Near Surface Geoscience 2016 - Second Applied Shallow Marine Geophysics Conference

Harmonic noise in seismic data is a single frequency noise caused by power line within the data acquisition systems and generally filtered by classical notch filter. However, the notch filter would attenuate all recorded data around notch frequency at especially 50 Hz (or 60 Hz) and so leads to loss available information that explains fine details in the seismic data. On the other hand, iterative trimmed and truncated mean filter method locally applied in spectral domain (LITTM) is a signal protector filter and is very effective to remove the harmonic noise in the signal. In this study we proposed to compare the quality of migrated sections after removing the harmonic noise by using classical notch and LITTM filters. We tested the applicability and reliability of the LITTM filter in removing the single frequency harmonic noise on synthetic data set and then applied it to a marine data from Turkey. When comparing the outputs from classical notch and LITTM filters, we found that the LITTM filter strikes a balance between the removing harmonic noise and protecting available frequencies. Our experiments show that the LITTM filter can effectively be used to eliminate other spike-like noise within the bandwidth of the data.

Marine geophysical techniques are a staple tool for shallow section site investigation. Very- and Ultra-High-Resolution (VHR and UHR) seismic reflection imaging using Chirp, boomer, sparker and small airgun sources is a particularly common technique, employed the world-over for site investigations in a huge range of water depths, substrates, and environments. Seismic reflection profiling is relatively fast, cheap, and flexible, permitting the acquisition of surveys to target everything from decimetre-scale object detection to large-scale geological ground model building. However, these data are almost always interpreted in a purely qualitative manner, providing information on the spatial variation in facies boundaries, but no quantitative information on the composition (and spatial variation in composition) of said facies. In both industry and academia, quantitative information regarding the particular geological and geotechnical makeup of the subsurface is predominantly derived from direct sampling with cores, boreholes, and cone penetrometers.

I present a summary of the published research on quantitative imaging of the shallow subsurface. I look at the different inversion methods that have been developed and the soil/sediment properties derived. I will conclude by discussing the major outstanding questions and propose some potential pathways forward that may bring site investigation more in line with hydrocarbon prospection.

A quantitative physical model of the shallow marine sediments is of crucial importance in broad range of environmental and engineering contexts, from the assessment of tsunamigenic landslides hazard and offshore structure stability, to the identification and monitoring of carbon capture and storage sites. However, in contrast to reservoir geophysics, where quantitative seismic interpretation and inversion are extensively employed tools, sub-seabed characterisation is still heavily reliant on direct samplings, using cores, boreholes and CPTUs. In this context, the role of seismic reflection is often limited to a mostly qualitative interpretation of the reflection architecture. Here we will present the first published application of pre-stack elastic full waveform inversion to a Very-High-Frequency (VHF, 0.4 – 2.5 kHz) multi-channel seismic reflection dataset. A custom-built local multi-parameter sequential waveform inversion method has been developed and tested on two common-shot gathers from a multi-channel Boomer seismic profile acquired in northern Norway. The decimetre-resolution impedance and Poisson’s ratio obtained models strongly agree with the a-priori geotechnical and geological information available in the area, proving the potential of the method in obtaining an accurate remote characterisation of the shallow sediments within a reasonable computational cost and using a traditional multi-channel sub-bottom profiler.

The quality of high-resolution marine 8-channel air gun seismic data can be enhanced by correcting for the effects of swell using the average of previously detected depth values. To apply this correction, we detected and calculated the average two-way travel times of sea bottom depths in seismic data acquired offshore near Yeosu, Korea, and then corrected for the differences between the detected and averaged values. Automatic sea bottom detection is required to obtain sea bottom locations from numerous seismic traces. However, it is difficult to detect locations when the sea bottom signal is unclear due to noise generated by the motion of the sea. We chose reliable sea bottom locations within a certain range around the averaged sea bottom depth. We achieved reasonable detection results using our method. Correcting for swell significantly improved noisy data and produced relatively high-quality seismic sections.

Several shallow submarines areas are characterized by considerable releases of hydrocarbon gases, which, during earthquakes or volcanic activity, may suddenly and violently occur. In some instances, changes in chemical and physical properties of gas emissions, which are generated by geothermal systems connected to the volcanoes’ plumbing system, are important precursors of volcanic activity and useful indices for the study of the geothermal field. Furthermore, some superficial structures such faults, landslide slopes or archaeological relics should be studied with such a high detail that geophysical and bathymetric surveys are unable to provide.

In response to the needs increasingly felt of a completely customizable low cost tool for geological/geophysical/geochemical investigation of shallow submarine system, in the framework of the ROVER project, starting from an international project called OpenROV, is being implemented a micro-ROV for multidisciplinary environmental investigation.

In this paper, we will describe the project progress, with particular reference to the first tests conducted in pool and in sea water, the geophysical/geochemical instrumentation of which will be equipped and the automatic control algorithms that are being developed for the semi-automatic navigation.

The Quaternary evolution of the Mediterranean-Black Sea connection area is strictly related to the global sea level variation during the Last Glacial Period.

An integrated analysis of a large amount of seismic, geotechnical and environmental data, collected during several marine surveys carried out by Fugro, shows how the private sector can corroborate and expand on researches of the scientific community.

In the last 2 decades this area has attracted the attention of geologists, anthropologists, historians, and teologists all around the world.

Did the transgressive sediments of the Black Sea, deposited between the Wurmian Low Stand and the Holocene High Stand, represent the expression of catastrophic Flood Hypothesis described on the Holy Bible?

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To perform environmental monitoring of deep-sea drill sites (e.g., gas leakage and seafloor deformation) and for optimal placement of monitoring equipment on the seafloor, the exact fault trace and any possible fault offsets need to known. We mapped the structures around a growth fault near the gas hydrate drill site. Then, based on the contour lines, we derived individual two-dimensional (2D) chirp data and merged 3D volumes. The chirp data revealed a clear growth fault slightly south (about 210 m) of drill site. The fault was mapped across all inlines of the 3D volumes. Fault offsets were determined for five individual layers and a clear change in offset with depth was observed. A reduced offset was noted below layer L2 (∼1 m) but it was to 2.5 m for both underlying layers L3 and L4. Based on five piston cores across the survey area between North Zone and South Zone, the linear sedimentation rate for the Holocene showed almost no difference (16.5–16.7 cm/kyr). The rate below the Holocene showed a difference between the North Zone (lower; 12.6–12.8 cm/kyr) and the South Zone (higher; 14.8–16.1 cm/kyr). This change is explained by a sudden reduction of fault activity during the Holocene.

The integration of a large set of geophysical, geological, geotechnical and geochemical data through a multi-disciplinary team approach provided new insights into the geological processes occurring along the Western Black Sea Bulgarian Margin. These results provided a solid basis and key input parameters for slope stability and debris flows runout analysis.

High resolution mapping of the continental slope revealed the presence of three main types of recent shallow slope instabilities. Each type of failure characterizes a geomorphic domain and, assuming a common seismic trigger, is inferred to be linked to peculiar preconditioning factors.

Small scale retrogressive slumps occurred below the shelf edge, and generated debris flows with limited (few km) runouts and velocities up to 9 m/s. Such failures were favored by the presence of shallow gassy sediments. A regional composite headscarp crossing the slope is continuously cut back by retrogressive failures, separated in time by several hundred years. A similar slope failure return period is expected for future slides based on seismic-based slope stability analysis.

Low-deformation superficial slumps occurred almost exclusively in finely laminated sapropels deposited in the last 8,000 years and formed widespread parabola-shaped sediment ridges.