Jeong Woo Kim

Analysis of anomaly correlations

Recognizing correlations between data sets is the basis for rationalizing geophysical interpretation and theory. Procedures are presented that constitute an effective process for identifying correlative features between two or more digital data sets. The procedures include the development of normalization factors from the mean and variance properties of the data sets. Using these factors, the data sets may be transformed so that they have common amplitude ranges, means, and variances, thereby allowing a common graphical representation of the data sets that facilitates the visualization of feature correlations. Anomaly features that show direct, inverse, or no correlations between data sets may be separated by the application of correlation filters in the frequency domains of the data sets. The correlation filter passes or rejects wavenumbers between coregistered data sets based on the correlation coefficient between common wavenumbers as given by the cosine of their phase difference. Standardizing and summing the filtered outputs where directly correlative features have been enhanced yields local favorability indices that optimize the perception of these features. Differencing the standardized outputs where inversely correlative features have been enhanced, on the other hand, provides favorability indices that improve the perception of the inverse correlations. This study includes a generic example, as well as magnetic and gravity anomaly profile examples that illustrate the usefulness of these procedures for extracting correlative features between digital data sets.

Antarctic crustal modeling from the spectral correlation of free‐air gravity anomalies with the terrain

We investigated the use of enhanced spectral correlation theory for modeling the crustal features of the Antarctic from regional observations of gravity and terrain. The analysis considered 1°-gridded free-air gravity anomalies and topographic rock, ice, and water components for the region south of 60°S. We modeled terrain gravity effects at 150-km altitude by Gauss-Legendre quadrature (GLQ) integration assuming densities of 2800 kg/m3 for rock, 900 kg/m3 for ice, and 1030 kg/m3 for seawater. These effects are substantial relative to the free-air anomalies and must be compensated by the effects of subsurface density variations. Significant terrain-correlated free-air anomalies were revealed by the wavenumber correlation spectrum between the free-air anomalies and the modeled terrain gravity effects, which we interpreted mostly to reflect possible isostatic imbalances of the crust. Subtracting the terrain-correlated free-air anomalies from the total free-air anomalies and topographic gravity effects yielded terrain-decorrelated free-air anomalies and the gravity effects of isostatically compensated terrain features, respectively, which are uncorrelated with each other. The compensating effects that annihilate the latter were attributed to undulations of the Moho, which we estimated by inversion using GLQ integration and a mantle-to-crust density contrast of 400 kg/m3. The inversion produced a Moho map with nearly 40 km of total relief that agrees very well with deep seismic refraction soundings. For East Antarctica, a bimodal variation in crustal thickness was found: the crustal wedge between the eastern Weddell Sea (≃330°E) and the eastern flank of the Gamburtsev Subglacial Mountains (≃90°E) has a mean thickness of about 37 km, whereas the mean crustal thickness is near 32 km for the northern half of the rest of East Antarctica up to the western flank of the Transantarctic Mountains (≃150°E). For West Antarctica and the oceanic regions, mean crustal thicknesses of about 30 km and 14 km, respectively, are inferred. The terrain-decorrelated free-air anomalies may be related to long-wavelength, large-amplitude subcrustal density variations and to much shorter-wavelength, smaller-amplitude intracrustal density variations.

GRACE gravity evidence for an impact basin in Wilkes Land, Antarctica

[1]xa0New details on the east Antarctic gravity field from the Gravity Recovery and Climate Experiment (GRACE) mission reveal a prominent positive free-air gravity anomaly over a roughly 500-km diameter subglacial basin centered on (70°S, 120°E) in north central Wilkes Land. This regional inverse correlation between topography and gravity is quantitatively consistent with thinned crust from a giant meteorite impact underlain by an isostatically disturbed mantle plug. The inferred impact crater is nearly three times the size of the Chicxulub crater and presumably formed before the Cretaceous formation of the east Antarctic coast that cuts the projected ring faults. It extensively thinned and disrupted the Wilkes Land crust where the Kerguelen hot spot and Gondwana rifting developed but left the adjacent Australian block relatively undisturbed. The micrometeorite and fossil evidence suggests that the impact may have occurred at the beginning of the greatest extinction of life on Earth at ∼260 Ma when the Siberian Traps were effectively antipodal to it. Antipodal volcanism is common to large impact craters of the Moon and Mars and may also account for the antipodal relationships of essentially half of the Earths large igneous provinces and hot spots. Thus, the impact may have triggered the “Great Dying” at the end of the Permian and contributed to the development of the hot spot that produced the Siberian Traps and now may underlie Iceland. The glacial ice up to a few kilometers thick that has covered the crater for the past 30–40 Ma poses formidable difficulties to sampling the subglacial geology. Thus, the most expedient and viable test of the prospective crater is to survey it for relevant airborne gravity and magnetic anomalies.

Lunar crustal analysis of Mare Orientale from topographic and gravity correlations

We investigated the use of spectral correlation analysis for modeling the crustal features of Mare Orientale from lunar 70th degree spherical harmonic topographic and gravity field models derived from Clementine satellite and earlier investigations. The analysis considered a 64°-by-64° region of the Moon centered roughly on Mare Orientale at an altitude of 100 km. The topography of the study region, which includes over 11 km of relief, was modeled for its gravity effects in lunar spherical coordinates by Gauss-Legendre quadrature integration assuming a terrain density of 2.8 g/cm3. We observed substantial positive and negative correlations between terrain gravity effects and free-air gravity anomalies that seriously limit the utility of simple Bouguer gravity anomalies for subsurface studies. Using the wavenumber correlation spectrum between the two data sets, we designed correlation filters to extract the common features. Possible interpretations for the terrain-correlated free-air gravity anomalies include isostatic crustal mass imbalances that may be equilibrated by radial adjustments of the Moho of up to 44 km, assuming Airy-Heiskanen compensation and a mantle density contrast of 0.5 g/cm3 with the crust. These Moho adjustments define mass variations that account for most of the mascon and flanking negative free-air gravity anomalies. Furthermore, their remarkable correlation with the topographic rings of Mare Orientale points to the possible influence of a strong local stress field of the crust in the development of the ring structures. Subtracting the terrain-correlated free-air anomalies from the free-air gravity anomalies and terrain gravity effects yielded terrain-decorrelated free-air and isostatically compensated terrain gravity anomalies, respectively, that show zero correlation. This lack of correlation may be interpreted for a Moho that involves over 100 km of relief assuming Airy-Heiskanen compensation of the crust. Beneath Mare Orientale, we observed a minimum crustal thickness of about 17 km. Corresponding terrain-decorrelated free-air gravity anomalies of Mare Orientale may be related to a central cone-shaped body of 0.5 g/cm3 density contrast with apex extending nearly 5 km below the surface, which is surrounded by a ringed-shaped body of −0.5 g/cm3 density contrast that may extend about 7 km below the surface. These bodies resulted possibly from meteorite impact that produced a roughly circular region of breccia and highly fractured crust with a higher density core where some remelting of the rocks about the impact site may have occurred.

Spectral correlation of magnetic and gravity anomalies of Ohio

Geologic interpretation of Ohios magnetic or gravity anomalies is hindered by the effects of anomaly superposition and source ambiguity inherent to potential field analysis. A common approach to minimizing interpretational ambiguities is to consider analyses of anomaly correlations. A spectral procedure is adapted which correlates anomaly fields in the frequency domain to produce filters separating positively and negatively correlated, as well as null correlated features. The correlation filter passes or rejects wavenumbers between coregistered fields based on the correlation coefficient between common wavenumbers as given by the cosine of their phase difference. This procedure is applied to reduced‐to‐pole magnetic and first vertical derivative gravity anomalies of Ohio for mapping correlative magnetization and density contrasts within the basement rocks. The analysis reveals predominantly positive correlations between anomaly maxima and minima. Correlative anomaly maxima may be generally modeled as maf...

Application of satellite magnetic observations for estimating near-surface magnetic anomalies

Regional to continental scale magnetic anomaly maps are becoming increasingly available from airborne, shipborne, and terrestrial surveys. Satellite data are commonly considered to fill the coverage gaps in regional compilations of these near-surface surveys. For the near-surface Antarctic magnetic anomaly map being produced by the Antarctic Digital Magnetic Anomaly Project (ADMAP), we show that near-surface magnetic anomaly estimation is greatly enhanced by the joint inversion of the near-surface data with Ørsted satellite observations compared to Magsat data that have order-of-magnitude greater measurement errors, albeit collected at much lower orbital altitudes. The CHAMP satellite is observing the geomagnetic field with the same measurement accuracy as the Ørsted mission, but at the lower orbital altitudes covered by Magsat. Hence, additional significant improvement in predicting near-surface magnetic anomalies can result as lithospheric magnetic anomaly data from the CHAMP mission become available. Our analysis also suggests that a further order-of-magnitude improvement in the accuracy of the magnetometer measurements at minimum orbital altitude may reveal considerable new insight into the magnetic properties of the lithosphere.

Spectral attenuation of track-line noise

Satellite, airborne, and marine geophysical surveys usually sample features better along-track than across-track. In maps, this aliasing together with errors in along-track data processing produce a systematic “washboard” effect across the survey lines known as track-line noise. In regions covered by two line surveys at different azimuths, the combined data generate maps with this noise orthogonal to both survey azimuths. Statistical efforts to minimize random noise will only be partially effective in attenuating systematic track-line noise. We present a method that exploits the spectral differences resulting from the different survey azimuths to reduce this systematic noise. In each survey, only two of the spectral quadrants are normally contaminated. Combining the four least contaminated quadrants from both surveys generates a map of minimal track-line noise. This method is used to improve the recovery of geoid undulations from satellite altimetry of the Barents Sea in the Arctic.

Ørsted verifies regional magnetic anomalies of the Antarctic lithosphere

[1]xa0Initial magnetic measurements from the Orsted satellite reveal lithospheric anomalies over the Antarctic that are similar to those obtained by Magsat. Accordingly, lithospheric anomalies can be extracted from the Orsted data, despite the much greater operational altitude of Orsted (650–865 km) relative to Magsat (350–550 km). Furthermore, these correspondences confirm the lithospheric origins for the resulting small-amplitude anomalies in the satellite data. In studies of the Antarctic lithosphere, the Magsat data particularly were limited by the large relative uncertainties of their lithospheric components. These uncertainties occurred because the short nearly seven-month mission more than 20 years ago collected data over austral high summer and early fall when the contaminating large-amplitude external field effects were at a maximum. Therefore, the recent and more numerous Orsted measurements greatly facilitate our efforts to separate effectively the core, lithospheric, and external field components for enhanced studies of the Antarctic lithosphere.

Crustal modeling from spectrally correlated free-air and terrain gravity data—A case study of Ohio

We investigate the use of spectral correlation theory to analyze terrain gravity effects and free‐air gravity anomalies of Ohio for possible constraints on the thickness variations and neotectonics of the crust. Terrain gravity effects are computed from the topography by Gauss‐Legendre quadrature integration and are compared against independent free‐air gravity anomaly observations for their wavenumber correlation spectrum. Spectral correlation filters are designed accordingly to extract terrain‐correlated free‐air gravity anomalies that are subtracted from the terrain gravity effects for estimates of the compensated terrain gravity effects. These effects are used to model the Moho by inversion, assuming they predominantly reflect crustal thickness variations. Our results characterize the middle third of Ohio as a broad zone of thickened Precambrian crust, which also may include rifted regions where the thickness of the prerift crust has been reduced greatly. Furthermore, we find that about 83% of the ins...

CHAMP, Ørsted, and Magsat Magnetic Anomalies of the Antarctic Lithosphere

We processed CHAMP, Orsted and Magsat mission data for the Antarctic using advanced spectral correlation theory to separate spatially and temporally static components from the dynamic ones. Ignoring measurement noise and processing errors, the dynamic components include strong external field effects, whereas the static components reflect lithospheric and core field effects. Consistency is apparent in the correlation of all the anomaly fields at degree and order 13 and higher. However, the longer wavelength components are corrupted by errors in the core field reductions that do not account for the crustal thickness magnetic effects. To assess these magnetic effects, we developed an Antarctic crustal thickness model from free-air and computed terrain gravity effects. The pseudo magnetic effects evaluated from the crustal model indicate quite strong crustal anomaly contributions for degrees lower than 13 in the satellite magnetometer observations. The Antarctic gravity field from which the crustal thickness effects were inferred is limited by the general lack of terrestrial gravity observations. However, CHAMP is providing significant new insight on the magnetic and terrestrial gravity fields for advancing our understanding of the Antarctic lithosphere and its tectonic evolution