Identification and classification of multiple reflections with self-organizing maps

Artificial neural networks can be used effectively to identify and classify multiple events in a seismic data set. We use a specialized neural network, a self-organizing map (SOM), that tries to establish rules for the characterization of the physical problem. Selected seismic data attributes from CMP gathers are used as input patterns, such that the SOM arranges the data to form clusters in an abstract space. We show with synthetic and real data how the SOM can identify and classify primaries and multiples, and how it can classify the various types of multiple corresponding to a certain generating mechanism in the subsurface.     

A simple model of seasonal open ocean convection

 Aspects of open ocean deep convection variability are explored with a two-box model. In order to place the model in a region of parameter space relevant to the real ocean, it is fitted to observational data from the Labrador Sea. A systematic fit to OWS Bravo data allows us to determine the model parameters and to locate the position of the Labrador Sea on a stability diagram. The model suggests that the Labrador Sea is in a bistable regime where winter convection can be either “on” or “off ”, with both these possibilities being stable climate states. When shifting the surface buoyancy forcing slightly to warmer or fresher conditions, the only steady solution is one without winter convection.  We then introduce short-term variability by adding a noise term to the surface temperature forcing, turning the box model into a stochastic climate model. The surface forcing anomalies generated in this way induce jumps between the two model states. These state transitions occur on the interannual to decadal time scale. Changing the average surface forcing towards more buoyant conditions lowers the frequency of convection. However, convection becomes more frequent with stronger variability in the surface forcing. As part of the natural variability, there is a non-negligible probability for decadal interruptions of convection. The results highlight the role of surface forcing variability for the persistence of convection in the ocean.     

Empirical and Numerical Modeling of T-phase Propagation from Ocean to Land

—?T-phase propagation from ocean onto land is investigated by comparing data from hydrophones in the water column with data from the same events recorded on island and coastal seismometers. Several events located on Hawaii and the emerging seamount Loihi generated very large amplitude T phases that were recorded at both the preliminary IMS hydrophone station at Point Sur and land-based stations along the northern California coast. We use data from seismic stations operated by U. C. Berkeley along the coast of California, and from the PG&;E coastal California seismic network, to estimate the T-phase transfer functions. The transfer function and predicted signal from the Loihi events are modeled with a composite technique, using normal mode-based numerical propagation codes to calculate the hydroacoustic pressure field and an elastic finite difference code to calculate the seismic propagation to la nd-based stations. The modal code is used to calculate the acoustic pressure and particle velocity fields in the ocean off the California coast, which is used as input to the finite difference code TRES to model propagation onto land. We find both empirically and in the calculations that T phases observed near the conversion point consist primarily of surface waves, although the T phases propagate as P waves after the surface waves attenuate. Surface wave conversion occurs farther offshore and over a longer region than body wave conversion, which has the effect that surface waves may arrive at coastal stations before body waves. We also look at the nature of T phases after conversion from ocean to land by examining far inland T phases. We find that T phases propagate primarily as P waves once they are well inland from the coast, and can be observed in some cases hundreds of kilometers inland. T-phase conversion at tenuates higher frequencies, however we find that high frequency energy from underwater explosion sources can still be observed at T-phase stations.     

Normal Mode Composition of Earthquake T Phases

— Understanding the nature of the coupling between the underwater acoustic field and the land seismic field is important for evaluating the performance of the T-phase stations in the International Monitoring System for the Comprehensive Nuclear-Test-Ban Treaty. For upslope propagation in an ocean environment, the places where underwater acoustic field energy couples into the land seismic field are determined to first approximation by the local water depth and the normal mode composition of the acoustic energy. Therefore, the use of earthquake-generated T phases as natural probes of water-to-land coupling characteristics is aided by knowledge of their modal composition. Data collected by a 200-element, 3000-m-aperture vertical hydrophone array during a 1989 experiment in the deep northeast Pacific Ocean are used to determine the mode composition of T-phase arrivals from two m_b 4.1 earthquakes near the w est coast of the U.S., one occurring offshore and the other on land. Results from an eigenanalysis approach and conventional mode decomposition for the two events are consistent and show that at 5?Hz, the offshore event's arrivals have higher-order mode content compared to those from the event on land. Single hydrophone recordings at Pt. Sur of two m_b 4.4 Hawaiian events in 1996 and 1997, one occurring offshore and the second on land, display time-frequency arrival structures that are explainable by the dispersion characteristics over the oceanic path. Although other effects due to complex source time functions and shear wave and dispersive propagation effects along the initial land path cannot be separated with these single element data, differences in these two events' arrival structures suggest differences in normal mode content consistent with those seen in the pair of 1989 events. Ocean-path dispersion also appears to play a significant role in determining the i n-water arrival structure from a 1995 French nuclear test at Mururoa. Recordings of two Hawaiian events in 1997 by the T-phase station VIB and the seismic station at Berkeley illustrate that the water-land coupling confuses the relative timing between normal modes, resulting in apparent loss of information about the source.     

Floating seaweeds and associated fauna in the White Sea

Floating seaweeds and associated fauna in Kandalaksha, Onega, and Dvina bays and in the Gorlo of the White Sea were studied. The bulk of the drifting objects were composed of various algae of the Fucus, Chorda, and Chordaria genera. In these drifting communities, 82 different animal and plant species were encountered. The drifting objects were most abundant in Kandalaksha and Onega bays. The minimal abundances were detected in the Gorlo and in Dvina Bay. The total abundance and biomass of the drifting seaweeds and animals were estimated as 25.5 million individuals and 24.2 thousand tons, respectively. The role of the phenomenon considered in the distribution of the animals and seaweeds and in the balance of matter and energy in the White Sea is discussed.     

Hydrothermal dolomite marbles associated with charnockitic magmatism in the Proterozoic Bamble Shear Belt,south Norway

Deformed and metamorphosed dolomite marbles in the Kragerø area of the Bamble Shear Belt occur as lenses within metasupracrustal sequences, as matrix in bodies resembling magmatic breccias, and as veins/dykes cutting amphibolites and metagabbros. A common origin is not evident from the field relationships, but is nevertherless probable due to great geochemical similarities between dolomite from the different occurrences. They are characterized by higher REE, Ni, Co, Cr and Sc, and lower Ba and Sr contents relative to metasedimentary marbles occurring nearby. Sm-Nd isotope data shows that the dolomites are of Sveconorwegian age (1175±37 Ma). The dolomite marbles are very weakly LREE-enriched and display in most cases positive Eu anomalies. Their stable isotope compositions are uniform (¹⁸O=+9.6 to +10.7; ¹³C=-8.5 to-6.2), their initial ⁸⁷Sr/⁸⁶Sr ratios are high (0.706–0.709), whereas their age corrected _Nd varies from +0.7 to-1.5. Geochemically the dolomitic marbles differ considerably from sedimentary or metasedimentary marbles as well as from carbonatites. The Kragerø dolomite marbles represent deformed and metamorphosed hydrothermal veins or vein-complexes deposited in tensional fractures in the deep crust. Although the dolomitic marbles regionally are of minor volume, the dolomite deposition represents a specific and important event in the geological evolution of the Bamble Shear Belt. Their geochemical and isotopic homogeneity on a regional scale suggest that the hydrothermal solutions were supplied from a very large, homogeneous reservoir. Trace elements, stable and radiogenic isotopes, and field and isotopic age relationships are consistent with a deposition from hydrothermal solutions which were exsolved from crystallizing charnockitic intrusions and subsequently interacted with the crust. The parental magma to these intrusions underplated the crust and raised the geotherm of a carbonated ultramafic uppermost mantle, and imposed decarbonation and fluid release. These fluids were channelled into a large degassing zone now found as a deformed, regional zone with hydrothermal dolomite deposits, albitites, apatite-veins and widespread scapolitization. Whether the CO₂-rich fluids, which precipitated the Kragerø dolomites, pervasively infiltrated crustal rocks at a deeper level and caused granulitization is ambiguous, but possible.     

Comments on ‘correction of errors associated with measurement of net all-wave radiation with double-domed radiometers’ by Oliver and Wright (1990)

In their note on correction of measurements with the REBS Q^*4 net radiometer, Oliver and Wright discuss a problem of high general interest, namely that of wavelength-dependent responsivities in net radiometry. They suggest that old measurements can be corrected by a simple addition of 40% of the net longwave radiation. It is shown that this is only the first of two necessary steps needed for a good correction. The 40% value suggested by Oliver and Wright leads to night-time radiation values that are typically 20–25% too negative. It is also suggested that their 40% correction varies with time and place of measurement as well as with the way the radiometer was originally calibrated. This percentage should be replaced by a correction factor based on a design-dependent responsivity difference of around 63% multiplied by a factor depending on the relative importance of longwave and shortwave radiation components prevailing for the measurements to be corrected. The second step in the correction procedure should be a recalibration of the signal achieved after equalizing responsivity for negative and positive values of net radiation. It is finally suggested that the cause of the malfunction of the double-dome radiometer has to do with the transmission properties of the polyethylene shield and not with properties of the heat-absorbing surface or entrapped hot air.     

Long cores from small lakes—recovering up to 100 m-long lake sediment sequences with a high-precision rod-operated piston corer (Usinger-corer)

We present a unique, versatile piston corer for recovering continuous lake and bog sediment sequences with superior accuracy and quality. The main components of the system and their function are described, with special focus on measures for obtaining long, continuous lake sediment sections up to a current maximum length of 95 m. Examples of lake sediment profiles obtained with this system from different climatic zones are presented.