MIGRATION IN THE PRESENCE OF A RUGGED INTERFACE WITH HIGH VELOCITY CONTRAST*

In the western coal-mining area of Ruhrkohle AG, reflection seismic prospecting for the Carboniferous coal measures is severely impaired by structures with halokinetic features. These structures make the interface between Mesozoic and Paleozoic layers, i.e., the top of Zechstein in general, very rugged. Unfortunately the velocity contrast at this interface is very high in that area, the ratio of velocities being 1.5 to 2.0. Therefore, migration and stacking become a problem. Three types of migration are presented:

• 1 (f, x)-time-migration with vertical time-to-depth conversion as a second step.
• 2 Kirchhoff migration down to a level determined approximately by the highest points of the top of Paleozoics, i.e., 0.35 s, and Kirchhoff-downward continuation for all times exceeding 0.35 s. Intermediate static corrections for these latter times with subsequent (f, k)-time-migration and final vertical time-to-depth conversion.
• 3 Direct depth migration in the (f, x)-domain using three interval velocities.

In all cases an intermediate picking of the velocity interfaces is necessary. In case 2 this occurs at an earlier stage of the process than in case 1, and in case 3 at a still earlier stage. The results of the second and third migration procedures are superior to those of the first. Possibilities for misinterpretation of faults are reduced considerably when the second or third migration procedure is applied.     

地震的感应磁效应（一）——三维电磁感应的数值理论

地震的感应磁效应包括两部分:（一）三维电磁感应的数值理论;（二）地震感应磁效应的模拟计算。本文是其中的第一部分,从Maxwell方程推得的扩散方程出发,给出了一般三维电磁感应的基本方程、边界条件和求解方法,建立了完整的三维电磁感应的数值理论。计算结果将另文讨论。     

Hot springs of Rhodesia: Their noble gases,isotopic and chemical composition

Four of Rhodesia's hottest spring complexes (54–100°C) were studied. Tritium contents were very low, compared to adjacent rivers, indicating that the samples studied were indigenous, deep-seated water which had undergone negligible intermixing with surface water.The noble-gas measurements revealed: (1) the waters are meteoric; (2) the noble gases were kept in closed-system conditions in the ground; (3) paleotemperatures are 26–31°C; (4) the boiling Binga springs lost part of their noble gases.Independently, measurements of stable isotopes indicate the meteoric origin of the springs. The chemical composition of the waters clearly reflects their origin from two groups of rocks — one from Karroo sediments and one from crystalline rocks.     

APPLICATION OF REFRACTION SEISMICS TO SUBSALT TECTONIC PROBLEMS IN A DEEP SALTDOME BASIN*

Despite the use of CDP and digital methods the Zechstein base is still the deepest horizon in the vast salt-dome basin of Central Europe for which continuous information can be obtained by reflection seismics. Thus in North-western Germany, in addition to reflection seismics, the refraction seismic method has been increasingly used for a reliable survey of deeper horizons. The first part of the paper deals with the investigation of the various possibilities and limitations of refraction seismics with regard to the investigation of Pre-Zechstein layers in a basin with a tectonically very complicated overburden. The recording techniques specially developed for continuous profiling of the desired refraction seismic arrivals and the data processing methods are described. The main problems of interpretation are then discussed, in particular with regard to depth representation. The advantages and disadvantages of the various methods, e.g. Gardner's, Hales' and Wyrobek's, and of the wave-front method, are compared. On account of the tectonically complicated overburden Thornburgh's wave-front method proved to be the most useful. In a further section the various possibilities for velocity determinations are mentioned, e.g. Wyrobek's determination of the overburden velocity, for which the wave-front method automatically furnishes the necessary corrections to a deep datum. Finally, some examples are given for the results obtained, including some incidental information on the deeper crust.     

Coupling of biological and physical regimes across the Antarctic Polar Front as reflected by nitrogen production and recycling

Oceanographic samples were collected across the Antarctic Polar Front (APF) region in the vicinity of 60°S, 170°W during the US JGOFS program from December 1997 to March 1998. This paper reports the uptake rates of new (nitrate) and regenerated (ammonium and urea) nitrogen measured by ¹⁵N tracer techniques together with the levels of ammonium, urea-N and dissolved free amino acids (DFAAs) during December and mid-February–March. The APF was an important biological boundary, and in December rates of new (nitrate) uptake were greatest south of the APF, exceeding 10 mmol m⁻² d⁻¹ near the retreating ice edge. In February, nitrate uptake rates were an order of magnitude lower. Rates of ammonium uptake in both periods were greater in the warmer water north of the front. Nitrogen f-ratios varied from 0.50 to less than 0.05, with larger values associated with the >5 μm size fraction at the ice edge and generally lower values north of the APF. Urea was an important nitrogen source north of the APF, and lowered f-ratios there by 22% on average when included as part of total nitrogen uptake. Urea uptake was less important south of the APF. Ammonium concentrations increased dramatically south of the APF later in the season, suggesting a system dominated by regeneration. Seasonal changes in the concentrations of regenerated organic compounds such as urea and DFAAs were less obvious, although DFAAs exhibited consistent maxima in the high flow regions of the APF. A mass balance based of ammonium fluxes suggests that nitrification was significant at locations south of the APF in February. In these nitrate-replete waters, light/mixing conditions in the surface water (the Sverdrup criterion) accounted for over 50% of the variance in nitrate uptake rates. The stability responsible for higher new production south of the APF is due both to the separation of this region from the maximum zonal wind field to the north as well as to melt-water contribution from the retreating ice field. Estimated new production and exportable carbon production exceeded 500 mmol nitrate m⁻² yr⁻¹ and 40 g C m⁻² yr⁻¹, respectively, south of the APF. Thus, new production in the marginal ice zone of the Southern Ocean rivals that in coastal systems and indicates that this is an important region for export production.     

X-ray and optical-to-infrared follow-up observations of the transient X-ray burster SAX J1810.8–2609

We have performed a ROSAT follow-up observation of the X-ray transient SAX J1810.8–2609 on 1998 March 24 and detected a bright X-ray source (named RX J1810.7–2609) which was not detected during the ROSAT all-sky survey in September 1990. Optical-to-infrared follow-up observations of the 10" radius ROSAT HRI X-ray error box revealed one variable object ( R =19.5±0.5 on March 13, R >21.5 on 1998 August 27) which we tentatively propose as the optical/IR counterpart of RX J1810.7–2609≡SAX J1810.8–2609.     

Order-disorder in Natural Nepheline Solid Solutions

The variation of the birefringence with temperature, from roomtemperature up to c. 900? C, was studied of 34 nephelines fromdifferent geological environments. The measurements were madewith a Berek compensator using a microscope heating stage. Two optical types of nepheline were distinguished on the basisof the absolute value for birefringence at room temperatureand on the basis of the optical -value defined as >=(w-E)₉₀₀?-(w-E)₂₀?.The optical type I is characterized by a high birefringenceat room temperature and by a highly negative optical -value,whereas the optical type II shows a low birefringence at roomtemperature and a highly positive optical -value. The nephelinespecimens studied represent optically a continuous gradationfrom type I to type IL Both optical types are represented amongmediopotassic and perpotassic nephelines. A nepheline of the optical type I is considered to representthe ordered form of the mineral and a nepheline of the opticaltype II the disordered form. All plutonic nephelines studiedbelong to the optical type I whereas the volcanic nephelinesrange from the type I to the type II. Using a high temperature attachment for the Norelco diffractometer,the thermal expansion was measured of a few nepheline specimens.A nepheline belonging to the optical type I differs slightlyin thermal expansion from that of the optical type 11. Thisdifference in thermal expansion may qualitatively explain thedifference between the two optical types. The crystal structural nature of the supposed order-disordertransition in nepheline is not known. The analogy with the feldsparswould suggest a varying degree of ordering in the distributionof the silicon and aluminium cations in the oxygen tetrahedra.     

Brown earth and podzol: soil genesis illuminated by microfossil analysis

Andersen, S. Th. 1979 03 01: Brown earth and podzol: soil genesis illuminated by microfossil analysis. Boreas , Vol. 8, pp. 59–73. Oslo. ISSN 0300–9483.
Pollen and other plant microfossils were studied in soil profiles from a Fagus-Quercus petraea woodland in eastern Jutland, Denmark. Fungal hypha fragments in the soil show characteristic length frequency distributions, which indicate comminution by large arthropods and microarthropods (Oribatei) during the burial. Distribution of the hypha fragments within the podzol indicates development from a former brown earth. The pollen sequence in a humus deposit reflects recent vegetational development. Modification of pollen assemblages in the mineral soils can be explained by mixing during burial by soil fauna. Development from brown earth to podzoloid and podzol follows the models assumed by pedologists. The various stages are related to vegetational changes and the processes were completed within a few hundred years.