Spatial Power Spectral Density Distribution of Magnetic Sources in the Gulf of Mexico and Caribbean Plate

Abstract:

Spectral methods are widely used in magnetism and gravimetry, including the estimation of the depth to the centroid of the anomaly source after Spector and Grant (1970). They aim to decompose the signal to its constitutive sources in the frequency domain. The power spectral density of a potential field decreases exponentially in function of the depth to the source and the wavelength. Hence, we usually relate deep sources to strong spectrum at long wavelength which decays rapidly. However, to estimate the depth to the source is not straightforward and requires a proper interpretation of local extrema in the spectrum. Such extrema can reflect either density or magnetic susceptibility contrasts in the subsurface or artifacts. Furthermore, complex geological settings may lead to ambiguous depth estimations.

Here we take advantage of the information provided by the radially averaged spectrum to explore the spatial distribution of the power spectral density at a regional scale. We calculate the radially averaged spectrum on 5°x5° sub-grids at intervals of 1.5° extracted from our recently compiled map of marine magnetic anomalies in the Caribbean and the Gulf of Mexico (Garcia et al., 2018) complemented by the WDMAM 2.0 data on land. We present a tridimensional image of the total power spectral density content and identify spatial correlations with the main geological structures. As expected, we observe low and homogeneous spectral energy for short wavelengths (0-~50 km), as these wavelengths are not present in the grid. We identify coherent zones of high spectral density for wavelengths higher than 50 km. These zones correspond to geological provinces, most often continental blocks: the Yucatan Block, the Northern Nicaragua Rise, the Guiana Craton, the Greater Antilles, and the Sierra Madre Range. We also observe such a zone of high spectral energy in the center of the Venezuela Basin.

Simple exploration of the Earth from the spectral content of potential field anomalies, before passing to depth in an initial stage, gives the opportunity to revisit and recognize useful spectral information, giving room to new perspectives in exploration.

References

Spector, A., & Grant, F. S. (1970). Statistical models for interpreting aeromagnetic data. Geophysics, 35(2), 293-302.