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IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy (S) - IASPEI - International Association of Seismology and Physics of the Earth's Interior JSS013 Poster presentation 2237 Evaluation of the effects by heterogeneously inserted thin/thick high/low velocity layers in the part of crust Dr. Kayoko Tsuruga Japan Contineltal Shelf Survey Co. Ltd. IASPEI Junzo Kasahara, Yoshihiro Naito, Azusa Nishizawa, Kentaro Kaneda 1. Introduction In the crustal structure studies using OBS and control sources, we frequently use arrival times and waveforms (e.g., refracted and wide-angle reflected arrivals, later phases of refracted and reflected arrivals, and P to S and S to P converted waves) and other data obtained by MCS and gravity data (Kasahara et al., 2007, this meeting). In our analysis, we use a forward modeling method simultaneously referring synthetic waveform and a travel time inversion. Real observed seismic records frequently show some complicated waveforms whose arrivals quickly decay with offset distance. They are usually interpreted by the presence of low/high velocity layer or negative gradient in velocity layers. Because such heterogeneous structures greatly influence the model construction, we have to explain these phases with geophysical interpretation of such phases. In this report, we, therefore, try to interpret particular seismic phases resulting from small-scale heterogeneous crustal structure in the ocean region by using FDM simulation of several models. 2. Numerical simulation Using horizontally inhomogeneous structure models, we synthesize waveforms by the FDM method (E3D developed by Larsen, 2000). We examined several numerical examples. Basic structural model was a horizontal multilayered structure with 200km (H) x 20km (D). An oceanic crust with 7 km-thick was divided into four layers: (a) 0-1 km, (b) 1-2.5 km, (c) 2.5-3.5 km and (d) 3.5-7.0 km deep. We here show the following velocity gradient models for the layers (b) and (c) while the layers (a) and (d) has same velocity in any cases: Case-1: Velocity monotonously increases with depth for a whole model space (basic model) Case-2: Velocity in the both or either of layers (b) and (c) is constant Case-3: Velocity gradient changes at the interfaces such (a)-(b), (b)-(c) and (c)-(d). Case-4: Velocity discontinuity at the interfaces of such (a)-(b), (b)-(c) and (c)-(d). These models provides us a lot of in formations about the behaviors of amplitude observed in the cases of decollman, velocity reversal with depth and horizontal pinch-out of high velocity layer on OBS-airgun seismic records. We can also evaluate the characteristics of waveforms for thin high velocity layers such as chart and/or limestone layers at the shallow part and the fluid-filled layers existing in the crust of decollman. These inserted layers are frequently found in DSDP and ODP deep-sea drillings. Through the present case studies, seismic phases particularly with amplitude decay with offset distance are troublesome to construct the correct crustal velocity model because such phases are strongly affected by extremely local structure near receivers. So, in such case, we need to discard / screen such arrivals for the interpretation of long-offset arrivals because time delay through such local thin layers is very small, but the arrivals are very strong for shorter offset distance. 3. Summary Some OBS-arigun records show some arrivals which reflect the inhomogeneous velocity structure in space. The interpretation of such phase is not carried out by the usual interpretation method. In order to find the best treatment of such phases, we use FDM waveform simulation for several cases and screen the adequate data. Our results may greatly help for the interpretation of such troublesome phases. Keywords: crustal structure, synthetic seismogram, fdm
IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy (S) - IASPEI - International Association of Seismology and Physics of the Earth's Interior JSS013 Poster presentation 2238 Evaluation of efficiencies of P-S conversion in THE oceanic crust and Vp/Vs estimation Dr. Kayoko Tsuruga Japan Contineltal Shelf Survey Co. Ltd. IASPEI Junzo Kasahara, Eiichiro Nishiyama, Kei Murase, Ryuji Kubota, Azusa Nishizawa 1. Introduction In the OBS-airgun records, P to S and/or S to P converted phases have been frequently observed by horizontal seismometers and vertical seismometer/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 required. In this paper, we propose an evaluation method for the conversion efficiency of P to S and S to P and obtain S wave structure in the hard rock part of the crust. 2. Method 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, here, simply evaluate a total energy flux of P-S conversion waves as follows: 1) We estimate efficiencies of transmission and conversion from P to S and/or S to V through the ocean bottom/sediments/hardrock interfaces. 2) We evaluate relative converted P or S wave square amplitudes at OBS relative to the incident P waves penetrated into the ocean bottom. 3. Results The conversion from P to S occurs at (a) sediments/hard-rock interface or (b) seawater/bare rock interface. The case (a) occurs at the presence of thin unconsolidated sediment layer (P-wave velocity, Vp is less than 2.2km/s, S-wave velocity, Vs is less than 1.0 km/s) more than tens meters underlined by sedimentary rocks or hard rock layer (Vp is greater than 2.5km/s, Vp/Vs ratio is about 1.78). Such interface is often called as an acoustic basement by reflection seismics. The case (b) corresponds to bare rock layer exposed at the ocean bottom. For the present study, we assume 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 converts to S wave only at the sediments/hard-rock interface just beneath an OBS. 4. Examples We examined real OBS-arigun data observed in the western part of the Pacific Ocean. It was found that most of conversions were observed on horizontal seismographs and they fit to the cases of (i) and (iii) for most of OBS records by comparing P-wave velocity crustal structure. The Vp/Vs ratios were estimated to be 3 to 20 for sedimentary layers whose Vs were to be 1.6-2.0 km/s beneath the western Pacific Ocean. Vs values were consistent with those measured by Hamilton (1976, 1979). We also found that Vp/Vs values of the upper and lower parts of the oceanic crust were extremely constant (i.e., 1.78) in the western Pacific and the ocean basins of NE Philippine Sea. We did not found Vp/Vs values of 2.0-2.5 which indicates the fractured crust and serpentinized mantle. On the other hand, Sn arrivals in two horizontally perpendicular directions suggest the presence of the anisotropy in the upper-most mantle. Although the Vp/Vs value in the upper-most mantle was 1.73, those in some areas were less than 1.70. We report the details and interpretation of Vp/Vs in the crust and the mantle. 5. Conclusion Because sea water can transmit only P wave, we have to use converted S waves in the crustal structure studies in the ocean region. Using converted phases from P to S and S to P waves, we can estimate the conversion rates and estimate the Vp/Vs in the crust. Keywords: crustal structure, p s converted wave, s wave velocity
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IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy<br />
(S) - <strong>IASPEI</strong> - International Association of Seismology and Physics of the Earth's<br />
Interior<br />
JSS013 Poster presentation 2238<br />
Evaluation of efficiencies of P-S conversion in THE oceanic crust and<br />
Vp/Vs estimation<br />
Dr. Kayoko Tsuruga<br />
Japan Contineltal Shelf Survey Co. Ltd. <strong>IASPEI</strong><br />
Junzo Kasahara, Eiichiro Nishiyama, Kei Murase, Ryuji Kubota, Azusa Nishizawa<br />
1. Introduction In the OBS-airgun records, P to S and/or S to P converted phases have been frequently<br />
observed by horizontal seismometers and vertical seismometer/hydrophone, respectively. Using such<br />
converted phases, we can estimate S wave structure in the crust and the mantle. In order to evaluate S<br />
wave velocities, the precise estimation of P and S velocity structures in the sediments is required. In this<br />
paper, we propose an evaluation method for the conversion efficiency of P to S and S to P and obtain S<br />
wave structure in the hard rock part of the crust. 2. Method In the oceanic region, S(V) waves observed<br />
in horizontal components are converted from P to S(V) at interfaces with large impedance contrast. We,<br />
here, simply evaluate a total energy flux of P-S conversion waves as follows: 1) We estimate efficiencies<br />
of transmission and conversion from P to S and/or S to V through the ocean bottom/sediments/hardrock<br />
interfaces. 2) We evaluate relative converted P or S wave square amplitudes at OBS relative to the<br />
incident P waves penetrated into the ocean bottom. 3. Results The conversion from P to S occurs at (a)<br />
sediments/hard-rock interface or (b) seawater/bare rock interface. The case (a) occurs at the presence<br />
of thin unconsolidated sediment layer (P-wave velocity, Vp is less than 2.2km/s, S-wave velocity, Vs is<br />
less than 1.0 km/s) more than tens meters underlined by sedimentary rocks or hard rock layer (Vp is<br />
greater than 2.5km/s, Vp/Vs ratio is about 1.78). Such interface is often called as an acoustic basement<br />
by reflection seismics. The case (b) corresponds to bare rock layer exposed at the ocean bottom. For<br />
the present study, we assume the conversion occurs only at the ocean bottom and sediment layer<br />
/hard-rock interface and calculate the conversion rates in terms of relative energies. Among possible<br />
seven phases, large conversions are expected for (i) sediments/hard-rock interface at the incident side,<br />
(ii) at the ocean bottom of incident side, and (iii) at the sediments/hard-rock interface just beneath an<br />
OBS. Both of (i) and (ii) travel through whole crust as S wave. In the case of (iii), P wave converts to S<br />
wave only at the sediments/hard-rock interface just beneath an OBS. 4. Examples We examined real<br />
OBS-arigun data observed in the western part of the Pacific Ocean. It was found that most of<br />
conversions were observed on horizontal seismographs and they fit to the cases of (i) and (iii) for most<br />
of OBS records by comparing P-wave velocity crustal structure. The Vp/Vs ratios were estimated to be 3<br />
to 20 for sedimentary layers whose Vs were to be 1.6-2.0 km/s beneath the western Pacific Ocean. Vs<br />
values were consistent with those measured by Hamilton (1976, 1979). We also found that Vp/Vs<br />
values of the upper and lower parts of the oceanic crust were extremely constant (i.e., 1.78) in the<br />
western Pacific and the ocean basins of NE Philippine Sea. We did not found Vp/Vs values of 2.0-2.5<br />
which indicates the fractured crust and serpentinized mantle. On the other hand, Sn arrivals in two<br />
horizontally perpendicular directions suggest the presence of the anisotropy in the upper-most mantle.<br />
Although the Vp/Vs value in the upper-most mantle was 1.73, those in some areas were less than 1.70.<br />
We report the details and interpretation of Vp/Vs in the crust and the mantle. 5. Conclusion Because sea<br />
water can transmit only P wave, we have to use converted S waves in the crustal structure studies in<br />
the ocean region. Using converted phases from P to S and S to P waves, we can estimate the<br />
conversion rates and estimate the Vp/Vs in the crust.<br />
Keywords: crustal structure, p s converted wave, s wave velocity