R. Herber

Microseismological evidence for a changing wave climate in the northeast Atlantic Ocean

One possible consequence of a change in climate over the past several decades is an increase in wave heights, potentially threatening coastal areas as well as the marine industry. But the difficulties in observing wave heights exacerbates a general problem of climate-change detection: inhomogeneities in long-term observational records owing to changes in the instruments or techniques used, which may cause artificial trends. Ground movements with periods of 4–16 seconds, known as microseisms, are associated with ocean waves and coastal surf , and have been recorded continuously since the early days of seismology. Here we use such a 40-year record of wintertime microseisms from Hamburg, Germany, to reconstruct the wave climate in the northeast Atlantic Ocean. For the period 1954–77, we detect an average of seven days per month with strong microseismic activity, without a significant trend. This number increases significantly in the second half of the record, reaching approximately 14 days of strong microseisms per month. The implied increase in northeast Atlantic wave height over the past 20 years parallels increased surface air temperatures and storminess in this region, suggesting a common forcing.

Does microseisms in Hamburg (Germany) reflect the wave climate in the North Atlantic

The possibility of monitoring climate variability on decadal scales by means of microseisms is investigated. For this purpose, digital microseismic data from Hamburg (Germany) of the six winters 1992/93 to 1997/98 are compared with model ocean-wave fields of the North Atlantic. A correlation coefficient of aboutr= 0.7 was found between the linear ocean-wave amplitude at the Norwegian coast and the square-root of the microseismic amplitude. Considering monthly means of the energy of both ocean waves and microseisms the correlation exceedsr= 0.8 in special areas. A correlation coefficient ofr = 0.6 was found between the monthly winter index of the North Atlantic Oscillation (NAO) and the respectively averaged microseismic energy. Encouraged by these results, further investigations are planned for analysing earlier microseisms which has been recorded in Hamburg since 1905.

Constraints on the shallow seismic structure at Ocean Drilling Program Site 1107, Ninetyeast Ridge, from implosive bottom sources and airgun shots

On Ninetyeast Ridge the seismological community drilled Ocean Drilling Program (ODP) Site 1107, coincident with ODP Site 757, for the future host of a long-term seismological observatory in the Central Indian Ocean. To study the shallow seismic structure at the drill site, we developed a unusual on-bottom seismic tool. Five implosive sources were placed along a 2-km line. Four ocean bottom sensors recorded seismic waves over epicentral distances of 400 m to 2200 m, revealing seismic velocities of the immediately subjacent sediments. Velocities within this portion of crust are generally not detectable by traditional marine seismic techniques. Airgun shots, however, can be used to determine velocities of basement rocks. One-dimensional travel-time inversion of both bottom sources and water path corrected surface shots yields a velocity structure which is in excellent agreement with ODP drilling results. Three lithological units are imaged. A 150 m thick sediment section with velocities of 1600–1720 m/s. Underneath, a layer of 230 m thickness with velocities ranging from 2080 to 2250 m/s, corresponding to volcanic ashes and tuffs. The volcanic basement occured at 380 m below seafloor. Velocities rapidly increase from 3500 m/s to values of more than 4500 m/s within a few hundred metres.

Relationship between Shot-Induced Noise and Shot Interval in Seismic Refraction Experiments at Sea

Seismic noise induced by the seismic source during continuous profiling reduces the signal-to-noise ratio and hence the data quality. This noise is largely dependent on the shot interval. In this paper, the noise amplitude of refraction seismic records from a special experiment is analysed as a function of the shot interval. An empirical exponential relationship between mean peak noise amplitude and shot interval is deduced. By increasing the shot interval, the induced noise can be minimized on all successive records. This results in an improvement of the data, predominantly the signal-to-noise ratio. Because the seismic signal and the shot-induced noise have nearly the same spectra, the chance of improving the signal-to-noise ratio by stacking is significantly reduced.

Ocean-bottom-seismograph of the Institut für Geophysik, Hamburg

The ocean bottom seismograph (OBS) of the Institut für Geophysik, Hamburg (IfG) is designed for refraction seismic experiments and for recording microseismic noise. Hydrophone signals are recorded directly on a casette tape recorder with a band width of 3–60 Hz. Signals from three component 1 Hz seismometers are recorded on a 2nd casette tape recorder in FM for a frequency range of 0.1–1 Hz. A telemetering buoy at the surface is connected with the OBS by a polypropylene rope.

A simple deep-towed vertical array for high-resolution reflection seismic profiling

A simple, low cost, deep-towed system for high-resolution reflection seismic profiling is described. It consists of a vertical array with two hydrophones having a separation of 2.2 m and rigidly mounted onto streamlined tow bodies. Improvement of the signal-to-noise ratio is attained by simple stacking of the hydrophone outputs after signal conditioning and travel time corrections. The suppression of side echoes and surface reflections is achieved by an analog procedure which in effect improves the directional characteristics of the array. A circuit for automatic gain control is included to enhance weak signals as well as to suppress ringing.Results in Kiel Bay and over the crest of the Jan Mayen Ridge (northern Atlantic) suggest that this simple vertical array may supplement air gun systems better than conventional, surface pinger-type equipment.

The UN Convention on the Carriage of Goods by Sea, 1978, Hamburg Rules, its Future and the Demands of Developing Countries

On 31 March 1978, the UN Conference held at Hamburg adopted the United Nations Convention on the Carriage of Goods by Sea, 1978 (Hamburg Rules). The Convention was adopted by Representatives of 72 States (78 States participated in the Conference), of which 68 voted in favour, none voted against and four (Canada, Greece, Liberia and Switzerland) abstained.

Increased efficiency of chemical charges in refraction seismic experiments at sea

The Techniques of the IFG Hamburg for the use and handling of chemical sources for seismic energy generation are described. The results of normal and dispersed charge shooting in marine and combined land/sea projects are presented and it is shown, that the introduction of a clock based shooting technique leads to more efficiency for dispersed charges. The increase of safety and the economizing of explosives are outlined.