S23A-1099
Development of Real-time Seismic Cable System
RSCS (Real-time seismic Cable System) is a new real-time seafloor reflection seismic observation system. It is a series of 3-component geophones and telemetry opto-electronics equipped into a high pressure resistant housing, which enables real-time seismic data acquisition at ultra deep sea. The first reflection seismic survey was carried out in March 2006. The geophone data show the high quality with the advantage such as precise timing with GPS link on board and superior sensor directivity. It is worthwhile for P- S converted wave processing as well as for P-P processing. The data processing is different from the conventional reflection seismic such as CDP method in the viewpoint of its geometry and P-S conversion. Precise velocity analysis and prestack migration solve the problem of its elevation difference between shot (sea surface) and receiver (sea bottom) point. The second seismic survey has been carried out in December 2006, equipped with accelerometer and hydrophone. These data was also very high quality as same as the geophone data. The accelerometer implies that the geophone will be replaceable with accelerometer. It means that there is a possibility to reduce the sensor size without gimbals mechanism. The hydrophone data is similar to the geophone data except the polarity change. This characteristic enables us to separate upgoing reflection from downgoing wave which includes only direct water waves and multiple reflections. The total data quality so far is very good compared to the conventional OBS systems. The RSCS has a great potential as a seabed observation system at deep sea.
S23A-1100
Developement of two ocean-bottom seismometers specialised in wide-angle seismic (MicrOBS) and seismological data acquisition (LotOBS).
A new generation of ocean bottom seismometers (MicrOBS) has been developed in a collaboration between ORCA instrumentation/SERCEL Brest and Ifremer (French Research Institute for Exploitation of the Sea) for wide- angle seismic data acquisition. For the first time, the acquisition and release electronics have been integrated to decrease the size, weight and cost of the equipment. Downloading of data by USB cable and recharging of batteries is accomplished from the outside allowing redeployment of the equipment without opening the glass sphere. The instrument is equipped with a hydrophone and a three channel internal geophone. The instrument weight is only 19 kg and it is housed in a 13 inch glass sphere. The instrument can record for up to 14 days during a 3 week period or 22 days during a one month period depending on the choice of rechargable or normal batteries. Data from the first seismological experiments performed off Sumatra and in the Mediteranean Sea demonstrate excellent quality of data from all four channels. Ifremer is now developing an OBS specialised for seismological data acquisition (LotOBS), using the same acquisition electronics as in the MicrOBS. In order to include sufficient batteries to be able to record for 8 months (during a deplyment period of up to 12 months) the instrument is housed in a 17 inch glass sphere with a weight in air of about 50 kg. The external 4.5 Hz geophones are deployed briefly after the arrival of the instrument on the sea-floor. The first tests of the instrument will be carried out in 2008.
S23A-1101
HOBIT-The new Hamburg Ocean Bottom Tiltmeter: A First Deployment at Columbo Seamount, Aegean Sea
Assessing the state of activity of subaerial volcanoes is already quite a difficult task, but assessing the activity of submarine seamounts is even more complicated due to the difficulty of deploying instruments. At land various techniques can be used including seismic networks, deformation studies, gas measurements and others. At sea, it is mainly seismological observations, which are used to assess volcanic activity safe for some measurements with tiltmeters. However, especially at land deformation studies using INSAR have proven to be very valuable in determining recharge of magmatic systems. We therefore developed a free fall, self leveling ocean bottom tiltmeter mounted in a 17 inch glass sphere to observe deformation on the ocean floor. For measuring tilt signals we use a two component high resolution tiltmeter manufactured by Lipmann Geophysikalische Messgeräte (www.l-gm.de). The instrument has a resolution of about 1nrad (0.15μ°) and a maximum signal of about 0.045 rad (0.5°) can be detected. It is mounted on a levelling stage, which can relevel the instrument between ± 5° down to an accuracy of 0.006°. During the measurement this leveling stage is standing on the bottom of the glass sphere. For releveling purposes the instrument can be pulled up by very thin nylon strings and then is looked to a gimbal system in order to compensate for tilt >5°. This releveling procedure is done once every 48 hours. The data are recorded on an 18bit (at 50Hz sampling rate) MLS Geolon logger (www.send.de). Once deployed the instruments sleeps for a prescribed amount of time before the first levelling procedure is initiated. In addition to the tiltmeter and a hydrophone for receiving seismic signals, temperature, absolute pressure (paroscientific pressure sensor) to measure possible uplift or subsidence, and orientation (electronic compass mounted to the system) are monitored. 4 HOBIT systems were deployed for ten months (Jun06-Mar07) at Columbo seamount, a submarine volcano north-east of Santorini island, Aegean Sea, Greece, on a 3 km long profile perpendicular to the first principal stress axis σ1 of the regional stressfield. Three of the instruments operated the whole time, one shut down due to a leak in the power supply leading to a short circuit. First data processing indicates that small regional earthquakes as well as major tectonic earthquakes are properly recorded by the system. Following some regional earthquakes occurring during a seismic swarm we find a small, but permanent deformation associated with the earthquakes. Later during the deployment we also observe slow deformation processes occurring over a period of days. The implications of these findings as well as the general operational principle of these instruments will be discussed in detail in the presentation.
S23A-1102
Locating Microseism Sources in Offshore Southern California
We use the broadband stations from the S. California network to locate the apparent origin of secondary microseisms energy (5-8 Hz band). The procedure is to grid the offshore region and using each grid point as the source point, predict the response of a Rayleigh wave at each station. These predicted waveforms are then correlated with the data over a time window that is typically a 1/2 hour in length and composited at the grid point. The length of the time window controls a tradeoff between the spatial-temporal resolution of the sources and the robustness on the image. The procedure is valid for multiple sources. This results show that during periods of high microseism activity the sources are distinct at several locations in a region approximately 50-100 km offshore. For an 11/09/2002 Southern Ocean storm, for example, two zones parallel to each other and perpendicular to the coast are imaged.
S23A-1103
A Year of Microseisms in Southern California
Microseisms are due to continuous harmonic forcing by ocean waves, whose sources vary in time, frequency, and azimuth. Using frequency-domain array beamforming, this variation is studied using one-year of continuous seismic data from 155 stations in Southern California. Detailed analysis of data delineates spatiotemporal variations of sources for the primary and secondary microseisms. Both types of microseisms are generated near the coasts but the locations of excitation are different and change with season. Often sources are multiply located but can be spread out over wider areas, especially in the case of secondary microseisms. Distant storms can also be seen occasionally in the frequency range between the primary and secondary microseisms where spectral amplitudes from nearby sources are low. http://www.mpl.ucsd.edu/people/gerstoft/
S23A-1104
Mechanism of Excitation and Propagation of the Longest-period Microseisms on Oceanic structures.
Microseisms are generated by storms at sea. We have recently shown that in the period range 4-20 seconds, the explicit mechanism of excitation is the associated cloud-to- "ground" (CG) lightning strikes on the sea surface. The mechanism of propagation is high-order P-SV (Rayleigh) modes whose principal energy is in the mid- to lower-earth mantle. The greatest microseism amplitudes are those at the longest periods---up to at least 2560 seconds---and these were not in evidence from our initial computations with an earth model lacking a core. We have therefore extended the earlier work to treat a complete oceanic structure: a standard oceanic model down to 1115 km of depth, with the recent full-earth model AK135F used from this depth down to the center of the earth. The addition of the core completely changes the modal characteristics at all periods, but the change is especially striking at long periods. Adding just the (liquid) outer section of the core introduces the mantle-(outer) core interface wave/mode that (1) is composed of all usual P-SV modes, and (2) concentrates the wave's/mode's energy more and more dramatically in the region of the interface as period decreases. Then, adding the (solid) inner section of the core introduces a whole new suite of inner-core modes. In our spectral range of current interest---periods in excess of 100 seconds---the complicated interaction of the "usual" P-SV modes, the mantle- (outer) core interface wave/mode, and the suite of inner-core modes is demonstrated with the multimode phase- velocity, group-velocity, and energy-density-integral spectra. Using these spectral results, the quantitative evaluation of CG lightning strikes from storms at sea, as the mechanism for explaining the longest-period microseismic excitation, is then presented graphically: as a function of CG lightning strikes per day, the upper envelope of the computed surface excitation (Fourier-amplitude) spectra, from this full, modal representation is compared with the experimental maximum and minimum amplitude measurements.
S23A-1105
Swell Propagation Along Japan Coast Detected by Associated Microseisms Observed With Seafloor Cabled Observatories
A prominent case of swell propagation along Japan coast which was detected through the observation of microseisms with deep seafloor cabled observatories is presented. In July 2001, a large swell which was generated by No. 4 typhoon attacked Japan coast, although the typhoon itself moved westward in the ocean more than 2000 km south of southern coast of Japan and it did not exert direct influence on weather over Japan. Microseisms with a spectral peak at 0.13 Hz which were associated with the arrival of the swell were observed by three seafloor cabled observatories; off Muroto Cape in Kochi Prefecture (western part of Japan), off Hatsushima Island in Sagami Bay (central part of Japan) and off Kushiro in Hokkaido (northern part of Japan). Time difference of the appearance of the microseism is roughly equivalent to the propagation time of deep water wave, with the same time period of a significant wave observed by a wave height meter at the coast, over the distance corresponding latitude difference between the observatories. In other words, the swell propagated from south and reached north to Hokkaido. The peak frequency 0.13 Hz of the microseism corresponds to double frequency of the significant wave frequency observed at the coast. In addition, considering microseisms observed with seismometers on land, the microseisms associated with the swell appeared to be generated at southern coast of both Honshu and Hokkaido where the coastline lies almost perpendicular to the propagation direction of the swell, but did not at eastern coast of Honshu where the coastline is almost parallel. This suggests topographical influence on the generation of microseisms by swells. http://www.jamstec.go.jp
S23A-1106
Observations of Infragravity Waves at the Ocean-Bottom Broadband Seismic Stations Endeavour (KEBB) and Explorer (KXBB)
The long-period background noise observed at ocean-bottom seismic stations is mainly due to deformation of the seafloor under the pressure forcing by long-period ocean surface gravity waves (infragravity waves; 0.002 to 0.05 Hz). Understanding the nature and characteristics of the coupling between the infragravity waves and the solid earth is important for the study of the infragravity wave generation and dissipation as well as for the study of the earth's hum and structure using non-seismic sources. Ocean-bottom broadband stations KEBB and KXBB were deployed as part of a three-year multidisciplinary experiment funded by the W. M. Keck foundation to monitor the linkages between seismic deformation and hydrothermal fluxes on the northern Juan de Fuca plate. The seismic component of the project was a collaboration between the University of Washington, the University of Oregon, and the Monterey Bay Aquarium Research Institute. Station KEBB was installed 247 km west of Vancouver Island (VI) at a water depth of 2376 m in August 2003. A year later station KXBB was installed 105 km offshore VI at a water depth of 2370 m. Each station comprised a Guralp CMG-1T three-component broadband seismometer, sensitive over a wide frequency range from 50 Hz to 2.8 mHz (360 sec), connected to a recording and battery package. Both seismometers were completely buried in the ocean floor sediments. The two stations recorded data continuously and stored them locally until retrieval once per year. Infragravity waves can be observed at KEBB and KXBB on stormy as well as quiet days in the period band from 30 to 400 sec. When compared to the energy of short-period ocean waves recorded at local buoys, the low- frequency seismic noise is found to be mainly generated when the short-period ocean waves reach the coast, and not when the storm passes directly above the stations. Two types of modulation of the infragravity signal are observed. First, a longer-period modulation of the infragravity signal is observed and is best correlated with the energy of the 14-16 sec period ocean waves. Second, the entire infragravity band signal is modulated in phase with the tides. The observations suggest that the process which results in the tidal modulation takes place in the nearshore region, before the infragravity waves propagate back from the shelf into the deeper water. We also present a comparison of the long-period seismic noise observed on the vertical and horizontal KEBB and KXBB seismic channels to the nearshore ocean current measurements near the surface, as well as to the ocean bottom current observations from the Endeavour Ridge region only a few kilometers from KEBB.
S23A-1107
Broadband Ocean Bottom Instruments Record Earth's Free Oscillations during the Hawaiian PLUME Experiment
Ocean islands are usually thought of as being noisy sites for the global seismic network (GSN). For example, in the microseism band between 15 and 5~s, noise levels can easily be 10-20~dB higher than at stations in the interiors of continents. On the other hand, and somewhat curiously, several Pacific island sites are some of the quietest to record vertical ground movement in the free oscillation band beyond 200~s. This includes station KIP (Kipapa, on Oahu/Hawaii) that is operated jointly by the USGS and the French GEOSCOPE group. During the Hawaiian PLUME (Plume-Lithosphere Undersea Melt Experiment) deployment, we collected continuous seismic data from January 2005 through June 2007, using a variety of seismic sensors deployed on land and on the ocean floor. Ten broadband land stations were equipped with Wielandt--Streckeisen STS--2 seismometers, and about 70 ocean bottom sites were occupied with Güralp CMG-3T, Nanometrics Trillium 40 or Trillium 240 seismometers, and a Cox--Webb differential pressure gauge (DPG). This experiment gives us the unique opportunity to assess the quality and variability of ultra--long period seismic signals, for specific sensors, and evaluate the benefit and limits of deploying broad--band sensors on the ocean floor. In 2005, we recorded five very large earthquakes with scalar seismic moments of M0=2x1020~Nm or larger. One of these was the MS=8.2 28 March aftershock of the great 26 December 2004 Sumatra--Andaman earthquake. At station KIP, which is equipped with a very broad--band STS--1, the free oscillation spectrum is of extremely high quality. We can identify mode 0S2 (~0.31~mHz) that is observed only for the largest earthquakes, and only at the quietest GSN stations. Even on the STS--2 record, mode 0S3 (~0.47~mHz) is clearly discernible. The quality of spectra recorded on the OBSs varies greatly, but at some sites we observe mode 0S6 (~1.04~mHz). These records are greatly superior to those at GSN stations POHA (island of Hawaii), whose record has no seismic signal, and MIDW (Midway Island), whose record is noisy for this event. With M0=111x1020~Nm, events such as the 28 March aftershock occur only once every few decades and high-quality spectra should be expected. On the other hand, we can observe modes on OBS records also for the other four 2005 events, down to 0S13 (~2.11~mHz) and at a noise level that is not met by many land stations in broadband seismic monitoring arrays. PLUME may well be the first OBS experiment recording free oscillation spectra since the OSN1 pilot experiment recorded the great 25 March 1998 Balleny Island earthquake. Higher ambient noise levels in the pressure signal as well as the lower sensor sensitivity at ultra--long periods hamper the observation of free oscillations on the DPG. For the 25 March event, we can observe modes down to 0S20 (~2.88~mHz) but not much beyond that. http://mahi.ucsd.edu/Gabi/plume.html
S23A-1108
Seafloor borehole seismic observatories in the western Pacific and upper mantle and crustal structure beneath the northwestern Pacific basin
Geophysical networks provide data for better understanding of the dynamics in the Earth. For uniform distribution of stations on the Earth, we need to construct observatories in the sea. The western Pacific area had been selected for installation of ocean-bottom observatories because it is ideal for problems related to plate subduction. Boreholes give best environment for geophysical observations in the sea. The observatories WP-1 and WP-2 are effectively located to complete a 1000-km span network in the western Pacific area. The WP-1 site is in the west Philippine Basin west of the Kyushu-Palau Ridge. The WP-2 observatory is situated in the northwestern Pacific Basin. Both the stations fill gap for global seismic networks. The WP-1 observatory was activated in March 2002 using an ROV and long-term observation started. In June 2006, an ROV dived to the WP- 1 (fourth visit) and recovered the data. At this visit, data recording was discontinued. Seismic records of 692-days (Mar. 2002 - Feb, 2004) have been obtained from the WP-1. The WP-2 observatory was activated in October 2000 using an ROV. In June 2005, an ROV made fourth visit to the WP-2 and recovered the data. Recording at the WP- 2 has been suspended from the fourth ROV visit. In total, 436-days data (Oct. 2000 - Jan. 2001, Aug. 2001 - July 2002) were retrieved. A detailed structure of an oceanic plate is important information to consider a dynamics of oceanic plate. The seismic experiments with ocean bottom seismometers (OBSs), the WP-2 and airguns were performed. To detect a seismic anisotropy in the uppermost mantle, the experiments had four profiles with different directions. Reflecting low noise environment, the WP-2 recorded many teleseismic events. A detail seismic structure of crust and uppermost mantle including a seismic anisotropy was obtained by seismic surveys and an upper mantle structure including depths of discontinuities is estimated using the WP-2 records. A thickness of sedimentary layer is approximately 0.4km. The layer 2 is divided into two layers which have different vertical velocity gradients (layer 2A and layer 2B). Total thickness of the layer 2 is about 1.4 km. The uppermost Layer 3 has P- and S-wave velocities of 6.8km/s and 3.8km/s, respectively. The layer 3 is about 5km thick. The Pn velocities are different in each profile. Average velocities of Pn and Sn are 8.2 km/s and 4.7 km/s, respectively. The velocity variations are about 5% for P-wave and about 3.5% for S-wave. From this result, a seismic anisotropy of the uppermost mantle is suggested, and the fast direction seems to be perpendicular to the magnetic lineations. Therefore the anisotropy below the WP-2 is suggested to originate in a preferred orientation of olivine crystals in the uppermost mantle. To perform receiver function analysis, we selected 16 events with a magnitude greater than 6 from the WP-2 data. After the theoretical Ps-P times are calculated, travel times for large amplitudes of the receiver function corresponding to 410 km and 660 km discontinuties were read. Averaging the reading times, conversion depths of 416 km and 666 km were obtained.
S23A-1109
Resolution Analysis for Experiment Planning of a Nonlinear Seafloor Acoustic Inverse Problem
Geoacoustic inversion is the estimation of physical properties of the ocean bottom as a continuous function of position (or depth) in the seafloor given acoustic receptions in the water column. It is closely related to marine reflection seismology but also has features of refraction seismology, and uses sonar equipment and less than ideal geometries because accurate scientific determination of the seafloor may not be the primary goal of the experiments. However, the authors show how a pre-measurement inverse theory resolution analysis can be used as part of experiment planning regarding sensor placement and ship tracks, such that a desire for an experimental configuration giving the most information in bottom inversion can be quantitatively balanced with that for other needs like tracking and communication. This nonlinear geoacoustic inverse problem is ill-posed, so that one can only estimate the continuous function of seafloor properties to a limited resolution. This limited resolution varies with experiment geometry, frequency, and other such factors, and can be quantified in either a frequentist or Bayesian framework. Given statistics of the measurement noise (but without any new measurements themselves), the resolution can be quantified exactly for a linear inverse problem, and compared between different experiment geometries. Nonlinear problems complicate this picture, but if the problem can be transformed into a weakly nonlinear form then the resolution may still be explored in an approximate sense and used as a tool in the planning phase. The ideal situation is when previous seafloor estimates exist for the same region in which a new experiment with new geometry and configuration is being planned. For the scenario without previous results, a somewhat more ad-hoc approach can still compare changes in resolution across different seafloor models. This presentation demonstrates the technique for a synthetic problem involving a single stationary source and a single vertical array, but the formulation can be adapted to virtually any other sensor configuration as well.
S23A-1110
New method for the evaluation of crustal structure analysis using OBS-control sources-MCS records
In order to obtain on accurate best crustal velocity structure model, it is very important to make the best fit between the major phases identified in waveforms of the OBS wide-angle reflection and refraction survey data, and the MCS reflection sections. An ideal result is a crustal structure satisfying the waveform characteristics in both survey data. Recently, we have developed a new interactive software module (Kasahara et al., 2007) based on the gModeling-Pasteuph crustal structure analysis tool (Fujie et al.,m 2007) for OBS survey data to make a comparison between the OBS data and the MCS data in terms of time-offset distance sections. This integrated analyzing method computes the two-way travel times of normal incident waves from the analyzed crustal structure model. The software module in this method can superpose the time contours of the layers in the crustal structure model over the migrated time section of MCS. By an independent analysis, we also compute travel times of wide-angle reflections and superpose on the OBS seismic record sections. We also confirm the result of forward processing by the travel time inversion method and comparison to synthetic waveforms. Using a huge volume of real OBS-airgun seismic data and MCS reflection records around Japan and in the western Pacific, we confirmed that strong reflections from layers in the crust and the Moho seen in the OBS wide- angle reflection records are fairly consistent to the reflectors in the MCS section. Through the above integrated processes, we proved the correctness of the resultant crustal structure model. Such integrated analysis method may give the best crusal model using all available data.
S23A-1111
Evaluation of P-S conversion in the oceanic crust and Vp/Vs estimation
Recently, in order to determine crustal structure in the oceanic region, a huge amount of seismic data has been collected using a set of digital OBSs and a large volume tuned airgun-array. In the OBS-airgun records, P to S and/or S to P converted phases have been frequently observed by horizontal and vertical seismometers/hydrophone, respectively. Using such converted phases, we can estimate S wave structure in the crust and the mantle. In order to evaluate S wave velocities, the precise estimation of P and S velocity structures in the sediments is important. In this paper, we evaluated the P to S and S to P conversions to obtain S wave structure, and applied to the S wave structures in the crust and the mantle around Japan and the adjacent region. In the oceanic region, S(V) waves observed in horizontal components are converted from P to S(V) at interfaces with large impedance contrast. We evaluated a total energy flux of P-S conversion waves on 1) efficiencies of transmission and conversion from P to S and/or S to V through the ocean bottom/sediments/hard-rock interfaces and 2) relative converted P or S wave square amplitudes at OBS relative to the incident P waves penetrated into the ocean bottom. The conversion from P to S occurs at (a) sediments/hard-rock interface and/or (b) seawater/bare rock interface. The case (a) corresponds to the presence of thin unconsolidated sediment layer (P-wave velocity, Vp < 2.2km/s, S-wave velocity, Vs< 1.0 km/s) more than tens meters underlined by sedimentary rocks or hard rock layer (Vp>2.5km/s, Vp/Vs ratio ~ 1.78). The case (b) corresponds to bare rock layer exposed at the ocean bottom. We assume that the conversion occurs only at the ocean bottom and sediment layer /hard-rock interface and calculate the conversion rates in terms of relative energies. Among possible seven phases, large conversions are expected for (i) sediments/hard-rock interface at the incident side, (ii) at the ocean bottom of incident side, and (iii) at the sediments/hard-rock interface just beneath an OBS. Both of (i) and (ii) travel through whole crust as S wave. In the case of (iii), P wave converted to S wave only at the sediments/hard-rock interface just beneath an OBS. We examined real OBS-arigun data and found that most of conversions are observed on horizontal seismographs. Comparing P-wave velocity crustal structure, the observed records fit to the cases 1) and 3) for most of OBS records. Vp/Vs are 3 ~ 20 and Vs values are consistent with those measured by Hamilton (1976, 1979). In the region around Japan and the western Pacific, we obtained fairly constant Vp/Vs for the crust, namely 1.78. On the other hand, the Vp/Vs varied from 1.60 to 1.80 in the upper most mantle. The reason for this wide variation of Vp/Vs in the mantle is partly due to serpentinization, and partly due to S-wave anisotropy.
S23A-1112
Depth Imaging of OBS Reflection Data With Wave Field Separation
We propose a newly-developed depth imaging approach for OBS (Ocean Bottom Seismometer) reflection data in active-source structural survey using wavefield separation and PSDM (Prestack Depth Migration). OBS data includes a lot of valuable signals not only reflection but refraction. However its wavefield is contaminated with a various kind of waves that degrade the quality of the depth imaging of OBS reflection. Water reverberations, for example, have been thought as a strong source of noise in the imaging. Surprisingly, we found that the multiples reflection waves have the wide spreads of reflection points and we take advantage of this feature to improve the depth imaging after careful processing acquired OBS data. We would like to demonstrate that multiples could be utilized to enhance signal-to-noise ratio. The processing of OBS data in this study is summarized as follows. First, we categorize the OBS wavefield into two parts, i.e., near and far offset data. The near and far offset data inhere are waves that arrive after and before the direct water arrival, respectively. Then we separate the both near and far wavefields into two parts at the arrival of the first-order multiple. The reflection signals before the multiple are primary and up-going waves, whereas the reflection after the multiple events are mainly multiple and down-going in a common receiver gather. After these time-based separations, we apply up/downgoing wave field separation using geophone vertical-component and hydrophone data. Hydrophone records water pressure and, hence, are omni-directional while the vertical component of geophone measures a component of the vector response. These characteristic difference leads us to the separation of upgoing primary reflections and downgoing multiples using the polarity differences due to propagation direction of incoming waves. Finally, we obtain the OBS reflections to 4 domains, near offset primary, near offset multiple, far offset primary and far offset multiple after all the above decompositions. Aside from applying PSDM straightforward to the primary reflections, we could image the same reflectors using decomposed multiples. In this processing, we use a method of mirror-image in PSDM to migrate the multiple reflections under the assumption that the data were acquired at a virtual receiver position located at the top of the virtual water layer above the sea surface whose thickness is exactly same as the sea depth. For OBS geometry in which both primary and multiples are acquired, we found that the multiples could give the significant improvement in the PSDM imaging especially for near offset data, because the spatial redundancy of each reflection point which is very narrow for primary reflection becomes improved after the inclusion of multiples.
S23A-1113
Overpressure Prediction From Seismic Data: Implications on Drilling Safety
High rate of sediment influx into the Niger Delta via river Niger coupled with high rate of basin subsidence, very thick clayey members of Agbada and Akata Formations as well as prevailing presence of growth faults had been identified as the main factors responsible for overpressure generation and preservation in the Niger Delta basin. Analysis of porosity dependent parameters such as interval transit times and interval velocities derived from the seismic records of a field in the Western Niger Delta revealed the presence of overpressured formation at depth of 8670 feet, which is the top of the overpressured zone. The plot of interval transit times against depth gave a positive deflection from normal at the region of overpressure while interval velocity plot gave negative deflection; the ratio of this deviation in both cases is as high as 1.52. Pressure gradient in the upper, normally pressured part of the field was determined to be 0.465 psi/ft., which is within the established normal pressure gradient range in Niger Delta, while the abnormal formation pressure gradient in the overpressured region was determined to be 0.96 psi/ft., and this is also within the published abnormal pressure gradient range of 0.71 to 1.1 psi/ft. in Niger Delta. Formation fracture pressure gradients were determined from the formation pressure information to be 0.66psi/ft. in the upper part of the field and 1.2psi/ft. in the overpressured horizon. Mud weight window (MWW); mud density range necessary to prevent formation kick without initiating hydraulic fracturing was determined to be 10.2 to 12.5lbm/gal in the upper part of the field and 22.1 to 22.63lbm/gal in the overpressured horizon. MWW is indispensable for the selection of the mud pump type, capacity, pumping rate and mud densities at different formation pressure regimes. Overpressure prediction is also requisite for drilling program design, casing design as well as rig capacity choice before spudding. It is necessary to reduce well construction risk, save drilling hour as well as cut down drilling cost. If adequate predictions are not taken however, drilling hazards known as blowout may occur. Blowout, an uncontrollable flow of formation fluid into the well has made oil exploration and exploitation activities in Niger Delta, Southern Nigeria, a curse for the people rather than a blessing because considerable numbers of wells blew out during well construction activities, hence the characteristic oil spill which had degraded the environment, making fishing operation, a source of livelihood of the people difficult. Therefore the need for overpressure prediction as a guide for safe drilling, especially in unfamiliar exploration environments.
S23A-1114
Seafloor Acoustic Properties Studies: Panama City, Florida
In 2007 we carried out the first of two field programs designed to evaluate variations in seafloor and shallow sub- surface acoustic properties at three sites in the northern Gulf of Mexico. This two-year program includes the acquisition, processing, and interpretation of high-resolution geophysical data including multibeam bathymetry, multibeam backscatter, side scan sonar, and sub-bottom profiling. In addition to temporal (seasonal) and spatial variations, we are examining the differences in acoustic signature of the seafloor at different frequencies. During June 2007 and in relatively benign summer conditions, high-resolution multibeam (bathymetry and backscatter) and sidescan sonar datasets were acquired off Panama City, Florida, using the R/V "Pelican". A challenging mobilization included mounting the Kongsberg Simrad EM3002 (300 kHz) and EM1002 (100 kHz) to the vessel. Side-scan sonar datasets were acquired with a dual frequency Benthos SIS-1000 working at 90-110 kHz. Each of the surveyed areas were 2km x 2km (approx.) and were chosen as potential sites of varying acoustic properties both outside and inside St. Andrews Bay. Three areas were surveyed inside the bay (BMHdavis, BMHvehicle, BMHmud) and two were surveyed outside the bay (Area14 and Area8). The resultant multibeam bathymetry was gridded using a weighted uncertainty method at .5 meters and the multibeam backscatter amplitude gridded at .25 meters. All the data were tide and SVP corrected. The preliminary analyses indicate spatial acoustic differences between the sites. Within individual sites there are differences in the data as a function of the frequencies used to map the seafloor. A follow-up cruise in the upcoming winter "08 will survey the same sites with the same systems, using the same boat, to determine whether there are any seasonal variations in the datasets as well.
S23A-1115
Seafloor and Shallow Sub-surface Acoustic Properties Studies: a Geophysical Baseline for the Fluid-Mud MURI Site
During June 2007 and in relatively benign summer conditions, high-resolution multibeam (bathymetry and backscatter), sidescan sonar, and sub-bottom profiling datasets were acquired off the coast of Louisiana, west of Atchafalaya Bay, where the multi-year research program "Mechanism of Fluid-Mud Interactions Under Waves" (MURI) is focused. This survey was the first of two field programs separated by several months designed to evaluate variations in seafloor and shallow sub-surface acoustic properties at three sites in the northern Gulf of Mexico. In addition to temporal (seasonal) and spatial variations, we are examining the differences in acoustic signature of the seafloor at different frequencies. The surveyed area covered an area of 12 kilometers2 in shallow water (5-26 meters) plus two 54-kilometers long transects perpendicular to the coastline. They were chosen as being a baseline for further studies for MURI- related scientists (e.g., giving information about the sediment properties where WHOI sensors are deployed). The resultant multibeam bathymetry was gridded using a weighted uncertainty method at 0.5 meters and the multibeam backscatter amplitude gridded at 0.25 meters. All the data were tide and SVP corrected. A preliminary analysis of high resolution bathymetric data shows detailed natural and man-made features on the present seafloor. The sub-bottom data shows up to 20 meters of sedimentary layers and several infilled channels topped by a strong unconformity. A follow-up cruise in the upcoming winter "08 will survey the same sites with the same systems, using the same boat, to determine whether there are any seasonal variations in the datasets.
S23A-1116
Modeling of Conversion of Seismic to Acoustic Waves at the Seafloor Interface
Hydroacoustic waves are generated at the seafloor interface, by conversion of seismic waves and travel in the water column within the SOFAR channel with little attenuation. Recording T-waves with widespread arrays of autonomous hydrophones moored in the SOFAR channel allows to detect and localize many small-magnitude earthquakes in oceanic areas. However, hydroacoustic data cannot be used straightforwardly in seismic interpretations. In particular, because the physics of the seismic to acoustic conversion and the acoustic propagation is not completely understood, no direct information on the event magnitudes, focal mechanisms and focal depths can be directly derived from the hydroacoustic signals. In order to overcome some of these limitations, we have developed a mechanical model of the conversion from seismic to acoustic waves at the seafloor interface. The modelling is achieved through major adaptations of the 2D- finite element code "FLUSOL", which was originally developed to model fluid to solid energy conversion. Velocity displacement module within fluids and solids are derived from the stress and pressure computed for each grid element. We are able to model successfully, over a 10 x 10 km-grid, the seismic to acoustic conversion of waves generated by a source in the crust. Our model shows that a source with a high S-wave content appear to be more efficient in producing T-waves than a simple explosive source that only generates P-waves. Future work include the modelling of the conversion by more realistic seafloor topographies. Finally, we will use the output of SOLFLU as input to standard long-range acoustic propagation codes made available by the marine acoustics community. The modelled T-waves generated by various source mechanisms (tectonic or magmatic) will then be compared with real data to validate our conversion model. http://www-sdt.univ-brest.fr/internet/
S23A-1117
Holocene Transgression on the Bengal Shelf and Built up of the Submarine Ganges-Brahmaputra-Meghna Prodelta
The Ganges-Brahmaputra-Meghna Delta is characterized by a low lying and strongly subsiding coastal zone and is therefore very sensitive to sea level variations affecting drainage capacity by enhancing flooding potential during monsoon as well as impact of storm surges on the coast during pre- and post-monsoon. Mean Sea Level changes could additionally alter the balance between the huge sedimentary input delivered by the rivers to the Bengal Shelf and the high subsidence rate due to plate subduction. Predictability of these issues will be studied in an appropriate time scale to detect interaction of coastal development and sea level changes. During the SO-188-2 Expedition to the Bengal Shelf in July 2006 in cooperation between Bremen University and the BGR, Hannover, a detailed seismostratigraphic data set of the sedimentary deposits was acquired and acoustic facies associated with the Holocene sea level variations were determined. The continental shelf offshore the G-B-M Delta have been investigated with high-resolution multichannel seismic, Parasound and bathymetric surveys, ground truthed with a vibrocoring system to analyze the sedimentation and transport processes. The Holocene Transgression could be reconstructed by identification of seismic units, discontinuities and erosive surfaces preserved in the almost unaltered composite sequences. Data suggest that the sea level rise flooded the low-stand deltaic plain from west to east where a wide estuary system developed; relict Ganges delta's distributaries channels are filled with distal sediments rather than eroded. Since the onset of the modern high-stand delta and the formation of the topset-foreset-bottomset submarine system, the prodelta front has been actively prograding. The internal structure of the clinoforms reveals shelf wide episodes of mass depositions identified by transparent units in the Parasound data and internal unconformities in the seismic data. Volumes, distribution and frequency of these units, which might represent a main sediment transport modality of the prodelta system, will be presented together with sedimentological parameters.
S23A-1118
Tectonic controls on sedimentation in the Eel River Basin: high-resolution CHIRP seismic data
High-resolution CHIRP seismic data reveal the details of shore-parallel variations in Late-Pleistocene and Holocene sediments along the tectonically active Eel River Basin region, offshore Northern California. Shore- perpendicular deformation due to the northward migration of the Mendocino Triple Junction creates a tectonic signal with varying sign and amplitude oriented parallel to the coastline. This geometry provides an ideal opportunity to examine how tectonic deformation affects stratigraphic architecture and facies assemblages. Sediments are thinnest over the Little Salmon Anticline and systematically thicken towards the Eel Syncline. Seismic reflectors diverge into the syncline where sedimentary sections are expanded indicating tectonic deformation has been concurrent with sedimentation since the Last Glacial Maximum (LGM ~21,000 ka). We also observe offset on the transgressive surface above the thrust fault of the Little Salmon Anticline indicating fault displacement has occurred post transgression. While previous multi-channel seismic data sets from this region have detailed variation in the arrangement of facies and associated stratal geometry over the last several million years, this high-resolution CHIRP data set provides an opportunity to observe with greater resolution the formation of strata during the most recent deformation along this active margin.
S23A-1119
Crustal thickness variations due to deep faulting and the velocity structure of the Lucky Strike segment (37N Mid-Atlantic ridge)
The 2005 SISMOMAR seismic survey, part of the MOMAR project (Monitoring the Mid-Atlantic Ridge), included 2D and 3D seismic reflection and refraction experiments covering the Lucky Strike segment. Prior results from this cruise demonstrate the presence an axial magma chamber beneath the volcano (1), under which earthquakes are detected. Here we present results from the 3D seismic refraction experiment, which reveals the interplay of tectonics and magmatism throughout the crust. We analyse data from 27 seismic lines extending approximately 40 km along- and across-axis, with inter-shot spacings of 75 and 150 m. The data were recorded by 25 ocean bottom seismometers (OBS) in a 18x18 km grid with 4.5-km inter-OBS spacing, centered on Lucky Strike volcano. The instruments recorded arrivals of crustal turning waves and reflections from the Mohorovi\check{c}i\acute{c} discontinuity (Moho). We performed a joint travel time tomographic inversion on the turning waves and reflections. The turning rays constrain crustal velocities, whereas the reflected rays constrain velocities and the depth to the Moho. The data reveal a low velocity zone beneath the Lucky Strike volcano, spanning the depths between the axial magma chamber reflector and the Moho. The velocity anomaly is at least 0.6 km/s, indicating that melt is present. We also show evidence that upper crustal velocities are slower in the median valley than beyond the valley walls and that the change in velocity occurs rather abruptly at the valley wall faults. This suggests that the crust outside of the valley was emplaced in a different manner than that within the valley or that crustal formation is broadly distributed across the valley and stops at the walls. Finally, the crust is 0.8 km thicker to the west of the volcano than to the east, suggesting that the magma source has recently moved westward relative to the segment or that asymmetric tectonism plays a major role in the formation and modification of the crust. References: (1) S. Singh et al.: Discovery of a magma chamber and faults beneath a Mid-Atlantic Ridge hydrothermal field. Nature 442, 1029-1032 (2006).
S23A-1120
Seismic Structure of the Northernmost Ryukyu Subduction Zone, Southward Offshore of Kyushu, Japan
The Philippine Sea plate subducts northwest beneath the Eurasian Plate along the Nankai Trough in the north and the Nansei-Shoto (Ryukyu) Trench in the south at a rate of about 50-60 mm/year. Magnitude 8-class earthquakes occurred repeatedly along the Nankai Trough in the northeastern neighborhood, while the largest earthquake ever recorded is at most 7.6 along the northernmost Ryukyu Trench in the southern neighborhood. We carried out five ocean bottom seismographic (OBS) and multi-channel seismic (MCS) profiles in the northernmost Ryukyu subduction zone and obtained seismic velocity structural models related to the characteristics of the earthquakes in this region. For each investigation in 2005 and 2006, we shot a tuned airgun array with a volume of 8,040 cubic inches at an interval of 200 m for the wide-angle seismic profiles and at 50 m for the MCS (480 channels, 60 folds) profiles. The OBSs were deployed at an average interval of 5 km, which provided us dense data of high quality. The OBS data were modeled by a tomographic inversion, two-dimensional ray tracing and synthetic seismograms. The obtained MCS records show distinct reflections from the subducting plate boundary beneath the landward slope of the Ryukyu Trench and low velocity (Vp < 4 km/s) and thick (~ 8 km) materials above and on the decollement zone are modeled from the OBS data. Both the MCS and OBS records give clear images of very rough sea bottom configuration of the northern extension of the Kyushu-Palau Ridge and Amami Plateau below the landward slope. Such strong undulations of the plate boundary may constrain the sizes and/or natures of the asperities in this region. In addition, the estimated position of the seismic asperity of 1968 Hyuga-nada earthquake (Mw 7.5) corresponds to high velocity materials ascending in our P-wave velocity model.
S23A-1121
Marine magnetotelluric survey of the crustal structure, off the Tokai region, Japan
Resistivity structure below the sea floor has useful information on the characteristics in the generation mechanism of earthquake, and it also has a key to investigate the distribution of natural resources such as oil and methane hydrate since they all could be related to interstitial fluid migration. To clarify the crustal and mantle resistivity structures around the Enshu Basin, off the Tokai region, Japan, marine magnetotelluric(MT) soundings were carried out. The Enshu Basin is located where unconsolidated sediments on the subducting Philippine Sea plate are underplated to the southwest Japan arc above a mega-thrust seismogenic zone. Five ocean bottom electro- magnetometers (OBEM) and four ocean bottom electrometers (OBE) were used in this survey to measure two marine electromagnetic field data sets over the subducting Philippine Sea plate for a couple of periods, one from January to February and the other April to July in 2007. Both time series data of horizontal electric and magnetic fields were obtained at five sites, and only horizontal electric field data were obtained four sites. In this survey, we recorded raw electromagnetic signals with 8 Hz sampling rate and estimated MT responses at the region after resampling the time series data with 0.1 Hz sampling rate. Careful de-spiking and noise reduction to maximize the coherence of the data at the five sites led us to obtain apparent resistivity structure. Our two-dimensional inversion model demonstrates the electrical resistivity structure in both the crust and mantle, and resembles to a seismic reflection section. In this study, we performed TE-mode MT inversion. The preliminary result taking in geographical features illustrates outlines of the characteristic resistivity structure. This result gives us explanations by what the elements and conditions the earthquake is caused in the region.