTP’s operator was amplified at higher frequencies (short wavelengths) can form a narrow pie-shaped; so it produces artifacts elongated along the direction of the magnetic declination. Therefore, many methods of RTP at low latitudes are given to solve this problem, but most of them are not efficiency. In this paper, we performed enhancing the quality of interpretation of magnetic data at low latitudes by some RTP methods for magnetic data at low latitudes and the analytic signal method using gradient operator and Hilbert transform. This method is applied to a model and to a real magnetic anomaly to find out the best method. Then this method was applied to enhance the quality of magnetic data interpretation in the Southern Vietnam. The result showed that the analytic signal method using Hilbert transform allowed enhancing the quality of interpretatio of magnetic data n at low latitudes is the best.

scholarly journals EVALUATION OF ENHANCEMENT TECHNIQUES OF MAGNETIC ANOMALIES APPLIED TO STRUCTURAL INTERPRETATION OF THE ITAITUBA REGION, STATE OF PARÁ – BRAZIL

2012 ◽  
Vol 30 (3) ◽  
Author(s):  
Alessandra De Barros e Silva Bongiolo ◽  
Francisco José Fonseca Ferreira
Keyword(s):  
Amazon Basin ◽  
Synthetic Data ◽  
Maximum Amplitude ◽  
Magnetic Anomalies ◽  
Magnetic Data ◽  
Horizontal Gradient ◽  
Structural Interpretation ◽  
Amazonian Craton ◽  
Reduction To The Pole ◽  
Tilt Derivative

The purpose of this article is to describe the work carried out for evaluating enhancement techniques of magnetic anomalies applying the reduction-to-the-pole method and its implications for structural interpretation of a region located in low magnetic latitude. With this objective, the answer given by several data enhancement methods with and without reduction-to-the-pole was analyzed. These methods were applied to synthetic prisms located at low magnetic latitudes similar to the area of analysis and the resulting anomalies were compared to those calculated at the magnetic pole. The synthetic data has been generated from a program that calculates the anomalies from prisms with arbitrary dimensions, susceptibilities and depths. The enhancement methods were also applied to magnetic data of rocks from the Amazon Basin and the Amazonian Craton, in the Itaituba region, Par´a state, northern Brazil. The reduction-to-the-pole algorithm applied to synthetic data during this work improved the performance of the enhancement methods, once, after its application, the maximum amplitude of the transformed anomalies were positioned over the edges of the sources, facilitating magnetic-structural interpretation. Good correlation among magnetic lineaments – particularly those inferred by the recently proposed tilt derivative of the total horizontal gradient method – and the already interpreted geologic structures back up the reduction to the pole indicating it may be applied even when data is collected in low magnetic latitudes.

Preliminary Interpretation of the magnetic anomalies of the Eastern Vietnam sea and adjacent region a

2018 ◽  
Vol 25 (2) ◽  
pp. 173-181
Author(s):  
Lê Huy Minh ◽  
Lưu Việt Hùng
Keyword(s):  
Magnetic Anomalies ◽  
Seafloor Spreading ◽  
Wiener Filtering ◽  
Adjacent Region ◽  
Low Latitude ◽  
Tectonic History ◽  
Geological Features ◽  
History Of ◽  
Reduction To The Pole ◽  
Filtering Approach

In this paper we showed that the reduction to the pole at low latitude based on a Wiener filtering approach of Hansen and Pawlowski could be applied on the magnetic equator if the magnetic boundaries are not paralel to magnetic meridians. Using this reduced-to-the-pole method, we have found that the geologic boundaries determined from the reduced-tothe-pole magnetic anomalies of the Eastern Vietnam sea and adjacent regions correlate remarkably well with the major geological features, including majorfaults, seafloor spreading segments, volcanic or intrusive blocks ... These results could be considered as very important informations to reconstruct the tectonic history of the region.

An improved technique for reduction to the pole at low latitudes

Geophysics ◽  
1996 ◽  
Vol 61 (1) ◽  
pp. 131-137 ◽  
Author(s):  
Pierre Keating ◽  
L. Zerbo
Keyword(s):  
Geomagnetic Latitude ◽  
Measured Data ◽  
Noise Model ◽  
Noise Power ◽  
Aeromagnetic Survey ◽  
Data Set ◽  
Low Latitudes ◽  
Reduction To The Pole ◽  
The Difference ◽  
Improved Technique

Reduction to the pole at low latitudes based on a Wiener filtering approach has been improved by introducing a deterministic noise model. It is assumed that the noise power is a fixed fraction of the signal power. This allows the method to be fully automated. Further improvement is obtained by requiring the reduced‐to‐the‐pole field to map into the observed field when projected to the geomagnetic latitude of the observed field. This is done by iteratively minimizing the difference between the measured field and the reduced‐to‐the‐pole field projected to the geomagnetic latitude of the measured data. This results in a reduced‐to‐the‐pole magnetic map that, when projected to the geomagnetic latitude of the given data, closely matches the measured data. The final reduced‐to‐the‐pole field does not show any of the artifacts typical of reduction‐to‐the‐pole at low geomagnetic latitudes. The method is demonstrated on a data set from an aeromagnetic survey flown over north‐central Burkina Faso, West Africa.

A stable truncated series approximation of the reduction‐to‐the‐pole operator

Geophysics ◽  
1993 ◽  
Vol 58 (8) ◽  
pp. 1084-1090 ◽  
Author(s):  
Carlos Alberto Mendonça ◽  
João B. C. Silva
Keyword(s):  
Synthetic Data ◽  
Magnetic Data ◽  
Theoretical Expression ◽  
Source Depth ◽  
Upward Continuation ◽  
Low Pass ◽  
Reduction To The Pole ◽  
Low Pass Filters ◽  
Infinite Sum ◽  
Taylor’S Series

We combine a stabilized reduction‐to‐the‐pole and an upward continuation filter to produce meaningful reduced‐to‐the‐pole fields at low magnetic latitudes. The stabilizing procedure is based on the development, in Taylor’s series, of the theoretical expression for the reduction‐to‐the‐pole filter in the wavenumber domain. The filter instability is caused by the huge filter amplitudes along the magnetization azimuth, which are expressed by the infinite sum of terms close to unity. The stabilizing procedure reduces to simply truncating the infinite series. The upward continuation filter attenuates the high wavenumber component of the noise and allows us to design a stabilized filter closer to the theoretical one. Besides, quantitative interpretations of source depth based on the filtered field are more reliable when using upward continuation as compared with arbitrary low‐pass filters. The proposed filter was applied to synthetic data of a single prism uniformly magnetized along a supposedly known direction, and it produced a reduced‐to‐the‐pole field very close to the theoretical field at pole. We also applied the filter to magnetic data from Dixon Seamount assuming induced magnetization only. We obtained, within the central part of the anomaly, roughly circular contours of the reduced‐to‐the‐pole anomaly due to the nearly circular shape of the Seamount (evidenced by topographic data).

The characteristics of transforming coordinates from the geocentric system WGS-84 for the Mercator projection under low latitudes conditions

2018 ◽  
Vol 940 (10) ◽  
pp. 2-6
Author(s):  
J.A. Younes ◽  
M.G. Mustafin
Keyword(s):  
Coordinate System ◽  
Satellite System ◽  
Programming Algorithm ◽  
Low Latitude ◽  
Model Calculations ◽  
Mercator Projection ◽  
Low Latitudes ◽  
Rectangular Coordinates ◽  
Global Navigation Satellite ◽  
Coordination Methods

The issue of calculating the plane rectangular coordinates using the data obtained by the satellite observations during the creation of the geodetic networks is discussed in the article. The peculiarity of these works is in conversion of the coordinates into the Mercator projection, while the plane coordinate system on the base of Gauss-Kruger projection is used in Russia. When using the technology of global navigation satellite system, this task is relevant for any point (area) of the Earth due to a fundamentally different approach in determining the coordinates. The fact is that satellite determinations are much more precise than the ground coordination methods (triangulation and others). In addition, the conversion to the zonal coordinate system is associated with errors; the value at present can prove to be completely critical. The expediency of using the Mercator projection in the topographic and geodetic works production at low latitudes is shown numerically on the basis of model calculations. To convert the coordinates from the geocentric system with the Mercator projection, a programming algorithm which is widely used in Russia was chosen. For its application under low-latitude conditions, the modification of known formulas to be used in Saudi Arabia is implemented.

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