Attenuation differences in layer 2A in intermediate- and slow-spreading oceanic crust

EPSL ELSEVIER

Earth and Planetary

Science Letters 150 (1997) 221-231

Attenuation differences in layer 2A in intermediate- and slow-spreading oceanic crust D. Goldberg Borehole Research Group, Lumont-Doherty

*, Y.F. Sun Earth Obsermtoq

Palisades, NY 10964, USA

Received 22 April 1997; accepted 30 April 1997

Abstract In situ seismic attenuation Q-’ logs are derived from borehole velocity profiles and reveal sharp boundaries between morphologies of the extrusive volcanic layers in intermediate- and slow-spreading oceanic crust. Q-l logs are calculated from the scattering attenuation associated with vertical velocity heterogeneity in Ocean Drilling Program Holes 504B and 896A and in Hole 395A, located in 5.9-7.3 Ma crust on the Pacific and Atlantic plates, respectively. Our results strongly tie crustal properties to seismic measurables and observed geological structures: we find that the scattering attenuation can be used to identify the extrusive volcanic sequence because it is closely related to changes in the degree of vertical heterogeneity. We interpret a distinct decrease in the Q- ’ log at the transition below the extrusive volcanic layer to correspond with the seismic layer 2A/2B boundary. The boundary is located at 465 m depth below the sea floor in both

Hole 395A and 504B, although this is likely to be a coincidence of the sediment thickness at these sites. Layer 2A is estimated to be approximately 150 m thick in Hole 504B and > 300 m thick in Hole 395A. Cyclic sequences of high-porosity pillows and low-porosity massive units in the uppermost 100 m of volcanics in Hole 395A result in large velocity heterogeneities which cause > 5 times more attenuation in this layer than in Hole 504B. In Hole 896A, by contrast, fewer pillows, more massive flows, and a greater volume of carbonate veins decrease the velocity heterogeneity and attenuation significantly over only 1 km distance from Hole 504B. We conclude that the attenuation in the extrusive volcanics of the ocean crust is largely controlled by variation in local heterogeneity and morphology as well as by subsequent hydrothermal alteration. The observed differences in Q- ’ profiles and layer 2A thickness at these sites may be attributed to variations in the volume and duration of volcanic activity at mid-ocean spreading centers for these Pacific and Atlantic ridge segments. 0 1997 Elsevier Science B.V. Kewordst

oceanic crust; Ocean Drilling Program;

attenuation;

seismic logging:

1. Introduction Investigations of the elastic properties of the uppermost oceanic crust have relied largely on the results of seismic refraction experiments [I], labora-

* Corresponding author. Fax: + 1 914 365 3182. E-mail: [email protected]

acoustical

logging

tory measurements on basalt samples [2], and downhole log data in Deep Sea Drilling Project and Ocean Drilling Program (DSDP/ODP) holes [3]. Hill [4] and Raitt 1.51 defined the structure of the oceanic crust in layers by discriminating in situ refraction velocities, where layer 1 corresponds to sediments, layer 2 to extrusive basalts and layer 3 to intrusive plutonic rocks. Talwani et al. [6] defined the upper-

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most portion of the basaltic crust as layer 2A, comprised of highly magnetic extrusive pillow lavas. Houtz and Ewing [l] related the thickness and velocity of layer 2A to crustal age and concluded that relatively uniform lithology exists below, observing a small variance and steeper gradient in velocities deeper in the crust. Numerous authors have since investigated the detailed velocity structure of layer 2A using long-offset seismic data and conclude that it thickens rapidly away from the ridge crest as pillow lavas accumulate and pore space fills over time [7-121. Laboratory studies on samples from dredges at mid-ocean ridge escarpments and from ophiolite complexes corroborate that the velocity of the layer 2A varies with both depth and age and often within a few kilometers spatially [2,13]. Seismic imaging near the intermediate-spreading Costa Rica rift [ 14-171 and near the slow-spreading midAtlantic ridge [ 18-211 have inferred lateral variability in the upper crust. Laboratory [22-241 and borehole studies [25-291 at DSDP/ODP drill sites near these ridge segments. therefore, have been essential in detailing the velocity profiles with depth through layer 2. Despite these numerous seismic and drilling experiments, models of seismic attenuation in the upper ocean crust remain poorly constrained. Seismic attenuation is particularly sensitive to the physical state of the formation, predominantly due to scattering from heterogeneities, cracks, and tine layering [30-331; however, direct field and laboratory rneasurements are difficult [34]. Some laboratory experiments [35] and borehole estimates [36] have correlated measurements of attenuation with the velocity model. These studies indicate that attenuation generally decreases with depth through layer 2 and can, in addition to seismic velocity, play a major role in distinguishing crustal layers. In this paper, we investigate the seismic attenuation profile in the extrusive volcanic sequence of the ocean crust at three N 6-7 Ma sites near the equatorial Costa Rica rift and the central mid-Atlantic ridge.

The intrinsic component of attenuation is due to fluid motion and frictional losses which reduce wave amplitudes exponentially in both frequency and propagation distance; the scattering component is the result of heterogeneity in the formation and strongly depends on the propagating wavelength. Seismic attenuation is usually quantified by the quality factor Q, which is used in seismology literature because it is sensitive to changes when attenuation is relatively low. In highly attenuating environments, Q-’ better represents changes in the physical state of the formation. Based on work by Chemov [37] and Aki and Richards [38], Wu [39,40] developed a model that consists of a linear combination of scattering and intrinsic components of attenuation in heterogeneous media, such that:

2. Attenuation

Fc( k) =

estimation

Attenuation is an inherent property of wave propagation and a measure of the total amplitude loss as a seismic wave propagates through a solid medium.

Q-’

= Q;l

+ Q;’

where Q-’ = total attenuation; Q,’ = scattering attenuation; and Q;’ = intrinsic attenuation. In many environments, scattering effects may dominate the total attenuation, such that Q, ’ > Q; ’ [30,41,42]). Wu’s model accounts for the attenuation due to multiple scattering through a distance x with a velocity distribution that varies around a mean value, LIP,with a small range of variation, AC. Through the fluctuation g(x) = Au( x)/u,,, the medium can be described by a correlation function: N(lx,

-+I)

= (g(xi)g(x*))/(gz)

in which (f) represents the ensemble average of f. Using this model, an isotropic random medium is divided into many slices of equal thickness in the propagation direction of the incident plane wave. The thickness of each slice, a, represents a typical correlation length of the velocity variation. For a plane incident wave of wavenumber k, and the scattering attenuation, Q,; ’ , through the medium over distance x(.x > a> can be expressed as:

Q;‘(k,)

=2(s’>k,(F,(fik,)

where FL(k) is the Fourier-Cosine correlation function:

LxiV( x)cos(

-E;(%J) transform

of the

&)d x

Goldberg and Yin [43] use this expression as a fundamental representation of the scattering model for computation of the attenuation. Once the correla-

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tion function is defined for a particular medium, the Q;’ spectrum can be calculated. The number, location, and magnitude of peaks in the spectrum depend on the form and complexity of the correlation function N(X), which is representative of the heterogeneity in the medium. Assuming that the vertical distribution of heterogeneity is reflected by the variations observed in a sonic velocity log 1391, Goldberg and Sun [36] outlined the application of the model and analytically computed the correlation function and Q; ’ spectrum as a function of depth. Sarns [44] used a similar approach based on the variation of velocity profiles to estimate formation heterogeneity. Using this method, the continuous Q-’ spectrum may be computed over a broad range of correlation lengths. The minimum and maximum spectral wavelengths are determined by the spatial sampling rate and length of the velocity log. The data used in this study allow for computations over a range of correlation lengths between 0.15 m (the spatial sampling of the log) and 600 m (the length of the log) using overlapping 500-point depth windows. Goldberg and Yin [43] approximated the relationship between the observed correlation length a and the incident wavelength A, to be: CI= A”/5 for the typical distributions of correlation lengths they observed. Using this relationship for correlation lengths a = 0.15-600 m, spectral wavelengths span between h = 0.75 and 3000 m. Seismic waves in the ocean crust typically propagate within this range of wavelengths. Fig. 1 illustrates the three-step procedure for the computation of Q- ’ : (a) windowing the Vp log to a 500-point (76.2 m) interval; (b) calculating the correlation function N(X); and (c) estimating the Q-’ spectrum through the F,(k) transformation. Using these log data, maxima in the spectrum usually occur between 1 and 10 m correlation length, indicating the scattering attenuation at the major length scales of heterogeneity. Plotting the maximum peak in each window versus depth yields the Q-’ profile that is characteristic of the dominant heterogeneity in the formation. While the other peaks reflect different correlation lengths, in these formations the maximum typically occurs at l-3 m and is 1.5-2 times larger than nearby maxima [36]. A coarser spectral compu-

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1

I

400.0

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Depth (m bsf) (b)

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0.0

20.0

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log W, m) Fig. 1. Computation of the continuous in situ attenuation spectrum Q-l. (a) A sonic Vp log over a 76 m depth interval. (b) The correlation function N(x) calculated from (a). (c) The attenuation Q-’ spectrum of correlation lengths H. Spectral peaks indicate the maxima due to scattering at the dominant wavelengths of heterogeneity; plotting these peaks versus depth yields the in situ attenuation Q- ’ profile.

tation or averaging approach would decrease the resolution of correlation lengths and the magnitude of attenuation, but it would not skew the spectra or change relative Q-’ values within a depth window.

3. Model limitations In the multiple backscattering model, the medium is assumed to be isotropic with smoothly varying velocities and no P to S mode conversions. Because this study focuses on planar compressional waves propagating in vertical boreholes, these assumptions are reasonable. Mode conversions that occur in sonic

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and Planetary Science Letters 150 (1997) 221-231

logs generally affect signals arriving late in the wave form and rarely influence the Vp computation. If mode conversion or lateral variability affect seismic data, however, subsequent comparisons to field measurements of Q-’ may be lower than those predicted using the model. In addition, Wu’s model assumes that the backscattered energy from each element is entirely lost, although both forward- and backwardscattered energy can be scattered in reverse directions. The magnitude of this effect is proportional to the square of the magnitude of backscattered energy, a second-order contribution that is significant only when k,a = 1 and ]g] = 1 [39]. Using log data, these second-order effects do not greatly reduce the predicted attenuation. Finally, in Aki and Richards’ derivation [38], it is assumed that single scattering effects are valid only in the ‘far field’ region (x S= a>, although Wu’s model assumes single scattering between neighboring elements. Although these limitations may overestimate the attenuation using this model, direct measurements in crustal rocks from laboratory, log and seismic data generally agree with predictions [43]. For this study, relative comparisons can be made reliably since the model will affect Q-’

01234567 0.1 w

Lithology 6

4. Results: attenuation profiles in ODP Holes We compute Q-’ profiles from sonic Vp logs acquired in the extrusive volcanic sequences in ODP Holes 504B and 896A, and in Hole 395A in the Pacific and Atlantic oceans, respectively. Hole 504B [45-471 penetrated to 2.1 km depth during seven drilling episodes through a thick sequence of pillow basalt, lava flows, breccias, and sheeted dikes near the Costa Rica rift (full spreading rate N 6.6 cm/yr). Hole 896A [47] was drilled in 1993 on a local heat flow high just 1 km southeast of Hole 504B to a depth of 0.47 km through pillow basalt, lava flows, and breccias. Hole 395A [22] was drilled in 1975 to 0.67 km depth through pillow basal& lava flows, and breccias on the upper western flank of the mid-Atlantic ridge (full spreading rate N 3.4 cm/yr). The crustal age at all three sites is similar, approximately 5.9-7.3 Ma, and the core recovery is low, ranging from 18% to 38% [22,45]. Overall understanding of the physical properties and lithostratigra-

896A

395A VP MW

values similarly as a function of depth and between different sites.

0123

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VP NW 7'6

VP mm

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012345676

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Fig. 2. Velocity logs acquired using the Schlumberger long-spaced sonic tool over 600 m depth below the seafloor in DSDP/ODP 395A (mid-Atlantic ridge) and Holes 896A and 504B (Costa Rica rift).

Hole

D. Goldberg, Y.F. Sun/Earth

ties [50,29]. A general increase in velocity occurs in all three holes versus depth and the velocity gradient remains relatively constant over these intervals. Fig. 3 shows a crossplot of Vp versus porosity using the unprocessed log data. The velocity-porosity relationship in Hole 504B (plusses) and Hole 896A (crosses) show a similar trend and scatter as the data from Hole 395A (dots), although the data from Hole 395A are systematically lower by about 2 km/s. The overall porosity and lithostratigraphy are similar in all three holes. We attribute the velocity offset to local differences in occurrence of pillow basalts, lava flows, breccias and cobbles. A relatively small number of low velocity values in Hole 395A (circles) correspond to two hole enlargements that could not be corrected [29]. These data are replaced with interpolated values in our analysis, but the resulting effect on the attenuation computation is minimal. The attenuation is affected mostly by sharp contrasts between low and high Vp, not by low absolute values. Overall, the velocity-porosity relationship is consistent in all three holes and with other studies [5 1,25.52,53], indicating similar trends over most of the logged intervals. With standard environ-

phy in these holes, therefore, has critically depended on continuous downhole logging [23,28]. Fig. 2 displays the unprocessed velocity log data and the lithostratigraphy in the three holes from 100 to 600 m below sea floor (bsf). This convention is used throughout the paper, rather than depth below basement, in order to illustrate differences in the thickness of the overlying sediments and facilitate depth correlation with seismic profiles. Porosity was derived from the resistivity log using the procedure outlined by Becker [48] and Pezard [49], which assumes that all the pores and porous cracks are connected and filled with sea water. Porosity averages lo%, but reaches > 40% in the uppermost intervals, and generally decreases with depth. The most recently recorded Vp logs using a Schlumberger long-spaced sonic tool (0.1524 m vertical resolution) were used [29,47]. The logs were edited and processed to correct for environmental effects (low values due to hole size) and cycle skips (erroneous high values) prior to further analysis. Excluding data in certain severely enlarged hole intervals (see below), most of the processed and corrected logs reliably represent the in situ formation proper-

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and Planetary Science Letters 150 (1997) 221-231

B

.* =-D

II

II

I

0.15

III,

,r

,I,,,,,#

0.20

0.25

,.,.,..,.I

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Porosity Fig. 3. Crossplot of porosity versus velocity for Hole 504B (plusses), 896A (crosses), and 395A (dots and circles). Porosity is derived from the resistivity log. The data from 395A that are unreliable in enlarged hole intervals (circles) were replaced by interpolated values (see text for details).

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mental corrections, therefore, we assume that the logs reliably indicate variation in velocity heterogeneity in these sequences. In Fig. 4, the processed and corrected Vp logs in Holes 504B and 395A, the porosity logs, and the computed Q- ’ profiles are compared as a function of depth. Above 465 m bsf, the Q-r logs are similar; Q-l values average about 25 and drop as low as 5 - 10. Below 465 m bsf, a sharp transition in both holes occurs where Q- ’ values decrease by a factor of 5 or more. In Hole 504B, the extrusive volcanic layer above 465 m bsf is approximately 150 m thick. In Hole 395A, the extrusive volcanic layer above this depth is > 300 m thick, more than twice as thick as in Hole 504B. Since the sedimentation near Hole 395A is quite variable, forming numerous sediment ponds [19], the occurrence of the layer 2A/2B boundary at 465 m bsf in both holes is likely to be coincidental. The Q-’ log also increases sharply

l/Q

VP WW 0 O.' c

2

4

above 220 m bsf within this interval and is > 5 times more attenuating than the top of Hole 504B. We interpret these sharp Q-’ discontinuities to be caused by large changes in velocity between higher porosity pillows and breccias and lower porosity massive and altered intervals. Such changes could not be observed from studies on core or seismic data, because they do not sample the intermediate scale of formation heterogeneity that is seen by the logs. Fig. 5 compares the Q- ’ logs in Hole 504B with Hole 896A. Although the velocity, porosity, and crustal age are nearly identical in both holes, Q-’ values are more than 5 times lower in Hole 896A. Both sites have similar lithostratigraphy, petrology, and alteration mineralogy; however, Hole 896A has 10 times greater carbonate volume contained in secondary veins and 15-20s fewer pillow basalts and more massive flows in the uppermost crust [54]. These differences reduce the vertical velocity hetero-

6

8

0.0 0.1 0.2

0.1 I"""""'

Porosity 0.3

0.4

0 10 20 0.1 ["""'I

30

40

Fig. 4. Comparison of sonic Vp logs; attenuation Q- ’ profiles: and porosity logs derived from resistivity between Hole 504B (dashed line) and Hole 395A (solid line). Q-’ profiles are similar between 220 and 465 m bsf, but are > 5 times greater in the upper interval of Hole 395A.

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D. Goldberg, Y.F. Sun /Earth and Planetary Science Letters 150 (1997) 221-231

Porosity

Vp (km/s) 0

2

4

6

0

6

10

20

30

40

0.1 i‘

s p E x

0.3 -

r E al 0.4 P

. ;;.--

_*g-

. -

465m 0.5 -

0.6 -

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Fig. 5. Comparison of sonic Vp logs: attenuation Q- ’ profiles; and porosity logs derived from resistivity between Hole 504B (dashed line) and Hole 896A (solid line). The Q- ’ profile is approximately 5 times lower in Hole 896A. indicating less heterogeneity in the extrusive volcanic sequence in this hole (see text for details)

geneity and likely cause less scattering attenuation. Ah et al. [54] also describe the proximity of Hole 896A to a basement fault, which may have contributed to hydrothermal sealing of open fractures and the localized reduction in heterogeneity. Hole 896A does not appear to penetrate the major Q-’ discontinuity associated with the transition zone below the extrusive volcanics that is observed in Hole 504B. Based on the logs, it is clear that large variations in crustal morphology and alteration may occur even in close spatial and age proximity to the same ridge segment.

5. Discussion In the uppermost portion of the oceanic crust, Q-’ profiles at Atlantic and Pacific sites indicate that scattering attenuation in the extrusive volcanic sequences is strongly controlled by variation in local

heterogeneity and morphology. Assuming that the contribution of intrinsic attenuation is likely to be small in oceanic basalts [55], a large contribution of scattering to the observed attenuation in seismic experiments can be anticipated [42.43,56]. The Q-’ profile could then be used to approximate the expected attenuation for seismic experiments in which wave modes propagate vertically through the upper oceanic crust. In field studies, observations suggest that seismic attenuation in the ocean crust is often dominated by scattering effects. Large attenuation values (Q = 8-16) were estimated using Vertical Seismic Profile (VSP) data near the highly deformed South West Indian ridge [57.43]. Amplitude modeling of seismic refraction and reflection data, which is often used to estimate attenuation, has shown high values (Q = 20-50) in the uppermost oceanic crust on the Juan de Fuca ridge [58]. Wilcock et al. [59] report high attenuation (Q = 10-20) from seismic studies in the shallow crust off the East Pacific Rise.

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High attenuation (Q = 60) was also revealed by seismic studies in young oceanic crust in Iceland, which was attributed to strong heterogeneity caused by fissures, faults, and extreme changes in porosity in volcanic rocks [60]. Goldberg and Sun [36] used the approach described here to estimate seismic attenuation as a continuous profile to the total depth of Hole 504B. They find that attenuation decreases step-wise at distinct layer boundaries and approaches deep seismic refraction estimates
the high-porosity extrusive volcanics are extremely heterogeneous and generate large amounts of scattering attenuation. Due to the strong influence of morphology and heterogeneity, changes in crustal structure versus depth can be deduced from the Q-’ log [36]. Based on the results shown in Fig. 4, consistent and distinct boundaries in Q-’ at 465 m bsf correspond to a decrease in velocity heterogeneity and can be reconciled by the presence of a morphologic change at this depth in both Hole 504B and Hole 395A. Based on the similarity of the Q values, we interpret this depth as representing the layer 2A/2B boundary in both holes; indicating the transition between strongly heterogeneous extrusive pillow basalts and massive flows to more homogeneous morphologies below. In Fig. 7 we show an interpreted Q structure of layer 2A in Hole 395A and Hole 504B; layer 2A is at least twice as thick in Hole 395A as in Hole 504B. Based on high Vp measurements that are attributed to pore-filling alteration minerals at the bottom of Hole 395A, Hyndman and Salisbury [23] concluded that the layer 2B/2C boundary was penetrated at about this depth. If so, the Q-’ discontinuity at 220 m bsf might represent the base of layer 2A, but this would imply that layer 2B is > 240 m thick. With similar lithologies and velocities over a considerable interval above and below 220 m bsf, we interpret this discontinuity as resulting from variation in the morphology of the extrusive volcanics within layer 2A. The layer

Fig. 6. Crossplot of porosity versus attenuation in Hole 504B (dots) and Hole 395A (crosses). The exponential relationship derived over the entire 2.1 km depth interval in Hole 504B (dotted line) shows that attenuation is essentially insensitive to porosity for these the heterogeneous extrusive volcanic rocks.

D. Goldberg,

and Planetnq

395A Slow

5040 Fast _--__-.._-_--1 z--TYlz __--

Y. F. Sun /Earth

.-SF G?;2-5

i

Q;5--35

2A

465m 2B

29

Fig. 7. Interpretation of the thickness of layer 2A from Fig. 4. The sharp decrease in Q -’ in both Hole 395A and 5048 at 465 m bsf is interpreted to be the layer 2A/2B boundary. Layer 2A is 2 times thicker in Hole 395A than in Hole 504B, although Q values are 5-6 times lower above 220 m bsf (see text for details).

2B/2C boundary may occur only 50-100 m below the base of layer 2A, near the bottom of Hole 395A. From estimates of seismic refraction velocities, Barrett and Purdy [IS] also interpreted a 400-500 m thick layer 2A near Hole 395A. Given their similarity in bulk properties and composition, the observed differences in Q ’ and layer 2A thickness at these sites may be attributed to variations in the volume or duration of volcanic activity between intermediateand slow-spreading mid-ocean ridge segments. A relationship between the crustal spreading rate and magmatic and volcanic processes at ridge segments has been suggested by several authors [ 11,62,63]. The resulting thickness of layer 2A has been associated with variations in the volcanic budget, which is thought to be largely controlled by the depth and thermal state of the underlying magma chamber 164-661. Hooft and Detrick 1641 suggest that this depth results from the balance of pressure in the magma chamber with the lithostatic pressure of the overlying rocks. Our observation of a thicker and more heterogeneous layer 2A in Hole 395A than in Hole 504B, may be evidence that such controls on local volcanism exist where a greater duration and volume of extrusion occurs from deeper magma chambers at slow-spreading than at intermediate-spreading ridges. The results from Hole 896A corroborate that the thickness and attenuation of the extrusive layer may indeed vary significantly over short lateral distances, which is probably also the result of changes in the volume of lava

Science Letters 150 (1997)

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flows and eruptive pillows during different episodes of extrusion at the same ridge segment. The thickness of layer 2A, therefore, observed by changes in the volcanic morphologies (and resulting scattering attenuation) with depth may reflect the magmatic processes which occur at spreading centers. This comparison between N 6 and 7 Ma sites at the Costa Rica and mid-Atlantic ridge segments suggests that these processes are related to differences in crustal spreading rate.

6. Conclusions We conclude that the in situ attenuation in the ocean crust, consisting of both intrinsic and scattering contributions, is high because it is dominated by scattering effects from velocity heterogeneities that reflect large changes in morphology. Using an analytical model for multiple backscattering and data from the observed Vp logs in three holes drilled through extrusive volcanic sequences, the in situ and attenuation, Q - ’ , is greatest in high-porosity heterogeneous pillow basalts and massive flows and decreases by a factor of 5 or more over a transition zone in both intermediateand slow-spreading oceanic crust of similar age. This discontinuity in the Q- ’ profile is interpreted to coincide with the seismic layer 2A/2B boundary, which is not easily observed using conventional seismic velocity profiles. As a consequence of the large scattering effects in the extrusive volcanics, the attenuation is essentially insensitive to variations in porosity above 8%, which is consistent with an asymptotic relationship between Q- ’ and porosity in the shallow crust. The Q-’ logs indicate a thicker, higher porosity, and more highly attenuating extrusive layer in the Atlantic than in the Pacific, which may result from a greater volume or duration of volcanism at this slow-spreading ridge segment. The variability in morphology within layer 2A also affects the scattering significantly and may cause local changes in Q- ’ over short lateral distances. Observation of the vertical heterogeneity of extrusive volcanics using seismic scattering as a tool can image the thickness of layer 2A, which, in turn, may reflect the magmatic processes forming the upper ocean crust at intermediate- and slow-spreading ridge segments.

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