Dennis Wilken

Application of 2D Fourier filtering for elimination of stripe noise in side-scan sonar mosaics

Different types of along-track stripe noise are common artifacts in side-scan sonar mosaics, hampering both visual image quality and automatic seabed classification attempts. In this study, a 2D Fourier domain slope filter is applied to remove stripes from side-scan sonar mosaics. This technique has the advantage of using only two simple tuning parameters, namely, the slope angle of the stripe noise, and a filtering width. Furthermore, the image processing tool presented here can be applied to non-optimally processed side-scan mosaics without the need of reprocessing the original data. The performance of the filter technique is demonstrated by two datasets in which stripe noise, including nadir stripes, has been successfully removed. To one of the filtered dataset a simple unsupervised seabed classification was then applied, leading to a much improved result with only little misclassification due to residual stripe noise.

Effects of using inclined parametric echosounding on sub-bottom acoustic imaging and advances in buried object detection

This study reports an adaptation of a parametric echosounder system using 15 kHz as secondary frequency to investigate the angular response of sub-bottom backscatter strength of layered mud, providing a new method for enhanced acoustic detection of buried targets. Adaptions to achieve both vertical (0°) and non-vertical inclination (1–15°, 30°, 45° and 60°) comprise mechanical tilting of the acoustic transducer and electronic beam steering. Data were acquired at 18 m water depth at a study site characterized by a flat, muddy seafloor where a 0.1 m diameter power cable lies 1–2 m below the seafloor. Surveying the cable with vertical incidence revealed that the buried cable can hardly be discriminated against the backscatter strength of the layered mud. However, the backscatter strength of layered mud decreases strongly at >3±0.5° incidence and the layered mud echo pattern vanishes beyond 5°. As a consequence, the backscatter pattern of the buried cable is very pronounced in acoustic images gathered at 15°, 30°, 45° and 60° incidence. The size of the cable echo pattern increases linearly with incidence. These effects are attributed to reflection loss from layered mud at larger incidence and to the scattering of the 0.1 m diameter buried cable. Data analyses support the visual impression of superior detection of the cable with an up to 2.6-fold increase of the signal-to-noise ratio at 40° incidence compared to the vertical incidence case.

Case history: integrated geophysical survey at Katarínka Monastery (Slovakia)

Katarinka (St. Catherine) is the ruin of an abandoned Franciscan monastery from the early 17th century located in the western Small Carpathians in Slovakia. Historical sources and paintings suggest that, beside the remains of the monastery that are still visible, a circle of eight chapels, a pilgrim’s hospice, a cemetery, and garden terraces originally surrounded the main building of the monastery. From 2009 to 2012, geophysical campaigns were performed to find evidences and positions of remains of these buildings of the monastery campus. An initial magnetic overview survey revealed multiple local accumulations of disordered dipole anomalies. Since these accumulations did not allow a structural interpretation, ground penetrating radar measurements were conducted. The ground penetrating radar results clearly showed wall structures beneath almost all magnetic anomaly accumulations. In between the remains of the monastery main building, ground penetrating radar and electrical resistivity tomography were performed at different areas that were difficult to access because of a strong cover of vegetation and steep topography.

From Etruscan urban centre to medieval fortified village: San Giuliano Archaeological Research Project

Ongoing excavations at San Giuliano in central Italy are providing detailed evidence for testing explanatory models of cyclical shifts in settlements and socio-economic organisation from the Etruscan to medieval periods (c. 800 BC–AD 1300).

Shallow 3D Reflection Seismics

High-resolution 3D reflection seismology for near-surface applications is a noninvasive geophysical approach that uses sound wave reflected from subsurface discontinuities to infer and delineate the discontinuities. It results in high-resolution 3D structural images of the subsurface displaying mainly the interfaces between geological units and faults. The attributes “near surface” or “shallow” refer to sounding depths of typically some 10 m to 1 km. The 3D seismic method has been developed since the late 1960s. It is applied on- and offshore. This article gives an overview of the aims, principles, and realizations of shallow 3D reflection seismics. More details on field technique and data processing can be found, for example, in Meunier (2011) and Biondi (2006).