A new variable‐magnetization terrain correction method for aeromagnetic data

Geophysics ◽  
1987 ◽  
Vol 52 (1) ◽  
pp. 94-107 ◽  
Author(s):  
V. J. S. Grauch
Keyword(s):  
Magnetic Field ◽  
Real Data ◽  
Theoretical Models ◽  
Correction Method ◽  
Terrain Correction ◽  
Aeromagnetic Data ◽  
San Juan Mountains ◽  
Terrain Effects ◽  
Terrain Corrections ◽  
Lake City

Terrain effects in aeromagnetic data are produced by rugged, magnetic topography. These effects mimic the shape of topography and can often be so large that they obscure anomalies of interest. Thus it is desirable to remove terrain effects from aeromagnetic data in order to isolate the anomalies to be investigated. However, removal of aeromagnetic terrain effects has been a longstanding problem. Previously developed methods have succeeded only in certain, specific geologic situations. I present a new aeromagnetic terrain‐correction method that is superior to the previously developed methods for the general case. This method takes into account the highly variable magnetic properties of rocks and can remove terrain effects whether the sources of interest are shallow or deep. The new method is based on the assumption that magnetic sources of interest are often geometrically unrelated to terrain. It finds the magnetization that gives a magnetic‐field residual with minimum correlation to terrain effects for a window of data within a grid of magnetic‐field values. By repeating the calculation for windows covering the entire grid, a grid of variable‐magnetization values is produced which is combined with topography to calculate a magnetic‐terrain correction. The variable‐magnetizaton method was extensively tested using theoretical models (where the answer is known) and using real data from the Lake City caldera area in the San Juan Mountains of southern Colorado. The tests demonstrated the method’s effectiveness in removing terrain effects from aeromagnetic data. Valid terrain corrections were not obtained where anomalies of interest correlated with terrain effects. However, these places are readily recognizable and easily corrected by editing some of the magnetization values.

scholarly journals Necessity of Terrain Correction in Magnetotelluric Data Recorded from Garhwal Himalayan Region, India

Geosciences ◽  
2021 ◽  
Vol 11 (11) ◽  
pp. 482
Author(s):  
Dharmendra Kumar ◽  
Arun Singh ◽  
Mohammad Israil
Keyword(s):  
Real Data ◽  
Terrain Correction ◽  
Himalayan Region ◽  
General Character ◽  
Period Range ◽  
Magnetotelluric Data ◽  
Heterogeneous Models ◽  
Corrected Data ◽  
Terrain Corrections ◽  
And Inversion

The magnetotelluric (MT) method is one of the useful geophysical techniques to investigate deep crustal structures. However, in hilly terrains, e.g., the Garhwal Himalayan region, due to the highly undulating topography, MT responses are distorted. Such responses, if not corrected, may lead to the incorrect interpretation of geoelectric structures. In the present paper, we implemented terrain corrections in MT data recorded from the Garhwal Himalayan Corridor (GHC). We used AP3DMT, a 3D MT data modeling and inversion code written in the MATLAB environment. Terrain corrections in the MT impedance responses for 39 sites along the Roorkee–Gangotri profile in the period range of 0.01 s to 1000 s were first estimated using a synthetic model by recording the topography and locations of MT sites. Based on this study, we established the general character of the terrain and established where terrain corrections were necessary. The distortion introduced by topography was computed for each site using homogenous and heterogeneous models with actual topographic variations. Period-dependent, galvanic and inductive distortions were observed at different sites. We further applied terrain corrections to the real data recorded from the GHC. The corrected data were inverted, and the inverted model was compared with the corresponding inverted model obtained with uncorrected data. The modification in electrical resistivity features in the model obtained from the terrain-corrected response suggests the necessity of terrain correction in MT data recorded from the Himalayan region.

Does draping aeromagnetic data reduce terrain‐induced effects?

Geophysics ◽  
1984 ◽  
Vol 49 (1) ◽  
pp. 75-80 ◽  
Author(s):  
V. J. S. Grauch ◽  
David L. Campbell
Keyword(s):  
Magnetic Field ◽  
Magnetic Behavior ◽  
The Other ◽  
Aeromagnetic Data ◽  
Total Magnetic Field ◽  
Vertical Derivative ◽  
Topographic Features ◽  
Other Hand ◽  
Terrain Effects ◽  
The Difference

Contrary to intuition, draped aeromagnetic surveys (when compared to typical level surveys) amplify, rather than reduce, the problem of magnetic‐terrain anomalies. Calculations of the total magnetic field of various simple magnetic topographies on level and draped surfaces support this conclusion. In cases where draped surfaces are lower than level surfaces, the draped profiles exhibit steeper gradients and deeper polarity lows over topography than do the level profiles. On the other hand, where draped surfaces are higher than level surfaces, all anomalies are attenuated, so that magnetic‐terrain effects might be reduced relative to subsurface sources (depending upon the magnetization of each). The difference in magnetic behavior between level and draped data can be explained by a contribution of a vertical derivative component in the draped case that is absent in the level case. The contribution is most significant near topographic features because both the observation surface and the topographic surface are changing vertically.

REDUCTION OF TERRAIN‐INDUCED AEROMAGNETIC ANOMALIES BY PARALLEL‐SURFACE CONTINUATION; A CASE HISTORY FROM THE SOUTHERN SAN JUAN MOUNTAINS OF COLORADO

Geophysics ◽  
1977 ◽  
Vol 42 (7) ◽  
pp. 1431-1449 ◽  
Author(s):  
J. C. Wynn ◽  
B. K. Bhattacharyya
Keyword(s):  
Hydrothermal Alteration ◽  
Original Data ◽  
Economic Consequences ◽  
San Juan ◽  
Aeromagnetic Data ◽  
San Juan Mountains ◽  
Anomalous Field ◽  
Terrain Effects ◽  
Geologic Interpretation ◽  
Topographic Relief

A method for reduction of terrain‐induced anomalies in aeromagnetic data collected at constant or variable elevation has recently been developed by Bhattacharyya and Chan (1977a). The method utilizes an equivalent‐source approach to continue the anomalous field data to a reference surface parallel to the terrain, thereby attenuating the topographic effect. This approach to the reduction of terrain effects requires no assumption about physical properties or distribution of causative bodies. We present a test of the method in the Chama‐southern San Juan Mountains wilderness study area of southwestern Colorado, a region of substantial topographic relief (exceeding 1600 m) and extensive volcanic cover. This study shows that in many places in this area terrain effects contribute appreciably to the inaccuracy in geologic interpretation of the original aeromagnetic data. Comparison of interpretations based on original and revised data shows that some anomalies are shifted with respect to the original data by as much as 6 km, while others are not. The process now permits the interpreter to easily separate terrain‐induced lows from lows caused by hydrothermal alteration. In one example, a northern extension of a magnetic low thought to be caused by topography was shifted and aligned with several small zones of hydrothermal alteration. Another magnetic low incursion previously thought to be alteration‐caused was removed entirely. Several terrain‐induced highs were removed or combined, and several lows associated with valleys became relatively more prominent, leading to significant reevaluation of earlier interpretations. Several of these revisions have possible economic consequences. We show that this technique cannot be duplicated by filtering or upward continuation, and in regions of significant topographic relief the value of this new method to exploration geophysicists may be substantial.

scholarly journals Micro-Topography Mapping through Terrestrial LiDAR in Densely Vegetated Coastal Environments

2021 ◽  
Vol 10 (10) ◽  
pp. 665
Author(s):  
Xukai Zhang ◽  
Xuelian Meng ◽  
Chunyan Li ◽  
Nan Shang ◽  
Jiaze Wang ◽  
...  
Keyword(s):  
Correction Factor ◽  
Laser Scanning ◽  
Accuracy Assessment ◽  
Correction Method ◽  
Terrain Correction ◽  
Surface Model ◽  
Coastal Environments ◽  
Terrestrial Lidar ◽  
Dense Vegetation ◽  
Micro Topography

Terrestrial Light Detection And Ranging (LiDAR), also referred to as terrestrial laser scanning (TLS), has gained increasing popularity in terms of providing highly detailed micro-topography with millimetric measurement precision and accuracy. However, accurately depicting terrain under dense vegetation remains a challenge due to the blocking of signal and the lack of nearby ground. Without dependence on historical data, this research proposes a novel and rapid solution to map densely vegetated coastal environments by integrating terrestrial LiDAR with GPS surveys. To verify and improve the application of terrestrial LiDAR in coastal dense-vegetation areas, we set up eleven scans of terrestrial LiDAR in October 2015 along a sand berm with vegetation planted in Plaquemines Parish of Louisiana. At the same time, 2634 GPS points were collected for the accuracy assessment of terrain mapping and terrain correction. Object-oriented classification was applied to classify the whole berm into tall vegetation, low vegetation and bare ground, with an overall accuracy of 92.7% and a kappa value of 0.89. Based on the classification results, terrain correction was conducted for the tall-vegetation and low-vegetation areas, respectively. An adaptive correction factor was applied to the tall-vegetation area, and the 95th percentile error was calculated as the correction factor from the surface model instead of the terrain model for the low-vegetation area. The terrain correction method successfully reduced the mean error from 0.407 m to −0.068 m (RMSE errors from 0.425 m to 0.146 m) in low vegetation and from 0.993 m to −0.098 m (RMSE from 1.070 m to 0.144 m) in tall vegetation.

scholarly journals Submillimeter Array Observations of Magnetic Fields in Star Forming Regions

2010 ◽  
Vol 6 (S270) ◽  
pp. 103-106
Author(s):  
R. Rao ◽  
J.-M. Girart ◽  
D. P. Marrone
Keyword(s):  
Magnetic Field ◽  
Star Formation ◽  
Magnetic Fields ◽  
Computational Models ◽  
Dominant Role ◽  
Theoretical Models ◽  
Mass Star ◽  
Star Forming ◽  
Star Forming Regions ◽  
Low Mass

AbstractThere have been a number of theoretical and computational models which state that magnetic fields play an important role in the process of star formation. Competing theories instead postulate that it is turbulence which is dominant and magnetic fields are weak. The recent installation of a polarimetry system at the Submillimeter Array (SMA) has enabled us to conduct observations that could potentially distinguish between the two theories. Some of the nearby low mass star forming regions show hour-glass shaped magnetic field structures that are consistent with theoretical models in which the magnetic field plays a dominant role. However, there are other similar regions where no significant polarization is detected. Future polarimetry observations made by the Submillimeter Array should be able to increase the sample of observed regions. These measurements will allow us to address observationally the important question of the role of magnetic fields and/or turbulence in the process of star formation.

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