The origin of fine scale acoustic stratigraphy in deep-sea carbonates
The origin of fine scale acoustic stratigraphy in deep-sea carbonates
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深海碳酸盐岩细尺度声学地层学的起源
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发表时间：
2019
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通讯作者：
L. Mayer
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作者：
L. Mayer
在本文中，我们研究了声、声识别了东太平洋紧密灰层的广泛起源和地质意义。Hersey[1965]和Ryan等[1965]，也研究了远洋碳酸盐的高反射特征。对赤道太平洋的一小片带浊积岩的第勒尼安深海平原进行了详细的调查，并对海相砂和灰层进行了研究。低频封装的分辨率降低，提供高分辨率的4 khz（气枪，火花）反射剖面，使得剖面和岩心地层对比的精确定位变得更加困难。样本。在深海钻探项目之前，声速和视界的深间隔只能在它们饱和体积密度的地方进行采样。声阻抗出现露头[Ewing et al., 1966；Saito等计算，a反射系数等，1966；Windisch et al., 1968]。该测井曲线是在接近的10米高的地方测定的，由于沉积物柱的不确定性而受到影响。反射系数测井表明，地震系统的分辨率较差，且未发现反射较大的界面，对样品的位置缺乏控制。钻井方案的结果在Deep-Tow 4 khz地震剖面上得到了证实。在确定计算出的反射系数方面，具有巨大价值的是地震与（通常为10 - 10 - 10）之间的关系非常低，并且在地质记录中变化很大。在Deep Sea Drilling项目的前两段波长中，Ewing等人的波长为4 khz脉冲，这意味着[1970]确定了层位A和A”，干扰在北大西洋地震记录中突出反射带的组成中起作用。输出和加勒比海分别由4 khz脉冲与反射系数为对数层的燧石引起。随后，在深海钻探项目的许多地点生成的合成地震图中，相关性与4khz反射剖面非常相似。在反射区和改变输出脉冲核材料的频率之间，但随着高频工作的振幅和位置的改变，这些相关性反射器在合成地震图上看到。因此，我们得出结论，在边界（例如，沉积基底，4 khz地震剖面）上看到的反射物不是由碳酸盐-燧石，浊积岩和灰烬引起的。这是离散的地质层，但阻抗是许多小层的干扰的结果，这当然不足为奇。与这些类型的界面相关联的对比反映了大量的介绍能量。然而，需要进一步研究的是，自地震反射反射器出现以来，它们不能直接与这种剖面联系起来，岩性对比的地质意义。这个问题反映了层位一直是一个关键问题，在纯远洋碳酸盐中尤为突出。从粗略的意义上说，这些剖面是了解岩心地质均匀性的宝贵工具，并且通常显示一个结构，但它们与大量紧密间隔的反射体非常相似。地质剖面已促使众多Schlanger和Douglas[1974]讨论了一种假设这些声波和地层记录之间直接相关的成因模型。反射镜。他们强调这种假设的有效性，然而，反射者和反射者之间的关系值得怀疑。正如Sheriff[1977]所指出的那样，古海洋学事件，如对地震剖面海平面变化和方解石移动的冰川沉降地层学解释，必须受到地球物理补偿深度知识的约束。如果限制之间的关系，为，为……大多数反射都是声波记录，此类事件可以是多干涉合成，没有一对一精确建立的地震剖面对应关系，因此地震事件之间的对应关系可以成为地球上重要的古海洋学界面。的工具。然而，在更精细的尺度上（在更高的频率上），这并不是说，碳酸盐岩继续显示出这种不可能的、直接的、与声学特征相关联的声学地层学。许多层位具有岩性层位。事实上，Worzel[1959]使用12 khz回声标准活塞芯，在沉积物柱中最早的应用中，这些高频反射器较浅，足以用剖面法达到；因此，它们可以被美国地球物理联合会研究，而不需要花费金钱和复杂的“深版权1979”。海上钻探。考虑到这一点，论文编号为8Bl113。Mayer：精细尺度声学地层学接收信号的计算机处理提供了几种形式的实时显示（图la-ld），并获得了震源的理论近底部位置，还减少了许多可能由声学结构的横向不均匀性引起的模糊性，并允许大量穿透（通常为100米）进入沉积物柱。因此，低分辨率的问题是
In this paper we investigate the sounder, acoustically identified a widespread origin and geologic significance of the closely ash layer in the eastern Pacific. Hersey [1965] spaced high-frequency subbottom acoustic and Ryan et al. [1965], also working at high reflectors characteristic of pelagic carbonates. frequencies, correlated reflectors in the A detailed survey was conducted of a small area Tyrrhenian Abyssal Plain with turbidite in the equatorial Pacific with the Marine sands and ash layers. Physical Laboratory's Deep-Tow instrument The decreased resolution of low-frequency package, providing high-resolution 4-kHz (air gun, sparker) reflection profiles makes profiles and precise positioning of core stratigraphic correlation even more difficult. samples. The cores were sampled at closely Before the Deep Sea Drilling Project, deep spaced intervals for sound velocity and horizons could only be sampled where they saturated bulk density. Acoustic impedances appeared to outcrop [Ewing et al., 1966; Saito were calculated, and a reflection coefficient et al., 1966; Windisch et al., 1968]. This log determined for the upper 10 m of the approach is plagued by uncertainty caused by the sediment column. The reflection coefficient log poor resolution of the seismic system and the revealed no interfaces with large reflection lack of control on the position of the sample. coefficients that correlated with the reflectors The results of the drilling program proved to be seen on the Deep-Tow 4-kHz seismic profile. The of tremendous value in determining the calculated reflection •oefficients were very low relationship between the seismic and the (typically 10 -•10 -•) and varied about a geologic record. During the first two legs of wavelength that was on the order of the the Deep Sea Drilling Project, Ewing et al. wavelength of the 4-kHz pulse, implying that [1970] established that horizons A and A", interference plays a role in the composition of prominent reflective zones in the North Atlantic the seismic record. Convolving the outgoing and Caribbean, respectively, were caused by 4-kHz pulse with the reflection coefficient log layers of chert. Subsequently, at many of the generated a synthetic seismogram that very Deep Sea Drilling Project sites, correlations closely resembled the 4-kHz reflection profile. have been drawn between reflective zones and Varying the frequency of the outgoing pulse cored materials, but as was also true with the changed the amplitude and position of the high-frequency work, those correlations reflectors seen on the synthetic seismograms. established have been with major lithologic Thus we conclude that the reflectors seen on the boundaries (e.g., sediment-basement, 4-kHz seismic profile were not caused by carbonate-chert, turbidites, and ash). It is discrete geologic horizons but rather are the certainly not surprising that the impedance result of the interference of many small layers. contrasts associated with these types of interfaces would reflect a substantial amount of Introduction energy. What requires further examination, however, is the significance of the numerous Since the inception of seismic reflection reflectors that cannot be directly tied to such profiling, the geologic significance of lithologic contrasts. This problem is reflecting horizons has been an issue of key particularly acute in pure pelagic carbonate importance. These profiles have served as an sections which, in a coarse sense, appear invaluable tool for understanding geologic homogenous in cores and, yet, typically show a structure, but their close resemblance to a large number of closely spaced reflectors. geologic cross section has prompted numerous Schlanger and Douglas [1974] discuss a investigators to assume a direct correlation diagenetic model for the origin of these between the acoustic and stratigraphic record. reflectors. They emphasize the possible The validity of this assumption, however, is relationship between •he reflectors and questionable. As Sheriff [1977] points out, paleoceanographic events such as glacioeustatic stratigraphic interpretation of seismic sections sea level changes and shifts of the calcite must be constrained by knowledge of geophysical compensation depth. If the relationship between limitations, for, as '... most reflections are the acoustic record and such events can be more interference composites, there is no one-to-one precisely established, then seismic profiling correspondence between seismic events and could become an important paleoceanographic interfaces in the earth.' tool. On a finer scale (at higher frequencies) This is not to say, however, that it is the carbonates continue to show this impossible, directly, to correlate an acoustic characteristic acoustic stratigraphy. Many of horizon with a lithologic one. Indeed, in one these higher-frequency reflectors are shallow of the earliest applications of subbottom enough in the sediment column to be reached with profiling, Worzel [1959], using a 12-kHz echo standard piston cores; thus they can be studied without the expense and complications of deep Copyright 1979 by the American Geophysical Union. sea drilling. With this in mind a study was Paper number 8Bl113. 0148-O227 / 79 / O08B-1113501.00 6177 6178 Mayer: Fine Scale Acoustic Stratigraphy Computer processing of the received signal provides a real-time display in several forms (Figures la-ld) and attains the theoretical near-bottom position of the source also reduces many of the ambiguities that might result from lateral inhomogeneities in the acoustic structure and permits substantial penetration (typically 100 m) into the sediment column. Thus the problem of poor resolution is