3-D gravity and magnetic interpretation for the Haifa Bay ... · of 2500 m in the Atlit-1 well...

Ž .Journal of Applied Geophysics 44 2000 353–367www.elsevier.nlrlocaterjappgeo

3-D gravity and magnetic interpretation for thež /Haifa Bay area Israel

M. Rybakov ), V. Goldshmidt, L. Fleischer, Y. Ben-Gai( )The Geophysical Institute of Israel GII , P.O. Box 2286, Holon 58122, Israel

Received 22 September 1998; accepted 28 March 2000

Abstract

Ž .Recently observed features in the subsurface geology of the Haifa Bay area northern Israel have been evaluated using3-D forward gravity and magnetic modeling and inversion schemes. The interpretation is based on updated petrophysicaldata of the Jurassic, Cretaceous and Tertiary sedimentary layers and volcanics. It has been shown that the Bouguer gravityanomalies correspond mainly to thickness variations in the Senonian to Tertiary sediments. The gravity effect of thesesediments was calculated using their actual densities and structural setting as interpreted from seismic reflection data. Thiseffect was removed from the Bouguer gravity in order to study the pre-Senonian geological structures. The pattern of

Ž .residual gravity anomalies named ‘‘stripped gravity’’ is essentially different from the pattern of the Bouguer gravity. Theprominent Carmel gravity high, clearly seen on the Bouguer gravity map, completely vanishes on the ‘‘stripped’’ gravitymap. That suggests that this relatively positive anomaly is caused by the considerable thickness of the low-density youngsediments in the surrounding areas and does not correspond to high-density magmatic rocks or crystalline basement uplift aspreviously suggested. The average densities of the Jurassic and Cretaceous volcanics are generally lower then those of thebackground sedimentary rocks. Volcanics are the main cause for magnetic anomalies onshore and offshore northern Israel.The magmatic root of the Asher volcanics is, most probably, located close to the Yagur fault. A large, deep-seated gabbroicintrusion is assumed to be located under the Mediterranean abyssal plain in the NW part of the study area. The Atlit marinegravity low appears to be caused by a thick Mesozoic and Tertiary sedimentary accumulation. The results presented shouldbe of considerable assistance in delineating some aspects of hydrocarbon exploration in the area. q 2000 Elsevier ScienceB.V. All rights reserved.

Keywords: Gravity; Magnetics; Subsurface structures; Magmatism; Northern Israel

1. Introduction

ŽThe Carmel structure in northern Israel Fig..1 is an elongated, tilted block extending from

the onshore into the shelf of the Haifa Bay area.

) Corresponding author. Tel.: q972-3-557-6050; fax:q972-3-557-2925.

Ž .E-mail address: [email protected] M. Rybakov .

This structure comprises a prominent NNEtrending folding system, traversed by severalNW trending faults. The folding is probablypart of the Late Cretaceous to Tertiary ‘‘Syrian

ŽArc’’ compressional phase Picard and Kashai,1958; Arad, 1965; Sass, 1980; Ginzburg et al.,1975; Neev et al., 1976; Ron et al., 1984;Rotstein et al., 1993; Ben-Gai and Ben-Avra-

.ham, 1995 . The Carmel structure onshore is

0926-9851r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved.Ž .PII: S0926-9851 00 00013-6

( )M. RybakoÕ et al.rJournal of Applied Geophysics 44 2000 353–367354

Fig. 1. Magnetic anomalies of the Eastern Mediterranean. The anomalies named after the major features, i.e., EratosthenesŽ . Ž . Ž .E , Carmel C and Hebron H are the largest anomalies of the region. The regional tectonic map is shown in the insert.

separated from the Qishon graben to the northŽ .by the Yagur fault Figs. 2, 4 and 6 with a

vertical displacement of more than 1000 mŽPicard and Kashai, 1958; Ginzburg et al., 1975;

.Neev et al., 1976 .Our present knowledge of the subsurface ge-

ology, both onshore and offshore, is based onŽ .the deep oil wells Fig. 2 and on limited and

scattered seismic reflection data. The Triassicand Jurassic sequences penetrated in the areaare, in general, carbonates with minor facieschanges. The Cretaceous strata are characterizedby an abrupt change from shallow platformcarbonates to open marine, shaly–marly slopefacies. The Upper Cretaceous and Cenozoicrocks have been eroded from the elevated areas

of Mount Carmel and its offshore extension.Thick Cenozoic sequences of chalks and marlsare only present in the Qishon graben and in theRamot Menashe–Hadera syncline located northand south of the Carmel structure. The geologi-cal data on both sides of the Yagur fault suggestthat it was non-existent until the mid-Cenozoicand its onset probably coincides with the left-lateral motion along the Dead Sea Transform

Žfault de Sitter, 1962; Freund, 1970; Ben-Gai.and Ben-Avraham, 1995 .

Mount Carmel is characterized by intensevolcanic activity which began in the EarlyJurassic and continued during the Cretaceousand Tertiary. The Liassic Asher volcanics of theolivine basalt with some gabbroid magmatic

( )M. RybakoÕ et al.rJournal of Applied Geophysics 44 2000 353–367 355

Ž . Ž .Fig. 2. Bathyorographical bottom, after Hall, 1993 and magnetic top maps of the Haifa Bay area. Insert: location map ofthe referred wells corresponding to a bigger area.

intrusions are intercalated with the Triassic andthe Jurassic carbonates and exceed a thickness

Žof 2500 m in the Atlit-1 well Gvirtzman and.Steinitz, 1983; Dvorkin and Kohn, 1989 . Simi-

lar basaltic rocks, 200 m thick, were penetratedin the Yagur-1 well, while the alternations oftuffs with basaltic lava flows of 270 m wereencountered in the Deborah-2A well located 30

Ž .km east of the Carmel structure Fig. 2 . TheŽ .Tayassir volcanics Mimran, 1972 overlie the

regional Base Cretaceous unconformity and arewidespread in northern Israel. These volcanicsconsist of basaltic lava and tuffs, alternatingoccasionally with sedimentary rocks, have been

penetrated in a number of wells. The youngestvolcanics, composed mainly of a series of tuffs,are interbedded in the Cenomanian–Senonianrocks and mapped on Mount Carmel and south

Ž .of it Sass, 1980; Arad, 1965 .The magnetic anomaly corresponding to the

Mount Carmel structure is one of the largestmagnetic anomalies in the Eastern Mediter-

Ž .ranean Fig. 1, Rybakov et al., 1994 . ThisŽanomaly was the subject of many studies Ginz-

burg, 1960; Domzalski, 1967, 1986; Folkman,1976; Ben-Avraham and Hall, 1977; Ben-Avraham and Ginzburg, 1986; Gvirtzman et al.,

.1990 . Interpretations performed prior to the


findings in the Atlit-1 well in 1981 suggested ahighly elevated crystalline basement while, af-terwards, the concept of an Early Jurassic shieldvolcano spreading over the area was adopted by

Ž .researchers. Garfunkel and Derin 1984 sug-gested that this volcanic phase belongs to anearly Mesozoic rifting of the Levant margin.For the moment, a pile of Asher volcanics, 2500m thick penetrated in the Atlit-1 well, is aunique feature that bears on the evolution of thecomplex east Mediterranean, so that even itscrustal composition is still in dispute.

As noted above, the evolution of the Carmelstructure is not fully understood. An extensionaltectonic regime, associated with widespreadvolcanism, is suggested for the Early Mesozoic,followed by the development of a shallow car-bonate platform during Mesozoic and EarlyCenozoic times. Later on, in the Middle Ceno-zoic, the left lateral Dead Sea transform faultproduced the modern faulted block of MountCarmel and the Qishon Graben.

Several questions relating to the cycled vol-canic activity in the Carmel area could be for-mulated as follows.

Ø Are there more intrusive bodies in the area?Ø Was the Asher Volcano fed from a single

neck or from a zone of weakness?Ø If such a zone of weakness is indeed present,

is it associated with the modern Yagur fault?

Two main problems may be defined with regardto the technical aspects of previous interpreta-tions: the three-dimensionality of the structureswas not accounted for and the densities of thevolcanics were overestimated. A new interpreta-tion of the gravity and magnetic data, based on3-D routines with updated density values andmagnetic vectors, has been performed and ispresented in this paper.

2. The data

The gravity and magnetic data used in thisŽstudy are part of the GII Geophysical Institute

. Ž .of Israel database Rybakov et al., 1997 . The

magnetic data were composed of the aeromag-netic measurements at a constant flight level of

Ž .about 1 km line spacing — 2 km and marinemeasurements. The marine magnetic data, con-tinued upward to an elevation of 1 km, are inagreement with the aeromagnetic data. Thismagnetic data set was checked for erroneousvalues, gridded and slightly smoothed using theinverse distance method. The International Geo-

Ž .magnetic Reference Field IGRF was removedand, therefore, the magnetic data used representmagnetic anomalies, provided that the magneticcore field was adequately removed. The result-

Ž .ing grid Fig. 2 was used for reduction to thepole and upward magnetic continuation,pseudo-gravity calculation and gravity–mag-netic correlation. 3-D magnetic modeling andinversion were also based on this grid.

All the graÕity data were reduced to aBouguer density of 2670 kgrm3. This value

Žwas chosen using the Nettleton technique Net-.tleton, 1971 applied to a number of typical

topographical sections. The terrain correctionsfor all land gravity stations were calculated upto a 20-km distance using a model with a 25-mgrid adopted from the Digital Terrain ModelŽ .DTM compiled by the Geological Survey of

Ž .Israel Hall, 1993 . The gravity data have areliability and accuracy that allows interpolationto a 2-mGal contour interval. The data set wasgridded and gently smoothed using similar tech-niques and parameters. This grid was used forregional–residual gravity separation, horizontaland vertical gravity derivatives calculation andgravity–magnetic correlation. The 3-D gravitymodeling and resulting map compilation werebased on this grid.

3. Petrophysics

All the available density and magnetic rocksusceptibility data were collected and incorpo-

Ž .rated in a data bank Rybakov et al., 1999 . Fig.3 presents a generalized petrophysical model ofthe Mesozoic and Cenozoic rocks.


ŽFig. 3. Generalized petrophysical model of the Haifa Bay area as inferred from a number of deep boreholes Rybakov et al.,. Ž 3 3.1999 , only four typical density logs are shown. The logging density variation 10 kgrm is shown along the stratigraphic

Ž y5 .units drilled by the boreholes. Magnetic susceptibility K in 10 SI is assessed only for volcanics.

The average magnetic susceptibility valuesŽfor all igneous rocks Early Jurassic Asher vol-

canics, Late Jurassic Deborah volcanics, EarlyCretaceous Tayassir volcanics and Late Creta-

.ceous Carmel volcanics are assumed to beŽ .0.02–0.03 SI units Rybakov et al., 1999 . The

relation between remanent and induced magne-tization, measured from samples of the Jurassicvolcanics, was defined as 0.03–0.3. This im-

plies that the magnetic anomalies caused bysuch bodies depend mainly on induced magneti-zation. The parameters of the induced magneti-zation vector were computed using the IGRFprogram.

The densities for the stratigraphic sequenceswere calculated using borehole log density data.Fig. 3 shows that the main density contrast

Žoccurs between the least dense about 2000–


3.2100 kgrm Senonian–Tertiary rocks and thepredominantly carbonate, older Mesozoic rocks.

Ž 3.The densest rocks up to 2850 kgrm arescattered anhydrite and dolomites of Late Trias-sic occurring in the Deborah-2A well. The den-sity of rocks older than the Triassic was esti-mated at 2670 kgrm3, corresponding to theBouguer density. The Early Jurassic Asher vol-canics have an average density of about 2550–

3 Ž .2600 kgrm Fig. 3 . This means that volcanicrocks have a negative density contrast relativeto the Mesozoic carbonate sequence, which havean average density of about 2750 kgrm3.

4. Analysis and interpretation of the gravityand magnetic data

A generation of initial subsurface geologicalmodel corresponding to the geophysical obser-vations is an important interpretation stage. Alladditional investigations mainly check and re-fine the results revealed during this stage whichincludes an analysis of both the potential fieldsand their transformations in an attempt to em-phasize the various frequency components ofthe gravity and magnetic fields. A variety offiltering techniques was employed in order toenhance the data sets prior to interpretationŽ .Cordell et al., 1992 : regional–residual gravityseparation; horizontal and vertical gravity andmagnetic derivatives; reduction to the pole andupward magnetic continuation; pseudo-gravityand gravity–magnetic correlation.

The Bouguer graÕity values in the area varyfrom 40 to 110 mGal and they generally de-crease to the southeast. The cause of the re-gional gravity trend could be the transition fromoceanic crust of the Eastern Mediterranean tothe continental crust of the Arabian plate, which

Žmay occur under the Levant margin Makris and.Wang, 1994 . For interpretation purposes, the

regional–residual separation of the gravityanomalies was conducted using the regionaltrend that was calculated as a third order poly-

nomial surface. Removing this trend, the resid-ual Bouguer gravity map has been compiledŽ .Fig. 4 . The main features of this map are aNW trending high with a magnitude of about 50mGal and two wide lows. The gravity high

Ž .consists of three separate anomalies Fig. 4 :a. The southern anomaly, located south of

Mount Carmel, is about 20 km in length alongits NNW oriented axis; its southern boundarycoincides with the northern edge of the Tertiarysedimentary embayment.

b. The central and most intense part of thepositive gravity ridge is steeply bounded on allsides.

c. The northern part, oriented NNE and per-pendicular to the central part, consists of a fewlocal highs. A small, but clearly observed, localgravity low coincides with the Carmel highelevation near Haifa. The wide gravity low inthe southwest has a magnitude of about y10mGal. No significant gravity anomalies havebeen observed in the northwest side of the studyarea.

As mentioned above, the lateral changes inthe thickness of low-density young sedimentsare the main reason for the residual anomaliesin the area. The thickness of the young sedi-ments was deduced from seismic reflection dataas the difference between the sea floor and the

Ž .structural depth to the top Turonian Fig. 4 .These values were used to calculate and subtractthe gravity effect of the low-density sediments.This stage of ‘‘gravity stripping’’ was per-formed using the PFGRAV3D program devel-

Ž .oped by Blakely Cordell et al., 1992 . Thisprogram calculates the gravity effect using threerectangular grids that define the source: the topsurface, the bottom surface and the density con-trast. In this case the first surface corresponds tothe sea floor and the second to the top Turonian

Ž .carbonates Fig. 5 . The density contrast wasŽ .derived from density well logs Fig. 3 . The

negative gravity effect of the low-density sedi-ments ranging from y60 to y10 mGal wasremoved from the observed gravity values. Thegravity effect of the Asher volcanics was calcu-


Ž .Fig. 4. Residual Bouguer anomalies of the Haifa Bay area contour interval — 5 mGal . Schematic structural map on top ofŽ . Žthe Turonian carbonates contour interval — 500 m . Residual ‘‘stripped’’ gravity map of the study area contour interval

.— 5 mGal . Densities of the young sediments and Asher volcanics were replaced by average Bouguer density.

Žlated using the GRAVPOLY program Godson,.1983a .

The geometry of the volcanic bodies wastaken from the 3-D magnetic interpretation de-


Fig. 5. Stages of ‘‘gravity stripping.’’

scribed below. The density contrast was as-sumed from density logs. The negative gravityeffect of the volcanic rocks, ranging from y0to y15 mGal, was also removed from theBouguer gravity values. Replacing the actualdensities of the young sediments and volcanics

Ž 3.to an average Bouguer density 2670 kgrm ,we cleaned the Bouguer gravity anomalies fromthe influence of the above mentioned geologicalbodies. The gravity anomalies obtained shouldbe named ‘‘stripped’’ gravity anomalies. Re-moving the regional trend from the ‘‘stripped’’gravity anomalies, we compiled the residual‘‘stripped’’ gravity anomalies. The stages of the‘‘gravity stripping’’ are illustrated in Fig. 5.Inasmuch as the final results are strongly de-pending on the accuracy of the calculated grav-ity effect we estimated this parameter by usingrealistic limitations of the top Turonian anddensity data as well as the accuracy of the 3-Dgravity calculation. The total error does notexceeding of 4 mGal.

The pattern of the residual ‘‘stripped’’ grav-Ž .ity anomalies Fig. 4 is essentially different

from that of the residual Bouguer gravity. Thestudied area consists of a number of local grav-ity lows and highs that are not apparent on theBouguer data. The elongated positive anomaly,with a magnitude of about of 15 mGal, isextended NW to about 20 km from the Qishongraben to the Haifa Bay area. The SW boundaryof this anomaly coincides with the present Yagurfault, while its NE boundary appears to be anewly discovered lineament. The high gravitygradients are related to these boundaries. Anelongated negative gravity anomaly with magni-tude of about y5 mGal, located SW of theabove-mentioned positive anomaly, is about 15km long. Only the SE part of this anomaly isseen inland as a small gravity low on the non-

Ž .stripped data Fig. 4 . A rounded gravity highwith a magnitude of about 5 mGal is locatedbetween the Atlit-1 and Foxtrot-1 wells. TheFoxtrot-1 well is located in a complex gravity


saddle. A high gravity anomaly occupies theNW corner of the study area. The magnitude ofthis wide anomaly reaches about 25 mGal. Awide gravity low is located in the SW corner ofthe area with a magnitude of about y25 mGal.

Ž .A relatively small, elongated N–S positivegravity anomaly with a magnitude of about 10mGal, oriented southward, is delineated in thenear shore close to the Atlit-1 well.

Gravity features, possibly fault or structurerelated, were drawn in the interpretation mapŽ . ŽFig. 6 using the residual gravity maps Bouguer

.and stripped and horizontal gravity gradients.Magnetic anomalies are a distortion of the

total geomagnetic field caused by local changesin the rock magnetization. In contrast to thegravity, the sedimentary strata are ‘‘transparent’’and the magnetic anomalies are caused only bybasic magmatism. The parameters of the Earth’s

Žtotal field vector for the area of study inclina-tions458, declinations38 and total field mag-

.nitude about 43,500 nT were calculated using aprogram developed by the US National SpaceScience Data Center. The theoretical magneticanomaly, caused by a body magnetized by thisnormal geomagnetic field, contains the conju-gated maximum and minimum, the latter lyingto the north. This signature is important forgeological understanding of a pattern of themagnetic anomalies. The central part of thestudied area is occupied by a few magneticanomalies as shown in Fig. 2. The easternmost

Žintensive Carmel magnetic anomaly Ml peak to.peak is about 240 nT , oriented WNW, is about

40 km long. The northern edge of this anomalyis marked by a sharp magnetic gradient. Itssouthern edge shows moderate gradients.

Ž .The next magnetic anomaly M2 is locatedoffshore west of the Carmel anomaly. It iselongated in shape and trends NNE, perpendicu-lar to the strike of the main direction of theCarmel anomaly. Its peak to peak reaches 210nT. This anomaly, extending over 20 km, isbounded to the south and north by steep hori-zontal gradients. The peak to peak of the M3

anomaly reaches 120 nT. Its marginal gradientsare more moderate than those of the otheranomalies mentioned above. The magneticanomaly M4, shown in the northeastern cornerof the studied area, is only a small part of a highmagnetic anomaly located in the southern off-

Ž .shore of Lebanon Fig. 1 . No magnetic anoma-lies are present in the southwestern part of thestudied area.

High frequency magnetic anomalies, locatedoffshore, have been mapped by the marine mag-netic survey. We speculate that these anomalieswere caused by shallow basic volcanics. Thelocation is marked on the interpretation map as

Ž .M5 Fig. 6 . These volcanic rocks probablybelong to the Cretaceous volcanic formationthat outcropped in Mount Carmel. The deep-

Ž .seated magmatics M3 are marked in the north-west part of the area using the pseudo-gravitytransformation.

The results obtained from potential field dataare inherently non-unique. The present interpre-tation should also be considered a member ofthe class of possible solutions that could pro-duce magnetic and gravity patterns matching theobservations. The appraisal values of the den-sity, magnetic susceptibility and the calculateddepths should be regarded as rough estimates.

In the first stage of the quantitative interpre-tation scheme, magnetic data were interpreted

Žusing the Werner deconvolution technique theUSGS potential fields software package, Cordell

.et al., 1992 . This 2-D program computes depthsassociated with magnetic basement dikes orfaults using the input magnetic profile for the

Ž .depth solutions dike model and the horizontalderivative of the input profile for depth solu-

Ž .tions fault model . In spite of the sprays ofŽsolutions are widely recognized features Fig.

.7 , these data appeared to be useful to compilean initial iteration for the 3-D magnetic model-ing. The modeling was carried out using the

Ž .MAGPOLY program Godson, 1983b whichcalculates the magnetic effect of polygonal bod-ies bounded by two horizontal planes and a


Žnumber of intersecting vertical planes Talwani,.1965; Plouff, 1976 . The magnetic effect of an

assemblage of polygonal prisms is calculatedfor all grid locations and should be compared

Fig. 6. Gravity and magnetic interpretation map of the Haifa Bay area.


with the interpolated values. The following as-sumptions were made for the quantitative inter-pretation.

1. The total magnetization vector coincides withthe vector of the Earth’s total field.

2. Magnetic susceptibility is the same as thesample measurements.

3. The anomalies examined are caused by themagmatic bodies, which are similar to the

ŽAsher shield volcano Gvirtzman et al.,.1990 .

The MAGPOLY program was combined withinteractive PC programs that permit digitizationof polygon corners as well as imaging of the

observed and calculated fields. The resulting3-D model was obtained after some experimen-tation. The modeling iterative process wasstopped when the main features of the calcu-lated anomaly had been adjusted to the observedone. Based on the 3-D modeling, it appears thatconcealed magnetic bodies located inside aswell as outside the investigated area cause themagnetic anomalies. The plane view of thesebodies is shown on Fig. 6. The best fit for themagnetic anomalies that were observed in thearea was obtained using the parameters shownin Figs. 6 and 7.

It is important to estimate the reliability ofthe model suggested. The deeper parts of the

Ž . Ž .Fig. 7. 3-D modeling results for the Carmel magnetic anomaly. Crosses dike model and triangles fault model show the2-D depth solutions obtained by using the Werner deconvolution technique.


model give rise to the small anomaliesand, hence, limit the resolution. The planeprojections of the magnetic body are morereliably defined than the upper and lowerlimits. The relatively simple geometry obtainedcan be altered using a more complicated

Žassemblage of polygonal prisms as in a pyra-.mid which could lead to bodies with smoother

slopes. The difference between the anomaliesof the simple and complicated models is negli-gible.

The top of the simple model was determined,after some experimentation, with an accuracy ofabout 0.5 km for the Carmel magmatics andabout 1 km for the western bodies. The Ashervolcanics were penetrated by the Atlit-1 and

ŽYagur-1 wells at a depth of 2.9 km estimated. Ž .as 3.5 km and 2.4 km estimated as 2 km ,

respectively. It should be noted that, by usingrealistic limitations for the initial parameters ofthe causative body, we could average the misfitbetween the observed and calculated anomaliesto less than about 25%. The magnetic data forthe deepest body has been interpreted by using

Ž .2-D magnetic modeling Rybakov, 1991 . Themagnetic effects of a large magmatic body withthick roots best fit the observed Carmel mag-netic anomaly. This root is probably located

Ž .close to the Yagur fault Fig. 7 .The difference between magnetic anomalies

caused by basic volcanic layers intercalatedwithin the sedimentary rocks and a solid mag-

Žmatic body gabbro intrusion with a similar.geometry and depth extension is small. The

assumption that a magnetic body is locatedbelow the Phanerozoic strata at a depth of about8-km was also checked. Modeling showed thata magmatic body could produce a magneticanomaly of the same magnitude as the measuredone, if an unrealistic magnetic susceptibility ofabout 0.15–0.2 SI units is used. However, evenin this case, the gradients of the observed andcalculated anomalies cannot be adjusted; there-fore, we suggest that a crystalline basementdoes not cause the magnetic anomalies in thestudied area.

The magnetic effect of the Early CretaceousTayassir volcanics and the Late Cretaceous vol-canics, which outcrop on Mount Carmel, wascalculated for various levels using 2-D model-ing. The magnitude of the calculated anomalieswas less than 8–10 nT for the 1-km elevation;therefore such bodies cannot be effectively ob-served using the available magnetic data.

5. Discussion

Three main directions can be seen on theŽ .interpretation map Fig. 6 . The first, which

strikes NW, generally coincides with the Yagurfault and the Carmel ridge and is probablyenhanced by young tectonic dislocations. Thesecond, oriented northward and parallel to the

Ž .coastline and the continental slope Fig. 2 ,reflects the thickness variations of the youngsediments. The third, oriented E–W and mainlyobserved in the southern part of the study area,probably corresponds to a density heterogeneityin the Mesozoic sequence. An areal distributionof the basic magmatics has a similar direction

Žcorresponding to an ancient probably Meso-. Ž .zoic weakness megazone. Ginzburg 1960

suggested that the density contrast between theCenozoic and the Mesozoic sequences is one ofthe main causes of gravity disturbances. Thecompiled ‘‘stripped’’ gravity map shows, forthe first time, the pattern of gravity anomalies

Ž .after removal of this effect Fig. 4 .The peak-to-peak of the gravity anomalies in

the area decreased from about 90 mGal on theBouguer gravity map to about 60 mGal on the‘‘stripped’’ gravity map. The maximum hori-zontal gradients are also less for ‘‘stripped’’

Žgravity. For example, a strip of the highest up.to 20 mGalrkm gravity gradients oriented

northward and located on the Atlit offshore, isvirtually invisible on the ‘‘stripped’’ map. The‘‘stripped’’ gravity map shows a simple patternand consists of fewer features than the Bouguer

Ž .map Fig. 4 . This reflects the deep position andlarge size of the causative bodies. The promi-


nent, positive Carmel gravity anomaly, clearlyŽ .seen on the Bouguer gravity maps Fig. 4 ,

completely vanishes on the ‘‘stripped’’ gravitymap, suggesting that this anomaly is producedby the great thickness of the low density rocksin the surrounding areas. In contrast, an elon-gated positive gravity anomaly appears north ofthe Yagur fault after replacing the low-densitysediments of the Qishon graben. In our opinion,this positive anomaly most probably expresseshigh-density gabbroic rocks of the magmatic

Ž .root of the Asher volcanics Fig. 7 .A large positive, rounded anomaly, occupy-

Ž .ing the whole NW corner of the area Fig. 4emerged after gravity stripping. Good correla-tion of this anomaly with the magmatic bodyinterpreted from the magnetic data leads us tosuggest a deep-seated gabbroic intrusion here.The top of the intrusion reaches a depth ofabout 8-km, as estimated by the quantitativeinterpretation of the magnetic data. It is impor-tant to emphasize that a number of featuresinterpreted from ‘‘stripped’’ gravity and locatednear previously delineated features, now show adifferent attitude as demonstrated, for example,by the Foxtrot-1 well. Its location coincideswith the maximum of the NW oriented positive

Ž .anomaly on the Bouguer map Figs. 4 and 6 ,while a negative NE oriented anomaly appearsin the same location in the ‘‘stripped’’ gravitymap. We believe that this demonstrates differenttectonic configurations for the Tertiary andMesozoic formations.

A large rounded gravity low, occupying theSW corner of the study area, is the only one thatdid not vanish in the ‘‘stripped’’ gravity map.We assume that this anomaly corresponds to thesubsurface geological structure, the Atlit em-bayment, with a series of thick Mesozoic andTertiary sediments. This structure was probablyinherited from the Mesozoic through the Ter-tiary.

The prominent geophysical features of thearea are the gravity and magnetic highs of theCarmel area. These anomalies were interpreted

Ž .by Gvirtzman et al. 1990 as being caused by

an Early Jurassic shield volcano below MountCarmel. Our interpretation, presented in Figs. 6and 7 is not very different apart from twoexceptions.

1. Analysis of the gravity and magnetic mapsclearly shows an inconsistency between thegravity and magnetic anomalies in the planelocation and strike direction. The correlationbetween the pseudo-gravity and the gravityanomalies was computed and the results ob-tained clearly suggest a lack of correlation be-tween these data and confirm that the causes ofthe gravity and magnetic anomalies are essen-tially different.

2. The possible location of a magmatic rootunder the Carmel ridge: the first hint for theexistence of such a root was obtained fromautomatic inverse programs that showed a fewdeep-seated magnetic heterogeneity locatednorth of the Yagur fault. An elongated positivegravity anomaly, also appearing north of the

ŽYagur fault in the ‘‘stripped’’ gravity map Fig..4 , probably reflects the high-density gabbroic

root of the Asher volcanics. 2-D and 3-D mag-netic modeling confirm the existence of such a

Ž .root at this location Fig. 7 .

6. Conclusions

The main results of the interpretation of theupdated gravity and magnetic data are as fol-lows.

1. This study confirms, as previously sug-Ž .gested by Gvirtzman et al. 1990 , that the Early

Jurassic Asher shield volcano, consisting ofbasaltic lava flows, is the causative body of theCarmel magnetic anomaly onshore and off-shore. This body oriented NNE is about 30=20km in size and its top reaches a depth of 2 kmin the central part and 4 km in the northern part.The thickness of the body is estimated at about2–3 km.

2. The root of the volcano is located immedi-ately north of the Yagur fault and not belowMount Carmel as previously suggested.


3. The rocks of the Asher volcanics have, ingeneral, relatively lower density values in com-parison to the Mesozoic rocks. This implies thatthis sequence cannot be the causative body forthe Carmel gravity high.

4. The second magmatic body, lying about 5km west of the Foxtrot-1 well and orientedN–S, is an elongated shape about 20 km long.These deep-seated magmatics, reaching a depthof about 4-km, are about 3.5 km thick. Thewesternmost magmatic body, expressed on themagnetic data, as well as a ‘‘stripped’’ gravity

Žhigh, probably correspond to a large 15–20 km. Ž .in diameter , deep-seated about 8 km deep

gabbroic intrusion. Another shallow magmaticbody about 20 km long is suggested south of theFoxtrot-1 well. The western and eastern bound-aries of this magmatic event are not defined.

5. Magnetic susceptibility of 0.02–0.035 SIunits and an effective vector of magnetizationparallel to the Earth’s present magnetic fieldwere measured for the above mentioned mag-matics. These bodies should be joined in acommon occurrence of deep-seated magmaticevents, which cover almost all the study area.

6. Most of the Bouguer gravity anomalies inthe area express the gravity influence of the lowdensity Senonian to Tertiary sediments. Thefeatures of the gravity field, such as the gradi-ents and elongated anomalies, can be dividedinto three groups according to direction. Thefirst, NW oriented group, corresponds to thedirection of the Yagur fault and the Carmelridge and is expressed by the young tectonicdislocations. The second group, oriented N–Sparallel to coastal line, corresponds mainly tothe thickness variations of the Tertiary sedi-ments. The third group, oriented E–W, probablycorresponds to the density heterogeneity of theMesozoic sequence.

7. The gravity effects of the young sedimentsand of the Asher volcanics were calculated andremoved from the observed gravity. The‘‘stripped’’ gravity map reflects, in general, theMesozoic subsurface geology. The structural

Žpattern of shallow geological formations Ter-

.tiary does not coincide with the pattern of theŽ .deeper Mesozoic formations. The ‘‘stripped’’

map should be of considerable help in delineat-ing some aspects for hydrocarbon exploration inthe area.

8. The Atlit marine embayment is suggested.A thick series of Mesozoic and Tertiary forma-tions appears to be deposited in this basin.

9. The present Yagur fault appears to be arejuvenation of an ancient weakness zone, basedon similar trend found on the ‘‘stripped’’ grav-ity and magnetic maps. These anomalies areinterpreted to be caused by a deep-seated basicintrusion of Early Jurassic age.

Acknowledgements

The authors are grateful to the Earth SciencesAdministration of the Ministry of National In-frastructures for their kind permission to use thegravity, magnetic and well data from Israel andfor their support of this study. We also thankDrs. G. Tsokas, A. Camacho and a third refereefor their valuable suggestions for improving theoriginal version of this paper. We are grateful tothe authors of the IGRF model and NASA,USA, for their kind cooperation. We would liketo thank Drs. Y. Rotstein, M. Goldman and I.Bruner for useful discussions as well as Mr. I.Goldberg, Ms. R. Gapsou and Mr. O. Siman-Tovfor their assistance.

References

Arad, A., 1965. Geological outline of the Ramot MenasheŽ .region northern Israel . Isr. J. Earth Sci. 14, 18–32.

Ben-Avraham, Z., Hall, J.K., 1977. Geophysical survey ofMount Carmel and its extension into the EasternMediterranean. J. Geophys. Res. 82, 793–802.

Ben-Avraham, Z., Ginzburg, A., 1986. Magnetic anoma-lies over the Central Levant continental margin. Mar.Pet. Geol. 3, 220–223.

Ben-Gai, Y., Ben-Avraham, Z., 1995. Tectonic processesin offshore northern Israel and the evolution of the

Ž .Carmel structure. Mar. Pet. Geol. 12 5 , 533–548.


Cordell, L., Phillips, J.D., Godson, R.H., 1992. US Geo-logical Survey Potential Field Geophysical Software,Version 2.0. Department of the Interior, US GeologicalSurvey, Boston, MA.

De Sitter, L.U., 1962. Structural development of the Ara-bian shield in Palestine. Geol. Mijnbouw 45, 116–124.

Domzalski, W., 1967. Aeromagnetic survey of Israel: in-terpretation. IPRG Report SMAr482r67, 62 pp.

Domzalski, W., 1986. Review and Additional Interpreta-tion of Selected Magnetic Data in Israel and Adjoining

Ž .Areas. Oil Exploration Investments Ltd., 55 pp.Dvorkin, A., Kohn, B.P., 1989. The Asher volcanics,

northern Israel. Isr. J. Earth Sci. 38, 105–123.Folkman, Y., 1976. Magnetic and gravity investigation of

the crustal structure of Israel. PhD Dissertation. Tel-Aviv University.

Freund, R., 1970. The geometry of faulting in the Galilee.Isr. J. Earth Sci. 19, 117–140.

Garfunkel, Z., Derin, B., 1984. Permian–Early Mesozoictectonism and continental margin formation in Israeland its implications for the history of the EasternMediterranean. In: Dixon, J.E., Robertson, A.H.F.Ž .Eds. , The Geological Evolution of the EasternMediterranean. Blackwell, Oxford, pp. 187–201.

Ginzburg, A., 1960. Geophysical studies in the central andnorthern coastal plane and the western Emeq. PhDThesis. Hebrew University, Jerusalem.

Ginzburg, A., Cohen, S., Hay-Roe, H., Rosenzweig, A.,1975. Geology of the Mediterranean shelf of Israel.AAPG Bull. 59, 2142–2160.

Godson, R.H., 1983. GRAVPOLY: a modification of athree-dimensional gravity modeling programs. Open-File Report 83-346, US Geological Survey, 53 pp.

Godson, R.H., 1983. MAGPOLY: a modification of athree-dimensional magnetic modeling programs. Open-File Report 83-345. US Geological Survey, 62 pp.

Ž .Hall, J.K., 1993. The GSI Digital Terrain Model DTMŽ .completed. In: Bogoch, R., Eshet, Y. Eds. , GSI Curr.

Res.. pp. 47–50, Jerusalem.Gvirtzman, G., Steinitz, G., 1983. The Asher volcanics —

an early Jurassic event in northern Israel. GSI Curr.Res., 28–33.

Gvirtzman, G., Klang, A., Rotstein, Y., 1990. Early Juras-sic shield volcano below Mount Carmel: a new inter-pretation of the magnetic and gravity anomalies andimplications for early Jurassic rifting. Isr. J. Earth Sci.39, 149–159.

Makris, J., Wang, J., 1994. Bouguer gravity anomalies ofthe Eastern Mediterranean Sea. In: Krasheninnikov,

Ž .V.A., Hall, J.K. Eds. , Geological Structures of theNortheastern Mediterranean. pp. 87–99, Jerusalem.

Mimran, Y., 1972. The Tayassir volcanics and LowerCretaceous formation in the Shomron, central Israel.GSI Bull. 58, 1–52.

Neev, D., Almagor, G., Arad, A., Ginzburg, A., Hall, J.,1976. The geology of the southern Mediterranean Sea.GSI, 68.

Nettleton, L.L., 1971. Elementary gravity and magnetic forgeologists and seismologists. SEG Monogr. Ser. 1, 121.

Picard, L., Kashai, E., 1958. On the lithostratigraphy andtectonics of the Carmel. Bull. Res. Counc. Isr. 7G,1–18.

Plouff, D., 1976. Gravity and magnetic fields of polygonalprisms and application to magnetic terrain corrections.

Ž .Geophysics 41 4 , 727–741.Ron, H., Freund, R., Garfunkel, Z., Nur, A., 1984. Block

rotation by strike slip faulting: structural and paleomag-netic evidence. J. Geophys. Res. 89, 6256–6277.

Rotstein, Y., Bruner, I., Kafri, U., 1993. High resolutionseismic imaging of the Carmel fault and its implica-tions to the structure of Mt. Carmel. Isr. J. Earth Sci.42, 55–69.

Rybakov, M., 1991. Interactiverautomated crustal model-ing using multiple data sets and multiple optimizedintuitiverautomated inversion techniques. In: Israel Ge-ological Survey, Annual Meeting, Abstracts. p. 90.

Rybakov, M., Goldshmidt, V., Folkman, Y., Rotstein, Y.,Ben-Avraham, Z., Hall, J., 1994. Magnetic anomalymap of Israel, scale 1:500,000. IPRG and Survey ofIsrael.

Rybakov, M., Goldshmidt, V., Rotstein, Y., 1997. A newcompilation of gravity and magnetic data from theLevant and their preliminary interpretation. Geophys.

Ž .Res. Lett. 24 1 , 33–36.Rybakov, M., Goldshmidt, V., Rotstein, Y., Fleischer, L.,

Goldberg, I., 1999. Petrophysical constraints on grav-ityrmagnetic interpretation in Israel. Leading Edge 18Ž .2 , 269–272.

Sass, E., 1980. Late Cretaceous volcanism in Mt. Carmel,Israel. Isr. J. Earth Sci. 29, 8–24.

Talwani, M., 1965. Computation with help of a digitalcomputer of magnetic anomalies caused by bodies of

Ž .arbitrary shape. Geophysics 30 5 , 797–817.

3-D gravity and magnetic interpretation for the Haifa Bay ... · of 2500 m in the Atlit-1 well...

Documents

Transcript of 3-D gravity and magnetic interpretation for the Haifa Bay ... · of 2500 m in the Atlit-1 well...