Günay Çifçi

Application of complex-trace analysis to seismic data for random-noise suppression and temporal resolution improvement

Time-dependent amplitude and phase information of stacked seismic data are processed independently using complex trace analysis in order to facilitate interpretation by improving resolution and decreasing random noise. We represent seismic traces using their envelopes and instantaneous phases obtained by the Hilbert transform. The proposed method reduces the amplitudes of the low-frequency components of the envelope, while preserving the phase information. Several tests are performed in order to investigate the behavior of the present method for resolution improvement and noise suppression. Applications on both 1D and 2D synthetic data show that the method is capable of reducing the amplitudes and temporal widths of the side lobes of the input wavelets, and hence, the spectral bandwidth of the input seismic data is enhanced, resulting in an improvement in the signal-to-noise ratio. The bright-spot anomalies observed on the stacked sections become clearer because the output seismic traces have a simplified appearance allowing an easier data interpretation. We recommend applying this simple signal processing for signal enhancement prior to interpretation, especially for single channel and low-fold seismic data.

Physical and Acoustic Properties of Gas-bearing Sediments in Jinhae Bay, the South Sea of Korea

High-resolution seismic survey and sediment core sampling were conducted to investigate acoustic characteristics of gas-bearing sediments in Jinhae Bay, the southeast of Korea. The sediment in Jinhae Bay is mostly homogenous mud deposited after the Holocene transgression. Along with the 410 km of chirp seismic profiling, five piston core samples were collected on the track lines. Gassy sediments are common and occur widely in the bay. Core samples were analyzed for sediment texture, physical properties (porosity, water content, bulk density, and grain density), acoustic properties (compressional wave velocity and attenuation), and electrical resistivity. X-radiograph image analysis was also performed to observe the shape of degassing cracks. There is no significant downcore variation on physical and sediment textures regardless of existence of gas bubbles. However, compressional wave velocity dramatically decreases from average 1480 to 1380∼739 m/s for the cores that penetrate the gas-bearing zones. This is probably due to degassying cracks that developed by escaping gases and free gas bubbles that are still trapped in the cores. Electrical resistivity is the only geotechnical property that increases in the gas-bearing zone where compressional wave velocity abruptly decreases. This indicates the possibility of using both electrical resistivity as an index variable as well as to compressional wave velocity to identify gassy sediment microstructure because there are little changes in texture and composition of sediment.

Mass Transport Deposits Periodicity Related to Glacial Cycles and Marine-Lacustrine Transitions on a Ponded Basin of the Sea of Marmara (Turkey) Over the Last 500 ka

The Sea of Marmara (SoM) is affected by large earthquakes occurring on the North Anatolian Fault. Numerous submarine mass movements have occurred and the most recent turbidites in the basins of the SoM have been related to historical earthquakes. Within the SoM, the occurrence of submarine mass movements and their size appears modulated by eustatic changes that can be accompanied by transitions between a salty marine environment and a brackish lake environment. Detailed analysis, using a 3D high-resolution seismic dataset, of stratigraphy over the last 500 ka, within a ponded basin of the Western High, shows that intervals of draped sedimentary reflectors alternate with onlap sequences that followed episodes of rapid sea-level rise, with a periodicity of approximately 100,000 years (corresponding to glacial cycles). Mass Transport Deposits (MTDs) occur within the onlapping sequences. Detail analysis of the youngest large slide, which probably followed the lacustrine transition to during Marine Isotopic Stage 4 is presented; and the possible triggering processes are discussed. The potential triggers of MTDs during this transition, in the context of the SoM are: (i) gas hydrate dissociation by pressure drop; (ii) changes in sediments supply and transport dynamics; (iii) variations in pressure and/or ionic strength in pores. The latter case appears the most suitable hypothesis, as salt diffuses out of the pores of the marine clay-rich sediment dominated by smectite at the beginning of low stand/lacustrine stages. The pore water freshening induces clay swelling, which can potentially drive sediment slope failure.

Towards permanent, multi-disciplinary seafloor observatories in the Sea of Marmara: Results from the Marmara Demonstration Mission of ESONET/NoE

The Marmara Demonstration Mission (april 2008 to september 2010) was conducted within the EU-funded ESONET Network of Excellence programme: i) to characterize the temporal and spatial relations between fluid expulsion, fluid chemistry and seismic activity in the SoM; ii) to test the relevance of permanent seafloor observatories for an innovative monitoring of earthquake related hazards, appropriate to the Marmara Sea specific environment; and iii) to conduct a feasibility study to optimize the submarine infrastructure options (fiber optic cable, buoys with a wireless meshed network, autonomous mobile stations with wireless messenger). A total of 6 cruises were conducted, allowing the selection of the optimum sites for the future multi-parameters sea-floor observatories: i) on the Istanbul-Silivri segment, located in the seismic gap immediately south of Istanbul where intense bubbling is observed; ii) on the Western High, where gas hydrates, oil and gas seeps from the Thrace Basin were found; and iii) at the entrance of Izmit Gulf near the western end of the surface rupture associated with the 1999 Izmit earthquake. A significant research effort has also been made during Marmara-DM for testing innovative sensors for monitoring variations in the geochemical and geophysical properties of gas emissions. The list of the sensors and the design of the observatories is described in a companion presentation, by Çagatay et al, [2011]. The results of the Marmara-DM demonstration mission support the necessity to monitor gas emission activity along with seismicity.

Gas and seismicity within the Istanbul seismic gap

Understanding micro-seismicity is a critical question for earthquake hazard assessment. Since the devastating earthquakes of Izmit and Duzce in 1999, the seismicity along the submerged section of North Anatolian Fault within the Sea of Marmara (comprising the “Istanbul seismic gap”) has been extensively studied in order to infer its mechanical behaviour (creeping vs locked). So far, the seismicity has been interpreted only in terms of being tectonic-driven, although the Main Marmara Fault (MMF) is known to strike across multiple hydrocarbon gas sources. Here, we show that a large number of the aftershocks that followed the M 5.1 earthquake of July, 25th 2011 in the western Sea of Marmara, occurred within a zone of gas overpressuring in the 1.5–5 km depth range, from where pressurized gas is expected to migrate along the MMF, up to the surface sediment layers. Hence, gas-related processes should also be considered for a complete interpretation of the micro-seismicity (~M < 3) within the Istanbul offshore domain.

Correction to: Gas occurrence and shallow conduit systems in the Western Sea of Marmara: a review and new acoustic evidence

The original version of this article unfortunately contained a mistake. The affiliation of Pierre Henry should have been the following: Aix Marseille Univ, CNRS, IRD, INRA, Coll France, CEREGE, Aix-en-Provence, France. This correction was requested by Aix-Marseille University.

Studies of deep seismic and modelling in Calabria (Italy)

The data of complete refraction-reflection seismic profiles were acquired with the goal to investigate the crustal structure of the Serre Mountains, Calabria (Southern Italy). The Calabrian arc is located at the place where continent-continent collision between the African and Eurapean plates had already been occured at the end of the Eocene.The tectonics are ruled by the convergence system under the forces of subduction and collision. The importante of this area is that it represents a nearly complete crustal section. The outcropping lithological sequence is a tilted block that comprises interlayered acidic, mafic and pelitic rocks metamorphosed to granulite and amphibolite facies. Outcropping and tilting veere occured afterwards during the Appennic orogeny. In the Serre area, the lower crustal rocks outcropped for an area of about 400 km2 and expose a 7-8 km thick lower trust.