Synthetic Aperture Source Research Articles

Synthetic aperture source localization

Many state-of-the-art methods in source localization require large numbers of sensors and perform poorly or require additional sensors when emitters of interest transmit highly correlated waveforms. We present a new source localization technique which employs a cross correlation measure of the time difference of arrival (TDOA) for signals recorded at two separate platforms, at least one of which is in motion. This data is backprojected through a process of synthetic aperture source localization (SASL) to form an image of the locations of the emitters in a region of interest (ROI) This method has the advantage of not requiring any a priori knowledge of the number of emitters in the scene. Nor does it rest on an ability to identify regions of the data which come from individual emitters, though if this capability is present it may improve image quality. We demonstrate that this method is capable of localizing emitters which transmit highly correlated waveforms, though complications arise when several such emitters are present in the scene. We discuss these complications and strategies to mitigate them.

Enhancing the detectability of a high-resistivity target by using a synthetic aperture source for 3D marine CSEM modelling of a rugged seafloor

When processing marine controlled-source electromagnetic (CSEM) data from a rugged seafloor, enhancing the reservoir response and suppressing the topographic effect and other interference are significant issues, especially in shallow water. We simulated the CSEM responses specific to these issues using an efficient finite-difference (FD) code. The synthetic aperture technique was applied to steer the EM field toward a high-resistivity target on the seafloor. A weighted 2D synthetic aperture source was constructed by imposing a real weighting factor on each source point. Numerical experiments showed that using the weighted 2D synthetic aperture source significantly enhanced the effective CSEM signals. Because of the destructive interference between bathymetric distortion and the airwave effect in shallow water, the synthetic aperture technique is useful for dealing with seafloor topography. Better results can be obtained before steering the distorted response with a bathymetric correction. However, the detectability results may exhibit a huge difference in numerical values if the background resistivity of the bathymetric model is estimated incorrectly.

Optimized 3D synthetic aperture for controlled-source electromagnetics

Locating offshore hydrocarbon reservoirs has become more challenging with smaller, deeper, or shallow-water targets in complicated environments. Controlled-source electromagnetics (CSEM) is a geophysical method used to find reservoirs in marine settings. The diffusive nature of CSEM fields means that the signal from the target is only a small part of the total field. To reduce the impact of the complicated settings and to improve the detecting capabilities of CSEM, we have applied a synthetic aperture to CSEM data. The synthetic aperture virtually increased the length and width of the CSEM source by combining the responses from multiple individual sources. Applying a weight to each source steered or focused the synthetic aperture source array in the inline and crossline directions. We have developed an optimization method to find the optimal weights for synthetic aperture arrays that adapted to the information about the reservoirs in the CSEM data. To demonstrate the benefits of a weighted synthetic aperture, we have applied a 2D synthetic aperture array and a crossline-only synthetic aperture array to noisy, simulated electromagnetic fields. Both synthetic aperture arrays reduced the noise and increased the detectability of the modeled reservoirs when compared with the original CSEM response. The crossline-only synthetic aperture array also preserved the structural information within the measurements.

A deep-water experiment to measure mid frequency attenuation

A deep-water experiment was performed off the west coast of the United States with a short vertical array cut for 7.5 Hz and a source transmitting tonals as well as chirps. The data were processed to identify eigenray arrivals out to a convergence zone for the ultimate purpose of revisiting mid frequency, deep water volume attenuation in the Pacific Ocean and comparing it to decades old measurements and models. Further, aside from the beamforming done at the receiver array, attempts were made to construct a synthetic aperture source array in order to perform “double beamforming” and thereby enhance the signal to noise ratio. We present our initial results of this experiment.

Research on 3D marine electromagnetic interferometry with synthetic sources for suppressing the airwave interference

In order to suppress the airwave noise in marine controlled-source electromagnetic (CSEM) data, we propose a 3D deconvolution (3DD) interferometry method with a synthetic aperture source and obtain the relative anomaly coefficient (RAC) of the EM field reflection responses to show the degree for suppressing the airwave. We analyze the potential of the proposed method for suppressing the airwave, and compare the proposed method with traditional methods in their effectiveness. A method to select synthetic source length is derived and the effect of the water depth on RAC is examined via numerical simulations. The results suggest that 3DD interferometry method with a synthetic source can effectively suppress the airwave and enhance the potential of marine CSEM to hydrocarbon exploration.

Electromagnetic interferometry in wavenumber and space domains in a layered earth

With interferometry applied to controlled-source electromagnetic data, the direct field and the airwave and all other effects related to the air-water interface can be suppressed in a data-driven way. Interferometry allows for retreival of the scattered field Green’s function of the subsurface or, in other words, the subsurface reflection response. This reflection response can then be further used to invert for the subsurface conductivity distribution. To perform interferometry in 3D, measurements on an areal grid are necessary. We discuss 3D interferometry by multidimensional deconvolution in the frequency-wavenumber and in the frequency-space domains and provide examples for a layered earth model. We use the synthetic aperture source concept to damp the signal at high wavenumbers to allow large receiver sampling distances. Interferometry indeed increases the detectability of a subsurface reservoir. Finally, we discuss the dependency of the accuracy of the retrieved reflection response on the two crucial parameters: the conductivity of the seabed at the receiver location and the stabilization parameter of the least-squares inversion.