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

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.

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Enhancement of the total horizontal gradient of magnetic anomalies using the tilt angle

Geophysics ◽

10.1190/geo2011-0441.1 ◽

2013 ◽

Vol 78 (3) ◽

pp. J33-J41 ◽

Cited By ~ 56

Author(s):

Francisco J. F. Ferreira ◽

Jeferson de Souza ◽

Alessandra de B. e S. Bongiolo ◽

Luís G. de Castro

Keyword(s):

Tilt Angle ◽

Real Data ◽

Magnetic Anomalies ◽

Detection Methods ◽

Magnetic Data ◽

Horizontal Gradient ◽

Geometrical Properties ◽

Different Depths ◽

Deep Sources ◽

Derivatives Of

Magnetic anomaly maps reflect the spatial distribution of magnetic sources, which may be located at different depths and have significantly different physical and geometrical properties, complicating the identification of the corresponding geologic structures. Filtering techniques are frequently used to balance anomalies from shallow and deep sources, and to enhance certain features of interest, such as the edges of the causative bodies. Most methods used for enhancing magnetic data are based on vertical or horizontal derivatives of the magnetic anomalies or combinations of them, and the edges or centers of the sources are identified by maxima, minima, or null values in the transformed data. Normalized derivatives methods are used to equalize signals from sources buried at different depths. We present an edge detector method for the enhancement of magnetic anomalies, which is based on the tilt angle of the total horizontal gradient. The notable features of this method are that it produces amplitude maxima over the source edges and that it equalizes signals from shallow and deep sources. The method is applied to synthetic and real data. The effectiveness of the method is evaluated by comparing it with other edge detection methods that have been previously reported in the literature and that make use of derivatives. The results show that our method is less sensitive to variations in the depth of the sources and that it indicates the position of the edges of causative bodies in a more accurate fashion, when compared with previous methods, even for anomalies due to multiple interfering sources. These results demonstrate that the proposed method is a useful tool for the qualitative interpretation of magnetic data.

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COMBINING TILT DERIVATIVE FILTERS: NEW APPROACHES TO ENHANCE MAGNETIC ANOMALIES

Brazilian Journal of Geophysics ◽

10.22564/rbgf.v36i3.1956 ◽

2018 ◽

Vol 36 (3) ◽

pp. 1

Author(s):

Fabrício Rodrigues Castro ◽

Saulo Pomponet Oliveira ◽

Jeferson de Souza ◽

Francisco José Fonseca Ferreira

Keyword(s):

Qualitative Methods ◽

Magnetic Anomaly ◽

Computational Cost ◽

Magnetic Anomalies ◽

Horizontal Gradient ◽

Local Phase ◽

Aeromagnetic Data ◽

Addition And Subtraction ◽

Deep Sources ◽

Tilt Derivative

ABSTRACT. We extend the concept of two earlier enhancement techniques based on the local phase of the magnetic anomaly, namely the vertical (TDR) and horizontal (TDX) tilt angles, which are defined by the inverse tangent of ratios involving the total horizontal gradient and the vertical derivative. These filters are useful to locate both shallow and deep sources, because they equalize the signal amplitudes. The proposed approach is based on the addition and subtraction of TDR and TDX. The TDR+TDX filter produces constant values over the causative bodies, while TDR-TDX generates peaks over the center of bodies and is constant out of them. By applying the proposed techniques to synthetic and aeromagnetic data we show that they locate more clearly the centers and edges of the sources in comparison to TDR and TDX, respectively. The combined filters have essentially the same computational cost as TDR and TDX and can replace them as auxiliary interpretation tools.Keywords: Qualitative Methods, Local Phase Filters, Aeromagnetic Data.RESUMO. Estendemos o conceito de duas técnicas de realce baseadas na fase local da anomalia magnética: as inclinações do sinal analítico (TDR) e do gradiente horizontal total (TDX), definidos pelo arco tangente de razões envolvendo o gradiente horizontal total e a derivada vertical. Estes filtros são úteis para localizar tanto fontes rasas quanto profundas. O método proposto baseia-se na adição e subtração dos filtros TDR e TDX. O filtro TDR+TDX produz valores constantes sobre as fontes causadoras, enquanto que o TDR-TDX produz picos sobre o centro dos corpos e é constante onde fontes causadoras não são verificadas. Aplicando as técnicas propostas aos dados sintéticos e reais mostra-se que elas localizam mais claramente os centros e as bordas dos corpos em comparação com o TDR e o TDX, respectivamente. Os filtros combinados têm essencialmente o mesmo custo computacional dos filtros originais, TDR e TDX, e podem substituí-los como ferramentas de interpretação.Palavras-chave: Métodos Qualitativos, Filtros de Fase Local, Dados Aeromagnéticos. Federal

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Reduction to the pole at low latitudes by Wiener filtering

Geophysics ◽

10.1190/1.1442628 ◽

1989 ◽

Vol 54 (12) ◽

pp. 1607-1613 ◽

Cited By ~ 58

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.

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Deciphering channel networks from aeromagnetic potential field data: the case of the North Sea Quaternary tunnel valleys

Geophysical Journal International ◽

10.1093/gji/ggz494 ◽

2019 ◽

Cited By ~ 2

Author(s):

S Brahimi ◽

P Le Maire ◽

J F Ghienne ◽

M Munschy

Keyword(s):

Magnetic Susceptibility ◽

North Sea ◽

Potential Field ◽

Magnetic Anomalies ◽

Channel Structure ◽

Magnetic Data ◽

Horizontal Gradient ◽

Finite Width ◽

The North ◽

The North Sea

Summary High-resolution magnetic data and potential field methods have been used to perform a detailed analysis of networks of late Quaternary subglacially-cut tunnel valleys (central Viking Graben, Norwegian sector of the North Sea). High-frequency, ribbon-like, sinuous, paired magnetic anomalies interpreted to be the signature of tunnel valleys are identified. Such magnetic anomalies have 1 to 8 nT amplitudes and reflect a magnetic susceptibility contrast between valley infills and the host sediments. Fractional vertical derivative and horizontal gradient transforms provide the best control on the accurately delineation of tunnel valleys by plotting automatically the extrema. The 2D forward modelling is a very effective approach to determinating the geometric parameters and magnetic susceptibility of the modeled valleys. It allows to determine the finite-width flat horizontal thin geometry as the most appropriate simple geometry to simulate the magnetic anomaly linked to a channel structure. The application of Euler deconvolution using complex algebra allows to substantiate the structural index (n = 1.5) for simple palaeovalley geometries and to determine fair valley depth estimates.

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Identifying remanent magnetization effects in magnetic data

Geophysics ◽

10.1190/1.1443449 ◽

1993 ◽

Vol 58 (5) ◽

pp. 653-659 ◽

Cited By ~ 118

Author(s):

Walter R. Roest ◽

Mark Pilkington

Keyword(s):

Magnetic Anomaly ◽

Remanent Magnetization ◽

Close Agreement ◽

Magnetic Anomalies ◽

Analytic Signal ◽

Magnetic Data ◽

Horizontal Gradient ◽

Impact Structure ◽

Magnetization Direction ◽

Additional Constraints

Remanent magnetization can have a significant influence on the shape of magnetic anomalies in areas that are generally characterized by induced magnetization. Since modeling of magnetic anomalies is nonunique, additional constraints on the direction of magnetization are useful. A method is proposed here to study the possible contribution of remanent magnetization to a particular anomaly, by comparing two functions that are calculated directly from the observations: (1) the amplitude of the analytic signal, and (2) the horizontal gradient of pseudogravity. From the amplitude and relative position of maxima in these derived quantities, we infer the deviation of the magnetization direction from that of the ambient field. The approach is applied to the magnetic anomaly in the center of the Manicouagan impact structure (Canada). Our results, based only on the magnetic anomaly observations, are in close agreement with constraints on the direction of remanent magnetization from rock samples.

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Three-dimensional distribution of igneous rocks near the Pebble porphyry Cu-Au-Mo deposit in southwestern Alaska: Constraints from regional-scale aeromagnetic data

Geophysics ◽

10.1190/geo2013-0326.1 ◽

2014 ◽

Vol 79 (2) ◽

pp. B63-B79 ◽

Cited By ~ 7

Author(s):

Eric D. Anderson ◽

Wei Zhou ◽

Yaoguo Li ◽

Murray W. Hitzman ◽

Thomas Monecke ◽

...

Keyword(s):

Porphyry Copper ◽

Regional Scale ◽

Copper Deposit ◽

Magnetic Data ◽

Horizontal Gradient ◽

Aeromagnetic Data ◽

Near Surface ◽

Magnetic Inversion ◽

Tilt Derivative ◽

Southwestern Alaska

Aeromagnetic data helped us to understand the 3D distribution of plutonic rocks near the Pebble porphyry copper deposit in southwestern Alaska, USA. Magnetic susceptibility measurements showed that rocks in the Pebble district are more magnetic than rocks of comparable compositions in the Pike Creek–Stuyahok Hills volcano-plutonic complex. The reduced-to-pole transformation of the aeromagnetic data demonstrated that the older rocks in the Pebble district produce strong magnetic anomaly highs. The tilt derivative transformation highlighted northeast-trending lineaments attributed to Tertiary volcanic rocks. Multiscale edge detection delineated near-surface magnetic sources that are mostly outward dipping and coalesce at depth in the Pebble district. The total horizontal gradient of the 10-km upward-continued magnetic data showed an oval, deep magnetic contact along which porphyry deposits occur. Forward and inverse magnetic modeling showed that the magnetic rocks in the Pebble district extend to depths greater than 9 km. Magnetic inversion was constrained by a near-surface, 3D geologic model that is attributed with measured magnetic susceptibilities from various rock types in the region. The inversion results indicated that several near-surface magnetic sources with moderate susceptibilities converge with depth into magnetic bodies with higher susceptibilities. This deep magnetic source appeared to rise toward the surface in several areas. An isosurface value of 0.02 SI was used to depict the magnetic contact between outcropping granodiorite and nonmagnetic sedimentary host rocks. The contact was shown to be outward dipping. At depths around 5 km, nearly the entire model exceeded the isosurface value indicating the limits of nonmagnetic host material. The inversion results showed the presence of a relatively deep, northeast-trending magnetic low that parallels lineaments mapped by the tilt derivative. This deep low represents a strand of the Lake Clark fault.

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

Geophysics ◽

10.1190/1.1443492 ◽

1993 ◽

Vol 58 (8) ◽

pp. 1084-1090 ◽

Cited By ~ 39

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).

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Euler’s differential equation and the identification of the magnetic point‐pole and point‐dipole sources

Geophysics ◽

10.1190/1.1441780 ◽

1984 ◽

Vol 49 (9) ◽

pp. 1549-1553 ◽

Cited By ~ 7

Author(s):

J. O. Barongo

Keyword(s):

Magnetic Field ◽

Differential Equation ◽

Magnetic Anomalies ◽

Field Vector ◽

Magnetic Data ◽

Dipole Source ◽

Point Dipole ◽

Main Subject ◽

Depth Extent ◽

Dipole Sources

The concept of point‐pole and point‐dipole in interpretation of magnetic data is often employed in the analysis of magnetic anomalies (or their derivatives) caused by geologic bodies whose geometric shapes approach those of (1) narrow prisms of infinite depth extent aligned, more or less, in the direction of the inducing earth’s magnetic field, and (2) spheres, respectively. The two geologic bodies are assumed to be magnetically polarized in the direction of the Earth’s total magnetic field vector (Figure 1). One problem that perhaps is not realized when interpretations are carried out on such anomalies, especially in regions of high magnetic latitudes (45–90 degrees), is that of being unable to differentiate an anomaly due to a point‐pole from that due to a point‐dipole source. The two anomalies look more or less alike at those latitudes (Figure 2). Hood (1971) presented a graphical procedure of determining depth to the top/center of the point pole/dipole in which he assumed prior knowledge of the anomaly type. While it is essential and mandatory to make an assumption such as this, it is very important to go a step further and carry out a test on the anomaly to check whether the assumption made is correct. The procedure to do this is the main subject of this note. I start off by first using some method that does not involve Euler’s differential equation to determine depth to the top/center of the suspected causative body. Then I employ the determined depth to identify the causative body from the graphical diagram of Hood (1971, Figure 26).

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Appraising structural interpretations using seismic data — Theoretical elements

Geophysics ◽

10.1190/geo2018-0128.1 ◽

2019 ◽

Vol 84 (2) ◽

pp. N29-N40

Author(s):

Modeste Irakarama ◽

Paul Cupillard ◽

Guillaume Caumon ◽

Paul Sava ◽

Jonathan Edwards

Keyword(s):

Seismic Data ◽

Structural Model ◽

Synthetic Data ◽

Target Region ◽

Structural Interpretation ◽

Vertical Seismic Profiling ◽

Seismic Image ◽

Vsp Data ◽

Phase Shift Data ◽

Seismic Images

Structural interpretation of seismic images can be highly subjective, especially in complex geologic settings. A single seismic image will often support multiple geologically valid interpretations. However, it is usually difficult to determine which of those interpretations are more likely than others. We have referred to this problem as structural model appraisal. We have developed the use of misfit functions to rank and appraise multiple interpretations of a given seismic image. Given a set of possible interpretations, we compute synthetic data for each structural interpretation, and then we compare these synthetic data against observed seismic data; this allows us to assign a data-misfit value to each structural interpretation. Our aim is to find data-misfit functions that enable a ranking of interpretations. To do so, we formalize the problem of appraising structural interpretations using seismic data and we derive a set of conditions to be satisfied by the data-misfit function for a successful appraisal. We investigate vertical seismic profiling (VSP) and surface seismic configurations. An application of the proposed method to a realistic synthetic model shows promising results for appraising structural interpretations using VSP data, provided that the target region is well-illuminated. However, we find appraising structural interpretations using surface seismic data to be more challenging, mainly due to the difficulty of computing phase-shift data misfits.

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Estimation of the total magnetization direction of approximately spherical bodies

Nonlinear Processes in Geophysics Discussions ◽

10.5194/npgd-1-1465-2014 ◽

2014 ◽

Vol 1 (2) ◽

pp. 1465-1507

Author(s):

V. C. Oliveira ◽

D. P. Sales ◽

V. C. F. Barbosa ◽

L. Uieda

Keyword(s):

Synthetic Data ◽

Magnetization Vector ◽

Spherical Shape ◽

Time Interval ◽

Alkaline Complex ◽

Multiple Sources ◽

Geological Time ◽

Magnetization Direction ◽

Reduction To The Pole ◽

Horizontal Grid

Abstract. We have developed a fast total-field anomaly inversion to estimate the magnetization direction of multiple sources with approximately spherical shape and known centres. Our method can be applied to interpret multiple sources with different magnetization directions. It neither requires the prior computation of any transformation like reduction to the pole nor the use of regularly spaced data on a horizontal grid. The method contains flexibility to be implemented as a linear or non-linear inverse problem, which results, respectively, in a least-squares or robust estimate of the components of the magnetization vector of the sources. Applications to synthetic data show the robustness of our method against interfering anomalies and errors in the location of the sources' centre. Besides, we show the feasibility of applying the upward continuation to interpret non-spherical sources. Applications to field data over the Goiás Alkaline Province (GAP), Brazil, show the good performance of our method in estimating geological meaningful magnetization directions. The results obtained for a region of the GAP, near from the alkaline complex of Diorama, suggest the presence of non-outcropping sources marked by strong remanent magnetization with inclination and declination close to -70.35° and -19.81°, respectively. This estimated magnetization direction leads to predominantly positive reduced-to-the-pole anomalies, even for other region of the GAP, in the alkaline complex of Montes Claros de Goiás. These results show that the non-outcropping sources near from the alkaline complex of Diorama have almost the same magnetization direction of that ones in the alkaline complex of Montes Claros de Goiás, strongly suggesting that these sources have emplaced the crust almost within the same geological time interval.

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