Boulder Detection in the Shallow Sub-Seafloor by Diffraction Imaging With Beamforming on Ultra-High Resolution Seismic Data-A Feasibility Study

Small-scale heterogeneities within the seafloor such as glacial boulders, concretions, or unexploded ordnance are of major scientific and economic interest. Because of their small size, such objects are hardly imageable by conventional seismic methods since they produce only faint diffractions. Despite the growing interest, reliable and efficient object detection methods are still not established. So, a marine acquisition system to image objects in the size range 0.3-4.1 m in the near-surface has been designed and workflows have been developed. Source signals with a central frequency of similar to 1,000 Hz are necessary to generate diffractions for the object size range. Performing beam pattern analyses, a rigid tow frame with the dimensions 8 x 3 m and attached hydrophones was designed. A synthetic aperture approach is realized to improve the resolution. Reflection events are suppressed in Normal-Move-Out corrected shot gathers by the muting of high singular values to enhance the diffractions. The diffractions are imaged with a beamforming algorithm with a high efficiency, as a total swath angle of about 80 degrees in across-track direction is covered. The processing of synthetic data sets allows an optimization and validation of the workflow and sensitivity analyses. Ground truthing is achieved with 1-2 m large boulders on the seafloor in 22 m depth, which were identified in a Multi-Beam Echo Sounder data set. Although diffractions are only weak events in seismic data sets, 3D object detection with seismic beamforming has been found to be an efficient and reliable method to perform derisking for offshore infrastructure installations, drillings, or geologic interpretation. Plain Language Summary Buried glacial boulders and other small, nonferrous objects like unexploded ordnance are hardly detected with standard geophysical methods. However, those objects are of special relevance being indicators for glacial processes and posing risks for infrastructure installations. So, we designed a marine acquisition system and a processing scheme to exploit the seldom used diffracted wavefield of a seismic record. The system includes a sound source emitting an acoustic signal with a dominant frequency of 1,000 Hz to the water, a rigid tow frame of 8 x 3 m with attached underwater sound recorders and 3D positioning. As a first step, the digital data processing aims to enhance the diffractions, which are the waves scattered by small-scale objects. Second, beamforming is used to detect the objects building on the constructive interference of the sound recorded and combined at calculated time shifts. As a swath of the seafloor greater than twice the depth of the water can be investigated in a single pass, our proposed method is very efficient. In this study, we show the development and validation of the method with computer models. Also, the practical feasibility is proven with the detection of 1-2 m large boulders on the seafloor of the North Sea in about 20 m depth.

Automated summary

Small-scale heterogeneities within the seafloor, such as glacial boulders, concretions, or unexploded ordnance, are of great scientific and economic interest. However, reliable and efficient object detection methods for these small objects are still lacking. In this study, a marine acquisition system and workflows were designed to image objects in the size range of 0.3-4.1 m. An efficient beamforming algorithm was used to image the diffractions, providing a reliable method for 3D object detection.