Attenuation of Seismic Multiples in Very Shallow Water: An Application in Archaeological Prospection Using Data Driven Approaches

Water-layer multiples pose a major problem in shallow water seismic investigations as they interfere with primaries reflected from layer boundaries or archaeology buried only a few meters below the water bottom. In the present study we evaluate two model-driven approaches (“Prediction and Subtraction” and “RTM-Deco”) to attenuate water-layer multiple reflections in very shallow water using synthetic and field data. The tests comprise both multi- and constant-offset data. We compare the multiple removal efficiency of the evaluated methods with two traditional methods (Predictive Deconvolution and SRME). Both model-driven approaches yield satisfactory results concerning the enhancement of primary energy and the attenuation of multiple energy. For the synthetic test cases, the multiple energy is reduced by at least 80% for the Prediction and Subtraction approach, and by more than 60% for the RTM-Deco approach. The application to two field data sets shows a significant amplification of primaries formerly hidden by the first water-layer multiple, with a reduction of multiple energy of up to 50%. The waveforms obtained from FD modeling match the true waveforms of the field data well and small deviations in time and amplitude can be removed by a time shift of the traces as well as an amplitude adaption to the field data. The field data examples should be emphasized, where the tested Prediction and Subtraction approach works significantly better than the traditional methods: the multiples are effectively predicted and attenuated while primary signals are highlighted. In conclusion, this shows that this method is particularly suitable in shallow water applications. Both evaluated multiple attenuation approaches could be successfully transferred to two other 3D systems used in shallow water near surface investigations. Especially the Prediction and Subtraction approach is able to enhance the primaries for both tested 3D systems with the multiple energy being reduced by more than 50

Magmatic accretion versus serpentinized mantle exhumation at ultraslow spreading rates: constraints from seismic data and Vp/Vs ratios, Mid-Cayman Spreading Centre, Caribbean Sea

About 57% of the Earth’s surface is covered by oceanic crust and new ocean floor is continuously created along the ~60.000 km long mid-ocean ridge (MOR) system. About 25% of the MOR spread at an ultra-slow spreading rate of 1.9. Here, we report results from a seismic refraction survey from the ultra-slow spreading Cayman Spreading Centre in the Caribbean Sea, sampling mature crust along a flowline from both conjugated ridge flanks. The ocean-bottom-seismometer and hydrophones provide both P-wave and S-wave refracted arrivals. Travel time data were inverted using seismic tomography. Resulting Vp/Vs ratios suggest that up to 25% of the lithosphere have high ratios of >1.9, supporting serpentinization and exposure of hydrated mantle at the seafloor. Further, the mode of accretion has changed over time, supporting both areas of mantle exposure and magmatic crust. Magmatic crust has a typical layer 2 and layer 3 velocity structure and a thin crust of 3 to 5 km thickness. However, a well-defined Moho boundary was not observed. Thus, crustal rocks are characterized by typical crustal-velocities (7.6 km/s. Domains of un-roofed mantle have high Vp/Vs ratios and velocities gradually increasing to 7.4-7.6 km/s. In addition, we will use our results to re-assess the depth distribution of local earthquakes at ultra-slow spreading ridges, including the Cayman Trough and the Southwest Indian Ridge. Most importantly, the high Vp/Vs ratio of >1.9 characterizing serpentinized mantle causes earthquakes to focus at much shallower depth when compared to location procedures using a global average for Vp/Vs of 1.73; the bias in depth might be in the order of 10 km

Application of advanced seismic methods in near-surface prospection of the amphibian zone

This dissertation focuses on the application of advanced seismic methods in the so-called amphibian zone, the transition zone between land and water, where the application of non-seismic methods such as electrical resistivity tomography or ground-penetrating radar is limited by the natural conditions in their depth penetration and resolution. In the marine domain, the application of seismic methods in shallow water poses challenges to both technology and data analysis. In particular, data recorded in shallow water are often contaminated by multiple reflections, so that primary onsets can only be insufficiently interpreted. In order to eliminate the water-layer multiples, the first part of this dissertation evaluates two data-driven approaches based on finite difference modelling of the acoustic wave equation and compares them with industry standard multiple elimination methods for both synthetic and real data examples in water depths between 5.0 and 0.25 m. On land, seismic methods based on shear waves offer the advantage that their propagation is independent of the water content. They provide an additional set of independent subsurface parameters, namely seismic velocity and density. In particular, seismic waveform inversion (FWI) of shear wave data offers the possibility to image the near subsurface with high resolution in the decimetre range by means of detailed velocity and density models. The advantages of seismic FWI over established methods are demonstrated using examples of an archaeological structure in a lagoon environment and a medieval dike. In both cases, the high structural resolution of FWI is demonstrated by ground truthing in the form of cores and archaeological excavations. Thereby, the importance of seismic methods and especially of FWI in near-surface geophysical prospection is highlighted, showing that these methods offer a non-negligible additional benefit in terms of resolution and interpretation reliability.Diese Dissertation befasst sich mit der Anwendung fortschrittlicher seismischer Methoden in der amphibischen Zone, der Übergangszone zwischen Land und Wasser, wo die Anwendung nicht-seismischer Methoden, z.B. elektrische Widerstandstomographie oder Bodenradar, durch die natürlichen Gegebenheiten in ihrer Tiefeneindringung und Auflösung begrenzt ist. Im marinen Bereich stellt die Anwendung seismischer Methoden in flachen Gewässern eine Herausforderung sowohl für die Technik als auch für die Datenanalyse dar. Insbesondere sind die in flachen Gewässern aufgezeichneten Daten oft durch Multiplen überlagert, so dass die Ersteinsätze nur unzureichend interpretiert werden können. Um die in der Wasserschicht entstehenden Multiplen zu eliminieren, werden zwei datenbasierte Ansätze, die auf der Finite-Differenzen-Modellierung der akustischen Wellengleichung beruhen, bewertet und mit Standardverfahren für synthetische und reale Datenbeispiele in Wassertiefen zwischen 5,0 und 0,25 m verglichen. An Land bieten auf Scherwellen basierte seismische Verfahren den Vorteil, dass ihre Ausbreitung unabhängig vom Wassergehalt ist und mit seismischer Geschwindigkeit und Dichte ein zusätzlicher Satz unabhängiger Untergrundparameter geliefert wird. Die seismische Wellenforminversion (FWI) ermöglicht ein Abbildung des nahen Untergrunds mit hoher Auflösung im Dezimeterbereich. Die Vorteile der FWI gegenüber etablierten Methoden werden an zwei Beispielen demonstriert und die hohe Strukturauflösung durch den Vergleich mit Bohrkernen und archäologischen Ausgrabungen nachgewiesen. Dies hebt die Bedeutung seismischer Methoden, insbesondere der FWI, in der oberflächennahen geophysikalischen Prospektion hervor und zeigt, dass diese einen nicht zu vernachlässigenden Zusatznutzen in Bezug auf Auflösung und Interpretationssicherheit bieten

Characterization of silty to fine-sandy sediments with SH waves: full waveform inversion in comparison with other geophysical methods

We apply seismic full waveform inversion to SH- and Love-wave data for investigating the near-surface lithology at an archaeological site. We evaluate the resolution of the applied full waveform inversion algorithm through ground truthing in the form of an excavation and sediment core studies. Thereby, we investigate the benefits of full waveform inversion in comparison with other established methods of near-surface prospecting in terms of resolution capabilities and interpretation security. The study is performed in a presumed harbour area of the ancient Thracian city of Ainos. The exemplary target is the source of a linear magnetic anomaly oriented perpendicular to the coast, which was found in a previous magnetic gradiometry survey, suggesting a mole. The SH-wave full waveform inversion recovered a subsurface SH-wave velocity model with submeter resolution showing lateral and vertical velocity variation between 40 and 150 m/s. To tame the non-linearity of the full waveform inversion, a sequential inversion of frequency bands has to be combined with time-windowing in order to separate the Love wave from the reflected SH wavefield. We compare the full waveform inversion results with multichannel analysis of surface waves, standard seismic reflection imaging, electrical resistivity tomography and electromagnetic induction. It turns out that the respective depth sections are correlated to a certain degree with the full waveform inversion results. However, the structural resolution of the other geophysical methods is significantly lower than for the full waveform inversion. An exception is the reflection seismic imaging, which shows the same resolution as full waveform inversion but can only be interpreted together with the full waveform inversion-based velocity model. An archaeological excavation as well as coring data allows ground truthing and a direct understanding of the geophysical structures. The results show that the target was a sort of near-surface trench of about 3-4 m width and 0.8 m to 1.0 m depth, filled with silty sediment, which differs from the layered surrounding in colour and composition. The ground truthing revealed that only SH-wave full waveform inversion and seismic reflection imaging could image the trench and sediment structure with satisfying lateral and depth resolution. We emphasize that the velocity distribution from SH-wave full waveform inversion agrees closely with the excavated subsurface structures, and that the discovered changes in seismic velocity correlate with changes in the sand content in the respective sediment facies sequences. The study demonstrated that SH-wave full waveform inversion is capable to image structural and lithological changes in the near subsurface at scales as low as 0.5 m, thus providing the high resolution needed for archaeological and geoarchaeological prospection

Characterization of silty to fine‐sandy sediments with SH waves: full waveform inversion in comparison with other geophysical methods

We apply seismic full waveform inversion to SH‐ and Love‐wave data for investigating the near‐surface lithology at an archaeological site. We evaluate the resolution of the applied full waveform inversion algorithm through ground truthing in the form of an excavation and sediment core studies. Thereby, we investigate the benefits of full waveform inversion in comparison with other established methods of near‐surface prospecting in terms of resolution capabilities and interpretation security. The study is performed in a presumed harbour area of the ancient Thracian city of Ainos. The exemplary target is the source of a linear magnetic anomaly oriented perpendicular to the coast, which was found in a previous magnetic gradiometry survey, suggesting a mole. The SH‐wave full waveform inversion recovered a subsurface SH‐wave velocity model with submeter resolution showing lateral and vertical velocity variation between 40 and 150 m/s. To tame the non‐linearity of the full waveform inversion, a sequential inversion of frequency bands has to be combined with time‐windowing in order to separate the Love wave from the reflected SH wavefield. We compare the full waveform inversion results with multichannel analysis of surface waves, standard seismic reflection imaging, electrical resistivity tomography and electromagnetic induction. It turns out that the respective depth sections are correlated to a certain degree with the full waveform inversion results. However, the structural resolution of the other geophysical methods is significantly lower than for the full waveform inversion. An exception is the reflection seismic imaging, which shows the same resolution as full waveform inversion but can only be interpreted together with the full waveform inversion–based velocity model. An archaeological excavation as well as coring data allows ground truthing and a direct understanding of the geophysical structures. The results show that the target was a sort of near‐surface trench of about 3–4 m width and 0.8 m to 1.0 m depth, filled with silty sediment, which differs from the layered surrounding in colour and composition. The ground truthing revealed that only SH‐wave full waveform inversion and seismic reflection imaging could image the trench and sediment structure with satisfying lateral and depth resolution. We emphasize that the velocity distribution from SH‐wave full waveform inversion agrees closely with the excavated subsurface structures, and that the discovered changes in seismic velocity correlate with changes in the sand content in the respective sediment facies sequences. The study demonstrated that SH‐wave full waveform inversion is capable to image structural and lithological changes in the near subsurface at scales as low as 0.5 m, thus providing the high resolution needed for archaeological and geoarchaeological prospection

Episodic magmatism and serpentinized mantle exhumation at an ultraslow spreading centre

Mid-ocean ridges spreading at ultraslow rates of less than 20 mm yr−1 can exhume serpentinized mantle to the seafloor, or they can produce magmatic crust. However, seismic imaging of ultraslow-spreading centres has not been able to resolve the abundance of serpentinized mantle exhumation, and instead supports 2 to 5 km of crust. Most seismic crustal thickness estimates reflect the depth at which the 7.1 km s−1 P-wave velocity is exceeded. Yet, the true nature of the oceanic lithosphere is more reliably deduced using the P- to S-wave velocity (Vp/Vs) ratio. Here we report on seismic data acquired along off-axis profiles of older oceanic lithosphere at the ultraslow-spreading Mid-Cayman Spreading Centre. We suggest that high Vp/Vs ratios greater than 1.9 and continuously increasing P-wave velocity, changing from 4 km s−1 at the seafloor to greater than 7.4 km s−1 at 2 to 4 km depth, indicate highly serpentinized peridotite exhumed to the seafloor. Elsewhere, either magmatic crust or serpentinized mantle deformed and uplifted at oceanic core complexes underlies areas of high bathymetry. The Cayman Trough therefore provides a window into mid-ocean ridge dynamics that switch between magma-rich and magma-poor oceanic crustal accretion, including exhumation of serpentinized mantle covering about 25% of the seafloor in this region