67. COMPRESSIONAL WAVE VELOCITIES, DENSITIES, AND POROSITIES OF BASALTS FROM HOLES 417A, 417D, AND 418A, DEEP SEA DRILLING PROJECT LEGS 51-53

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1 67. COMPRESSIOAL WAVE VELOCITIES, DESITIES, AD POROSITIES OF BASALTS FROM HOLES 417A, 417D, AD 418A, DEEP SEA DRILLIG PROJECT LEGS I. Chistensen, 1 S.C. Bli, 2 R.H. Wilkens, 3 nd M.H. Slisuy 4 ITRODUCTIO It hs long een suspected tht the seismic velocity of t lest the uppe level of Lye is contolled not only y the intinsic velocity of the ocks of which it is composed, ut y the pesence of wte-filled ccks nd voids. By comping lotoy mesuements of compessionl wve velocity though slts ecoveed fom Hole 417D, with the esults of the olique seismic expeiment (Stephen et l., this volume) nd logging (Slisuy et l., this volume) fo the sme hole, it should e possile fo the fist time to evlute the ole of such ccks upon velocity in the uppe few hunded metes of the ocenic cust. Since seismic velocities e extemely sensitive to confining pessue (e.g., Bich, 1960) ny such evlution must e sed on lotoy mesuements of velocity t elevted pessues ppopite in situ conditions. In this ppe, we pesent compessionl wve velocities to hydosttic confining pessues of 6 k fo smples of slt fom Holes 417A, 417D, nd 418A. In ddition, ulk densities nd poosities hve een mesued nd e shown to coelte well with the compessionl wve velocities. Of pticul significnce, slt velocities fom these sites e shown to decese linely with incesing poosity. EXPERIMETAL TECHIQUES AD DATA Pio to velocity mesuement, ech smple ws cut to ight cylinde ( cm in dimete nd 3 to 5 cm in length) nd then i-died, mesued, nd weighed to detemine its dy-ulk density. The smples wee then wte-stuted nd eweighed to otin wet-ulk densities nd effective poosities. Afte weighing, the stuted smples wee jcketed with coppe foil nd 1-MHz ium titnte tnsduces wee ttched to the coe ends. The compessionl wve velocity though ech smple ws then detemined using the pulse tnsmission technique of Bich (1960). Poe pessues wee mintined t vlues lowe thn extenl pessues y plcing 100 mesh sceen etween the smples nd coppe jckets. Pessue ws pplied to the smples y mens of two-stge intensifie with low viscosity oil s the pessue medium. Pessue ws mesued using clited mngnin coil. 1 Deptment of Geologicl Sciences, Gdute Pogm in Geophysics nd Deptment of Ocenogphy, Univesity of Wshington, Settle, Wshington. 2 Gdute Pogm in Geophysics, Univesity of Wshington, Settle, Wshington. 3 Deptment of Geologicl Sciences, Univesity of Wshington, Settle, Wshington. 4 Scipps Institution of Ocenogphy, Univesity of Clifoni, L Joll, Clifoni. Velocities t -k intevls to 1 k nd t lge intevls to 6 k, togethe with wet-ulk densities nd effective poosities t tmospheic pessue e given in Tle 1. Aove 1 k, the velocities e ccute to within 1 pe cent (Chistensen nd Shw, 1970). At lowe pessues, the velocities cn only e estimted to ±3 pe cent ecuse of signl ttenution. VELOCITY-DESITY RELATIOS Pevious investigtions hve demonstted n excellent coeltion etween compessionl wve velocity nd ulk density in ocks unde confining pessues to 10 k (e.g., Bich, 1960; Chistensen nd Slisuy, 1975). It is desile to fomulte velocity-density coeltions so tht seismic velocities oseved in situ my e used to detemine the distiution of density with depth. Compessionl wve velocities fo the sltic ocks studied fom Holes 417A, 417D, nd 418A nge fom 3.11 to 6.68 km/s nd wet ulk densities nge fom 2.10 to 2.96 g/cm 3. Figue 1 pesents the compessionl wve velocity (V P ) t 1 k vesus wet ulk density (p) fo these smples. Included in Figue 1 e line nd polic egessions of velocity vesus density fo 77 DSDP slts studied y Chistensen nd Slisuy (1975) with pmetes: V p = p V P = p 3 " Dt fom the cuent smples usully fll within ± km/s of the non line pedicto except fo the smple fom Section 417D-28-5, geenstone (p = 8 g/cm :! nd V P = km/s) which flls ove the sltic tends. Tle 2 includes line egession pmetes fo V P vesus p using vlues of V P mesued t, 1, nd 6 k fo smples. Mny smples fom Hole 418A tht wee coed t depths etween 710 nd 862 metes su-ottom exhiit densities gete thn 2. g/cm 3 nd ccount fo most of the high density points in Figue 1. Smples fom Hole 417D, on the othe hnd, wee ecoveed fom depths etween 345 nd 696 metes su-ottom nd hve modete densities; slts coed fom etween 218 nd 412 metes in Hole 417 A nge in density fom 2.90 g/cm 3 to vlues s low s 2.10 g/cm 3 ne the top of the sement. Thus, genel incese in density with su-ottom depth is oseved. Exmintion of the coed smples suggests tht this incese in density with depth coesponds to decese in ltetion. VELOCITY/POROSITY RELATIOSHIPS Figue 2 epesents plot of compessionl wve velocity t 1 k vesus effective poosity. Coeltion etween the 1467

2 . I. CHRISTESE, S. C. BLAIR, R. H. WILKES, M. H. SALISBURY TABLE 1 Velocities, Densities, nd Poosities Smple (Intevl in cm) 417A-24-1, A-25-1, A-27-1, A-28-6, A-30-4, A-31-3, A-32-4, A-34-3, A-37-1, A-38-4, A-42-6, A-44-3, A-46-4, D-22-1, D-22-6, D D-274, D-28-2, D-28-7, D D-30-1, D-31-1, D-32-5, D-33-2, D-34-2, D-35-2, D-37-2, D-39-1, D-394, D-40-1, D-42-2, D-43-2, D44-1, D-45-2, D-48-6, D-50-2, D-52-2, D-554, D-58-3, D-60-2, D-62-1, D-66-3, D-67-4, A-15-1, A-16-3, A-18-1, A-20-2, A-25-1, A-26-3, A-28-1, A-30-4, A-33-1, A A-35-5, A-39-2, A-41-1, A41-3, A424, A44-l, A-46-1, A48-l, A-4 9-2, A-51-2, A-52-3, A-54-2, A-55-2, A-56-1, A-57-2, A-60-6, A A-63-5, A-64-l, A-65-l, A A-69-l, A-71-1, A-73-1, A-74-1, A A-78-6, A A-80-2, A-81-5, A-82-1, A-83-2, A-84-2, A--2, A-86-2, Bulk Density (g/cm3) Poosity _ _ Compessionl Wve Velocity t Vying Pessues (k)

3 BASALT VELOCITIES, DESITIES, AD POROSITIES E ;* DESITY Figue 1. Compessionl wve velocity t k plotted ginst wet-ulk density. Line nd non-line solutions fom Chistensen nd Slisuy (1975). two pmetes is quite good, not only t 1 k, s illustted, ut lso t nd k. Sttistics nd egession pmetes e given fo ll thee coeltions in Tle 2. The coeltion is even moe emkle given the low ccucy of the poosity detemintion (±%). Vlues of poosity nge fom 0.0 to 11.3 pe cent, with lmost ll of the smples (83) hving vlues less thn pe cent. As expected t low confining pessues, the high-density, high-velocity smples (notly those fom the lowe sections of Hole 418A) exhiit low poosities. Wht is significnt is the fct tht the velocity/poosity coeltion is not sustntilly educed t 6 k, s might e nticipted if the confining pessue wee effective in closing the ccks nd voids in the slt which e esponsile fo its poosity. An ttempt to coelte the incese in mesued velocity with smple poosity in the to k nd to k intevls shows the incese in velocity to e lmost totlly independent of poosity, the coeltion coefficients eing on the ode of 0.10 to 0. The independence of the velocity/poosity eltionship fom confining pessue my e esult of the mechnism y which voids e fomed in ocenic slts. Plutonic ocks which hve een exposed t the eth's sufce wee fomed in egimes of high confining pessue. A lge pt of the poosity in such smples cn poly e elted to the effects of pessue elese; convesely, incesing confining pessue duing mesuements mde on such ocks should tend to evese these effects. This is not the cse fo ocenic slts which wee extuded in molten stte t ocen idge cests whee the confining pessue due to the ovelying column of se wte is genelly on the ode of only 5 to 0.35 k. Voids fomed in this egime cn e expected to e eithe vesicles due to tpped mgmtic gses o ccks which e the esult of theml contction s the slt cools. eithe of these void types need close completely unde the ppliction of high confining pessue. Futhemoe, the effect of ltetion of the slts t the idge cests might futhe tend to inhiit the closue of ccks t high confining pessues. Since the ccks esponsile (in pt) fo the poosity of sumine slts wee fomed y theml contction the thn pessue elese, it would e inteesting, especilly in the feshe smples, to exmine the effect of elevted tempetues on the velocity/poosity eltionship. DESITY/POROSITY RELATIOSHIPS The plot of density vesus poosity (Figue 3) exhiits tend which, s expected, indictes lowe ulk density fo the smples with highe poosity. The lge mount of sctte out the egession line in compison to the velocity/density nd velocity/poosity compisons in Figues 1 nd 2 is poly due oth to the low ccucy of the poosity mesuement nd to the extent nd minelogy of ltetion, which would tend to lowe density vlues independently of poosity. ACKOWLEDGMETS J. Hull, G. Bussod, nd R. Pio ssisted in the velocity mesuements. Finncil suppot ws povided y the Office of vl Resech Contct C REFERECES Bich, F., The velocity of compessionl wves in ocks to 10 kilos, 1,7. Geophys. Res., v. 65, p

4 . I. CHRISTESE, S. C. BLAIR, R. H. WILKES, M. H. SALISBURY TABLE 2 Regession Line Pmetes Vp = +p Pessue, k (k«/s i/g/cm 3 ) S (V,p) Vp = +0 Pessue, k (km/s/%) S (V,tf>) p = (g/cm 3 /%) S (p,φ) Φ ~ +p (%/) S (ΦP) p = +Vp Pessue, k (g/cm 3 /km/s) (p,vp) Φ = +Vp Pessue, k %/ S (Φ,Vp) ote: is the nume of dt points; S(,), stndd eo of estimte of on ;, coeltion coefficient;, coefficient of detemintion; p, wet ulk density; Φ, poosity; V p, compessionl wve velocity. Chistensen,.I. nd Slisuy, M.H., Stuctue nd constitution of the lowe ocenic cust, Rev. Geophys. Spce Phys., v. 13, p Chistensen,.I. nd Shw, G.H., Elsticity of mfic ocks fom the Mid-Atlntic Ridge, Geophys. J. Roy. Aston. Soc, v. 20, p

5 BASALT VELOCITIES, DESITIES, AD POROSITIES 7.0 \ ' > A + 417D Δ 418A POROSITY i i i i i i, i i Figue 2. Compessionl wve velocity t k plotted ginst effective poosity. Line solution fom Tle 1. I ' i i i ' i ' i I A + 417D Δ 418A POROSITY Figue 3. Wet-ulk density plotted ginst poosity. Line solution fom Tle