Modern and Holocene carbonate sedimentology of two saline volcanic maar lakes, southern Australia   总被引：4，自引：0，他引：4  

WILLIAM M. LAST  PATRICK DE  DECKKER† 《Sedimentology》1990,37(6):967-981

The Basin Lakes are two adjacent maar lakes located in the centre of the Western Volcanic Plains District of Victoria, Australia. Both lakes are saline and alkaline; West Basin Lake is meromictic whereas East Basin is a warm monomictic lake. The carbonate mineral suite of the modern offshore bottom sediments of these Basins consists mainly of dolomite and calcite, with smaller amounts of hydromagnesite and magnesite in West Basin and monohydrocalcite in East Basin. The dolomite, hydromagnesite, magnesite, and monohydrocalcite are endogenic in origin, being derived by primary inorganic precipitation within the water columns of the lakes or at the sediment-water interface. The calcite is biologically precipitated as ostracod valves. In addition to the carbonates in the modern offshore (deep-water) sediments, the lakes also contain a girdle of nearshore carbonate hardgrounds. Both beachrock and microbialites (algal boundstones) are present. These modern lithified carbonate units exhibit a wide range of depositional and diagenetic fabrics, morphologies and compositions. In West Basin, the hardgrounds are composed mainly of dolomite, hydromagnesite, and magnesite, whereas dolomite and monohydrocalcite dominate the East Basin sediments. Aragonite, high-Mg calcite, kutnahorite, siderite, and protohydromagnesite also occur in these lithified carbonate units. Stratigraphic variations in the carbonate mineralogy of the Holocene sediment record in the lakes were used to help decipher the palaeochemistry and palaeohydrology of the Basins. These changes, in conjunction with fluctuations in organic remains and fossil content, indicate a pattern of lake level histories similar to that deciphered from other maar lakes in western Victoria.  相似文献   

A NEW APPROACH FOR DERIVING PSEUDOVELOCITY LOGS FROM RESISTIVITY LOGS1     

W. L. BRITO DOS SANTOS  T. J. ULRYCH  O. A. L. DE LIMA 《Geophysical Prospecting》1988,36(1):83-91

This paper describes a method of generating pseudovelocity logs using measurements of electrical resistivity. A theoretical relation between electrical resistivity and transit time, which is applicable to a wide range of lithologies, has been developed. The application of this relation using a method which defines lithoresistivity zones as lithological intervals related to the same formation and showing small resistivity variations, has been tested in the Recôncavo sedimentary basin in Bahia, Brazil. A comparison of derived pseudovelocity logs with actual sonic logs for five wells whows the validity of the present approach.  相似文献   

LARGE-OFFSET APPROXIMATIONS IN THE MODIFIED CAGNIARD METHOD FOR COMPUTING SYNTHETIC SEISMOGRAMS: A SURVEY1     

A. T. DE HOOP 《Geophysical Prospecting》1988,36(5):465-477

Large horizontal offset and large vertical offset approximations in the modified Cagniard method for computing synthetic seismograms in a horizontally layered model of the earth are derived. They apply to each of the generalized-ray constituents into which the seismic wave motion is decomposed. For the results applying to large horizontal offset, which are known in the seismological literature, a simplified derivation is given. The results for large vertical offset, which are of particular interest to vertical seismic profiling, are new. The use of the large-offset approximations, both horizontal and vertical, leads to a considerable reduction in computation time for synthetic seismograms compared with the full three-dimensional version of the modified Cagniard method.  相似文献   

THE PHASE PROPERTY OF A FINITE REALIZATION OF RANDOM NOISE AND ITS INFLUENCE ON DECONVOLUTION *     

N. DE VOOGD 《Geophysical Prospecting》1978,26(3):509-524

A finite realization of a discrete random noise process may be considered as a one-sided energy signal. Its phase property can then be described by means of the center position. The samples of such a realization are the components of a random signal vector and the center position is therefore a random variable. A statistical analysis shows that the expected value of the center position equals half the time duration of the realization. This implies that the Z-transform of the realization may be expected to have an equal number of poles and zeros inside and outside the unit circle. The standard deviation from the expected value of the center position is shown to depend on the time duration of the realization and on the autocorrelation of the process. It follows that, for processes that can be described by the convolution of a white series and a disturbance wavelet, the center position is independent of the phase property of the wavelet. A conclusion based on these results is that the homomorphic technique of wavelet estimation through cepstrum stacking must give questionable outcomes. Another conclusion is that the super-position of a realization of random noise on a minimum phase wavelet will in general give a mixed phase resulting signal. It is pointed out that schemes for the derivation of deconvolution filters do not take account of this phenomenon.  相似文献   

Wave-generated structures and sequences from a shallow marine succession, Lower Carboniferous, County Cork, Ireland   总被引：2，自引：0，他引：2  

J. F. M. DE  RAAF J. R. BOERSMA  A. VAN  GELDER 《Sedimentology》1977,24(4):451-483

In a Lower Carboniferous formation south of Cork, Ireland, characterized by sand/mud alternations at varying scales, several lithotypes could be singled out on the basis of relative sand content and primary sedimentary structures. The lithotypes are arranged in several types of sequences: coarsening upwards, coarsening-fining upwards, random, and in rare instances: fining upwards. With increased gross sandcontent the sand intercalations thicken and show a richer array of structures, reflecting a wider range of energy conditions. The majority of the structures is interpreted as wave generated. Current formed structures play an insignificant role. Tidal effects could not be ascertained. The alternation of low- and high-energy lithotypes, occurring in sandy as well as muddy parts of the sequences is explained as resulting from the interplay of storm and fair weather conditions. The effect of shoaling bathymetry is reflected in sequences by an increasing importance of high-energy lithotypes. The environment of deposition can be visualized as a muddy shallow marine platform on which longshore directed sandy shoals were formed under the influence of wave action. Depending on the degree of wave agitation and the availability of sand the shoals reached different stages of maturity: incipient, submerged or emergent. The sequential type encountered in vertical sections depends on the location of the line of logging and varies laterally as sandlenses thicken, amalgamate or die out.  相似文献   

ELECTROMAGNETIC PROSPECTING FOR GROUNDWATER IN PRECAMBRIAN TERRAINS IN THE REPUBLIC OF UPPER VOLTA*     

G.J. PALACKY  I.L. RITSEMA  S.J. DE JONG 《Geophysical Prospecting》1981,29(6):932-955

The “Autorité des amenegements des valées des Voltas (AVV)” is establishing new rural settlements in the Volta valleys. First, a survey of available water supplies is performed. Economic aquifers in Precambrian terrains are deep (15–50 m) and usually occur in fractured zones accompanying faults. Such zones can be identified on aerial photographs, but their precise location on the ground is virtually impossible by visual means. Because of the small size of the aquifers, a location error of 5 m can make the difference between a productive well and a dry hole. Traditionally, resistivity profiling has been used as the means of locating the fractured zones in the field. Our studies suggest that the task can be performed faster, cheaper and more accurately by VLF and EM methods. Because of the limited choice of transmitting stations reccivable in Upper Volta, the VLF method is not sufficiently sensitive to detect conductors with a strike between 45° and 105°. The results obtained with a multifrequency, horizontal-loop EM (HLEM) system were satisfactory in all investigated areas. During the 1980 field season, 35 target areas were surveyed. Of the 24 holes drilled so far, 23 are productive. The weathered layer is a source of distinctive HLEM anomalies, which are characteristic of the underlying rocks. Therefore, different interpretational procedures had to be developed for granitic and volcano-sedimentary areas. Despite the high background level of in-phase and quadrature components, which varied with thickness and conductivity of the weathered layer, aquifers could be detected at a depth greater than 30 m. Attempts were made to interpret the HLEM results quantitatively using two models: a three-layer medium and a valley discontinuity. The latter model is more realistic, but more scale modelling will have to be performed to permit development of viable interpretational procedures. Meanwhile, phasor diagrams based on drilling and resistivity sounding data can be used to estimate the aquifer depth.  相似文献   

Convolute lamination in modern sands of the estuary of the Oosterschelde,the Netherlands,formed as the result of entrapped air*     

P. L. DE BOER 《Sedimentology》1979,26(2):283-294

Soft-sediment deformation structures similar to convolute lamination were found at the sandy rim of an intertidal shoal in the Oosterschelde estuary, the Netherlands. Abundant air filled cavities within upward penetrated domes suggest that reversed density stratification resulting from trapped air in some sand layers plays a role in the deformational process. Field observations show that air-filled cavities are formed within fine-grained sand; at rising tide the ground water level does not rise quickly enough to replace the interstitial air before flood water covers the sediment surface. Thus, air is trapped in the sand between the ground water level and the sediment-water interface. Because of the weight of the overlying water column and the slow downward movement of water due to capillary action, the entrapped air is compressed and eventually may attain a pressure which enables it to lift the overlying sediment. At this time bubbles are formed, often to a depth of 20 cm. Observation and experiments show that, in layers of fine sand, bubbles develop preferentially in better sorted and coarser zones. This is probably because capillary forces are greater in finer-grained and less sorted sand. Thus, water will penetrate by preference into the latter, pressing the interstitial air into the better sorted and coarser sand. The high content of air cavities in certain layers then provides the density instability responsible for the deformational process leading to the formation of convolute lamination. In the intertidal zone this appears to be a slow process that covers a number of ebb and flood cycles. Convolute lamination has been described from a number of ancient sandstones that are thought to have been deposited in shallow water, near-shore environments. In many of these cases sand size compares with that found in the Oosterschelde. Air entrapment might have played a role in the formation of certain of these occurrences of convolute lamination.  相似文献   

Trace Element and Strontium Isotope Constraints on the Origin and Evolution of Paran? Continental Flood Basalts of Santa Catarina State (Southern Brazil)     

MANTOVANI  M. S. M.; MARQUES  L. S.; DE SOUSA  M. A.; CIVETTA  L.; ATALLA  L.; INNOCENTI  F. 《Journal of Petrology》1985,26(1):187-209

Forty-five selected samples from two different sequences, andfrom different sites spread over the Parana Basin have beenanalysed for major and trace elements and Sr isotope ratios.Results demonstrate the existence of two geochemically and isotopicallydistinct groups, namely a high phosphorus and titanium (HPT)and a low phosphorus and titanium (LPT) group. HPT rocks are relatively rich in LIL elements and have morehomogeneous and markedly lower isotopic Sr ratios than LPT rocks.Since no relationship between the two groups was found, theymay derive from different mantle sources, the HPT source beingmore enriched in most incompatible trace elements except Th,U and Rb than the LPT one. The LPT group, comprising basalts to rhyodacites, displays alarge range in Sr isotopic ratios. This is interpreted as aresult of a fractional crystallization process occurring athigh crustal levels associated with assimilation of crustalmaterial.  相似文献   

INFLUENCE OF VELOCITY ERRORS ON THE FOCUSING ASPECTS OF MIGRATION*     

D. DE VRIES  A. J. BERKHOUT 《Geophysical Prospecting》1984,32(4):629-648

In migration procedures, the velocity profile of the subsurface is the most important input information. Since, in general, this information is only approximately known, errors in the migration output due to errors in the velocity input occur in all practical applications. In migration, velocity errors and depth errors can be interchanged. This interchange property is perfect in the paraxial approximation. From this result it follows that migration with incorrect velocities may still yield correctly migrated data if the imaging principle is modified. This attractive property can be used in the stripping version of migration (recursive migration).  相似文献   

Dr. Marjorie Sweeting     

Andrew Goudie  Derek Ford  David Sugden 《地球表面变化过程与地形》1987,12(5):445-451

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