GP54A-01 INVITED
Global magnetic anomaly maps from satellite, marine and aeromagnetic data
Recent years have seen rapid advances in global magnetic anomaly mapping. The CHAMP satellite continues to
provide highly accurate magnetic field measurements at steadily decreasing altitudes under solar-minimum
conditions. The latest MF6 crustal magnetic field model from CHAMP data extends to spherical harmonic (SH)
degree 120, resolving wavelengths down to 180 arc minutes, or 330 km. At this resolution, sea floor spreading
anomalies become visible throughout the oceans. Due to the strong attenuation of shorter wavelengths with
altitude, higher resolution maps must rely on additional near-surface data. Expanding the GEODAS marine
trackline database and bringing further aeromagnetic surveys into the public archives is therefore pursued with
high priority at NOAA's National Geophysical Data Center. Marine and aeromagnetic data
are merged with the satellite-derived models to a World Magnetic Anomaly Map (WMAM) at 3 arc minute (5 km)
resolution. The full vector of the magnetic anomaly field to 15 arc minute resolution is given by a SH model,
NGDC-720, estimated from the WMAM grid. These new global magnetic data sets, maps and SH models provide
exciting new possibilities in tectonic interpretation and crustal modeling.
http://www.geomag.org/
GP54A-02 INVITED
Looking for Relations Between Magnetization and Electrical Conductivity Variations in Limestones
Airborne magnetic and electromagnetic Tensor VLF (Very Low Frequency, frequencies around 20 kHz) data have been collected over the island of Gotland in the Baltic Sea. The island is covered by Paleozoic limestones and sandstones reaching thicknesses between 300-600 m overlying crystalline basement. The limestones can be characterized as either pure reef limestones or argillaceous limestones. The reef limestones are sometimes heavily fractured with a dominance of vertical fractures.´Such fractures systems couple very well to plane wave electromagnetic waves as used in the VLF method, and since the penetration depth in limestones is around 100 m the VLF responses are entirely caused by lateral changes in electrical conductivity in the limestones. The magnetic anomaly map of Gotland is dominated magnetic sources in the Precambrian basement.Intrasedimentary variations in magnetization contribute to the small wavelength signal observed along the profiles. Measurements on rock samples indicate that average susceptibilities of reef limestones, argillaceous limestones and sandstones are 0.3, 9.6 and 6.6×10-5 SI units, respectively, giving rise to magnetic anomalies of the order 1 nTesla. Airborne data were collected with a 400 m line spacing, a 60 m flight altitude, and a 16 m sampling interval. VLF anomalies are entirely related to the conductivity variations in the limestones and sandstones, but the magnetic anomalies are dominated by the magnetization variations in the crystalline basement for wavelength greater than the depth to the basement, i.e. wavelengths greater than about 500 m. The shortest wavelength is 32 m which means that about 94% of total range of wavenumbers corresponds to magnetization variations in the limestones and sandstones and possible noise contributions due to airplane influence, infrastructure on the ground and incomplete removal of time variations in the magnetic field. In conventional analysis of airborne data most of the wavenumber range is neglected because the line spacing sets the limit for horizontal resolution. Alternatively, densely sampled data along flight lines can be used as they are, either by assuming that the underlying geology is 2D or by assuming that the signals represent the responses of self-similar distributions in magnetization or electrical conductivity. VLF responses are particularly sensitive to linear structures, whereas point like sources couple very weakly to the plane wave electromagnetic field used. Furthermore tensor VLF data collected along profiles contain local strike information at all measurement points. Thus it becomes possible to partly compensate for a variable strike direction, such that distances are measured along hypothetical flight lines orthogonal to the local strike. We have developed some simple 2D models – using the Born approximation – relating the power spectrum of the VLF response to the power spectrum of the lateral conductivity distribution. For small wavenumbers the relation is similar to that relating magnetization and magnetic field anomalies, but with increasing wavenumber electrical conductivity variations and corresponding VLF anomalies are related by more complicated expressions. Statistical models will be compared with deterministic inverse models using both airborne and ground based high resolution data to answer the question as to what extent the small scale magnetic field variations can be correlated with VLF anomalies.
GP54A-03 INVITED
Integrated Potential-field Studies in Support of Energy Resource Assessment in Frontier Areas of Alaska
In frontier areas of Alaska, potential-field studies play an important role in characterizing the geologic structure of sedimentary basins having potential for undiscovered oil and gas resources. Two such areas are the Yukon Flats basin in the east-central interior of Alaska, and the coastal plain of the Arctic National Wildlife Refuge (ANWR) in northeastern Alaska. The Yukon Flats basin is a potential source of hydrocarbon resources for local consumption and possible export. Knowledge of the subsurface configuration of the basin is restricted to a few seismic reflection profiles covering a limited area and one well. The seismic profiles were reprocessed and reinterpreted in preparation for an assessment of the oil and gas resources of the basin. The assessment effort required knowledge of the basin configuration away from the seismic profiles, as well as an understanding of the nature of the underlying basement. To extend the interpretation of the basin thickness across the entire area of the basin, an iterative Jachens-Moring gravity inversion was performed on gridded quasi-isostatic residual gravity anomaly data. The inversion was constrained to agree with the interpreted basement surface along the seismic profiles. In addition to the main sedimentary depocenter interpreted from the seismic data as having over 8 km of fill, the gravity inversion indicated a depocenter with over 7 km of fill in the Crooked Creek sub-basin. Results for the Crooked Creek sub-basin are consistent with magnetic and magnetotelluric modeling, but they await confirmation by drilling or seismic profiling. Whether hydrocarbon source rocks are present in the pre-Cenozoic basement beneath Yukon Flats is difficult to determine because extensive surficial deposits obscure the bedrock geology, and no deep boreholes penetrate basement. The color and texture patterns in a red-green-blue composite image consisting of reduced-to-the-pole aeromagnetic data (red), magnetic potential (blue), and basement gravity (green) highlight domains with common geophysical characteristics and, by inference, lithology. The observed patterns suggest that much of the basin is underlain by Devonian to Jurassic oceanic rocks that probably have little or no potential for hydrocarbon generation. The coastal plain surficial deposits in the northern part of ANWR conceal another frontier basin with hydrocarbon potential. Proprietary aeromagnetic and gravity data were used, along with seismic reflection profiles, to construct a structural and stratigraphic model of this highly deformed sedimentary basin for use in an energy resource assessment. Matched-filtering techniques were used to separate short-wavelength magnetic and gravity anomalies attributed to sources near the top of the sedimentary section from longer-wavelength anomalies attributed to deeper basin and basement sources. Models along the seismic reflection lines indicate that the primary sources of the short-wavelength anomalies are folded and faulted sedimentary beds truncated at the Pleistocene erosion surface. In map view, the aeromagnetic and gravity anomalies produced by the sedimentary units were used to identify possible structural trapping features and geometries, but they also indicated that these features may be significantly disrupted by faulting.
GP54A-04 INVITED
Quantitative Correlation Analysis of Gravity, Magnetic, and Thermal Potential Fields
The correlations between the effects of different potential fields can greatly limit ambiguities of interpretation and improve the signal-to-noise ratio in anomaly mapping. In addition, correlative anomalies are well suited from estimating the physical properties of sources because the phase differences in the anomalies completely determine the correlations. The basis for quantitatively correlating geological variations in potential fields is the inverse distance function that describes the geometry between source and observation points. Poisson's theorem relates correlative density and magnetization contrasts, whereas the correlations of these contrasts with thermal conductivity contrasts may be quantified by the heat production equivalence of thermal sources. We present several examples in both data and spectral domains to illustrate the advantages and limitations of quantifying correlations between potential fields. These examples include analyzing free-air gravity anomaly observations for isostatically disturbed crustal components, modeling regional magnetic anomaly observations for the Curie isotherm within the crust, and quantifying correlative crustal density and magnetization contrasts from gravity and magnetic anomaly observations.
GP54A-05 INVITED
Improved Receiver Function Estimates of Moho Using Spatial Statistics and Gravity
Analyses of isostatic response have yielded several exciting advances in knowledge of lithospheric bending strength, rheology and loading processes in the past few years. However, coherence analysis of gravity and topography data hinges on an assumption that surface and internal load processes are uncorrelated. Until recently, that assumption has been difficult to evaluate independently, but EarthScope's transportable array (TA) of seismograph stations allows us to envision a time when assumptions about load statistics may be downweighted or even unnecessary in isostatic analyses of the conterminous US. Seismic velocities (from Pn, body and surface wave tomography) and layer thicknesses (from receiver function analysis of P-S converted phases) may be used to arrive at independent estimates of internal loading that can then be used to better constrain lithospheric strength and rheology. Moho estimates are especially useful because they directly constrain lithospheric flexural deflections, placing stringent limits on possible loading scenarios. Moho estimates should be relatively error-free and sample the ~100 to 2000 km spatial wavelengths relevant to flexure. The nominal 70-km spacing of the TA is ideal for flexural sampling but poses a problem for error processes. The iterative deconvolution and H-K stacking approach used in EARS (Crotwell & Owens, 2005) receiver function estimates, for example, identifies the Moho from the crustal thickness H and Vp/Vs ratio K that maximizes phase arrival stack amplitudes. However, oftentimes several different lithospheric impedance contrasts will yield significant stack amplitudes for a given site. At 70 km spacing, standard spatial correlation approaches do not help to distinguish which corresponds to the Moho, resulting in significant errors where the Moho is incorrectly assigned to another stack amplitude maximum. In this talk I will present a methodology for assessing likelihood of a given H-K combination at a site using optimal interpolation (OI) from values at surrounding sites. Boot-strap weighting of stack amplitudes using likelihood ratios results in reduced incidence of outlier estimates and greatly improved spatial statistics. However that approach fails to overcome ambiguities in problem areas such as western Wyoming, where impedance at the top of a crustal 7x layer occurs at similar depth to the nearby Basin and Range Moho. Gravity variations offer an additional source of constraint on likelihood that may help us to better distinguish the Moho from ambiguous stack amplitude maxima.
GP54A-06
Focusing inversion of marine full-tensor gradiometry data in offshore geophysical exploration
Recent technological developments make it possible to accurately measure all the independent tensor components of the gravity gradient field from a moving platform. The technology that enables such rapid and accurate data acquisition motivates the research to further develop methods for processing and interpreting gradiometer data. In this paper we consider a method of marine full-tensor gradiometry (FTG) data interpretation. Inversion of the tensor gravity gradient data is complicated by the fact that the gravity data are invariably contaminated by noise and are acquired at a limited number of observation points. Therefore, inversion of these data represents a typical ill-posed problem, which requires the application of corresponding regularization methods. The traditional regularized inversion algorithms providing smooth solutions for geological structures have difficulties, however, in describing the sharp boundaries between different geological formations. In these situations, it is useful to search for a stable solution within the class of inverse models with sharp petrophysical boundaries. The mathematical technique for solving this problem is based on introducing a special type of stabilizing functionals, the so-called minimum-support or minimum-gradient support functionals. This technique is called a focusing regularized inversion to distinguish it from the traditional smooth regularized inversion. In this paper we consider an application of the focusing inversion to interpretation of the marine full-tensor gradient (FTG) data collected by a sea-bottom survey in the Barents Sea. We conduct inversion of the different individual components of the FTG data. The numerical results demonstrate a possibility for resolving the complex geological structures of the salt diapir formation using gravity gradiometry data.
GP54A-07
3D gravity inversion through an adaptive learning procedure
We develop a gravity anomaly inversion to estimate a 3D density-contrast distribution that gives rise to an interfering gravity field. We use an interpretation model consists of a grid of 3D vertical, juxtaposed prisms in both horizontal and vertical directions. Iteratively, our approach estimates the 3D density-contrast distribution that fits the observed anomaly within the measurement errors and favors compact gravity sources closest to pre- specified geometric elements such as axes and points. This method retrieves the geometry of multiple gravity sources with prescribed density contrasts (positive and negative values) assigned to each geometric element. At the first iteration, we set an initial interpretation model, specifies the first-guess geometric elements and the corresponding target density contrasts. Each geometric element operates as the first-guess skeletal outline of a presumed gravity source (or its homogeneous section) to be reconstructed. From the second iteration on, our method redefines automatically a new set of geometric elements, the associated target density contrasts and a new interpretation model whose number of prisms is greater than the previous iteration. The iteration stops when the geometries of the estimated sources are invariant along successive iterations of our method. The examples with synthetic data illustrate the good performance of the method in retrieving the geometries of multiple gravity sources that gives rise to an interfering gravity field. Our method can be used to interpret gravity data from any complex geological settings, e.g., sedimentary environments, shallow or deep intrusions (laccoliths and sills) with complex shapes, mineral environments. Test on field data collected over a mafic-ultramafic body and a volcano-sedimentary sequence, located in the Tocantins provinve that lies between the Amazonian and São Francisco Cratons, Brazil, illustrate that our method makes its possible to reconstruct a sharp image of multiple and adjacent bodies. Finally, our method can be extended to invert magnetic, MT, EM, and seismic data.
GP54A-08
Inversion of potential-field data for layers with uneven thickness
Inversion of large-scale potential-field anomalies, aimed at determining density or magnetization, is usually made in the Fourier domain. The commonly adopted geometry is based on a layer of constant thickness, characterized by a bottom surface at a fixed distance from the top surface. We propose a new method to overcome this limiting geometry, by inverting in the usual iterating scheme using top and bottom surfaces of differing, but known shapes. Randomly generated synthetic models will be analyzed, and finally performance of this method will be tested on real gravity data describing the isostatic residual anomaly of the Southern Tyrrhenian Sea in Italy. The final result is a density model that shows the distribution of the oceanic crust in this region, which is delimited by known structural elements and appears strongly correlated with the oceanized abyssal basins of Vavilov and Marsili. As a possible future improvement we show the implication for simultaneous inversion of gravity data, both for density distribution and for bottom interface, under the hypothesis of local compensation.