Analysis and interpretation of the gravity and magnetic data

3 . 2100 kgrm Senonian–Tertiary rocks and the predominantly carbonate, older Mesozoic rocks. Ž 3 . The densest rocks up to 2850 kgrm are scattered 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 kgrm 3 , corresponding to the Bouguer density. The Early Jurassic Asher vol- canics have an average density of about 2550– 3 Ž . 2600 kgrm Fig. 3 . This means that volcanic rocks have a negative density contrast relative to the Mesozoic carbonate sequence, which have an average density of about 2750 kgrm 3 .

4. Analysis and interpretation of the gravity and magnetic data

A generation of initial subsurface geological model corresponding to the geophysical obser- vations is an important interpretation stage. All additional investigations mainly check and re- fine the results revealed during this stage which includes an analysis of both the potential fields and their transformations in an attempt to em- phasize the various frequency components of the gravity and magnetic fields. A variety of filtering techniques was employed in order to enhance the data sets prior to interpretation Ž . Cordell et al., 1992 : regional–residual gravity separation; horizontal and vertical gravity and magnetic derivatives; reduction to the pole and upward magnetic continuation; pseudo-gravity and gravity–magnetic correlation. The Bouguer graÕity values in the area vary from 40 to 110 mGal and they generally de- crease to the southeast. The cause of the re- gional gravity trend could be the transition from oceanic crust of the Eastern Mediterranean to the continental crust of the Arabian plate, which Ž may occur under the Levant margin Makris and . Wang, 1994 . For interpretation purposes, the regional–residual separation of the gravity anomalies was conducted using the regional trend that was calculated as a third order poly- nomial surface. Removing this trend, the resid- ual Bouguer gravity map has been compiled Ž . Fig. 4 . The main features of this map are a NW trending high with a magnitude of about 50 mGal 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 along its NNW oriented axis; its southern boundary coincides with the northern edge of the Tertiary sedimentary embayment. b. The central and most intense part of the positive gravity ridge is steeply bounded on all sides. c. The northern part, oriented NNE and per- pendicular to the central part, consists of a few local highs. A small, but clearly observed, local gravity low coincides with the Carmel high elevation near Haifa. The wide gravity low in the southwest has a magnitude of about y10 mGal. No significant gravity anomalies have been observed in the northwest side of the study area. As mentioned above, the lateral changes in the thickness of low-density young sediments are the main reason for the residual anomalies in the area. The thickness of the young sedi- ments was deduced from seismic reflection data as the difference between the sea floor and the Ž . structural depth to the top Turonian Fig. 4 . These values were used to calculate and subtract the 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 . This program calculates the gravity effect using three rectangular grids that define the source: the top surface, the bottom surface and the density con- trast. In this case the first surface corresponds to the sea floor and the second to the top Turonian Ž . carbonates Fig. 5 . The density contrast was Ž . derived from density well logs Fig. 3 . The negative gravity effect of the low-density sedi- ments ranging from y60 to y10 mGal was removed from the observed gravity values. The gravity 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 of Ž . Ž the 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. Ž lated using the GRAVPOLY program Godson, . 1983a . The geometry of the volcanic bodies was taken 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 gravity effect of the volcanic rocks, ranging from y0 to y15 mGal, was also removed from the Bouguer gravity values. Replacing the actual densities of the young sediments and volcanics Ž 3 . to an average Bouguer density 2670 kgrm , we cleaned the Bouguer gravity anomalies from the influence of the above mentioned geological bodies. The gravity anomalies obtained should be 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 using realistic limitations of the top Turonian and density data as well as the accuracy of the 3-D gravity calculation. The total error does not exceeding 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. The studied area consists of a number of local grav- ity lows and highs that are not apparent on the Bouguer data. The elongated positive anomaly, with a magnitude of about of 15 mGal, is extended NW to about 20 km from the Qishon graben to the Haifa Bay area. The SW boundary of this anomaly coincides with the present Yagur fault, while its NE boundary appears to be a newly discovered lineament. The high gravity gradients are related to these boundaries. An elongated negative gravity anomaly with magni- tude of about y5 mGal, located SW of the above-mentioned positive anomaly, is about 15 km long. Only the SE part of this anomaly is seen inland as a small gravity low on the non- Ž . stripped data Fig. 4 . A rounded gravity high with a magnitude of about 5 mGal is located between the Atlit-1 and Foxtrot-1 wells. The Foxtrot-1 well is located in a complex gravity saddle. A high gravity anomaly occupies the NW corner of the study area. The magnitude of this wide anomaly reaches about 25 mGal. A wide gravity low is located in the SW corner of the area with a magnitude of about y25 mGal. Ž . A relatively small, elongated N–S positive gravity anomaly with a magnitude of about 10 mGal, oriented southward, is delineated in the near shore close to the Atlit-1 well. Gravity features, possibly fault or structure related, were drawn in the interpretation map Ž . Ž Fig. 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 changes in the rock magnetization. In contrast to the gravity, the sedimentary strata are ‘‘transparent’’ and the magnetic anomalies are caused only by basic magmatism. The parameters of the Earth’s Ž total field vector for the area of study inclina- tion s 45 8, declinations38 and total field mag- . nitude about 43,500 nT were calculated using a program developed by the US National Space Science Data Center. The theoretical magnetic anomaly, caused by a body magnetized by this normal geomagnetic field, contains the conju- gated maximum and minimum, the latter lying to the north. This signature is important for geological understanding of a pattern of the magnetic anomalies. The central part of the studied area is occupied by a few magnetic anomalies as shown in Fig. 2. The easternmost Ž intensive Carmel magnetic anomaly Ml peak to . peak is about 240 nT , oriented WNW, is about 40 km long. The northern edge of this anomaly is marked by a sharp magnetic gradient. Its southern edge shows moderate gradients. Ž . The next magnetic anomaly M2 is located offshore west of the Carmel anomaly. It is elongated in shape and trends NNE, perpendicu- lar to the strike of the main direction of the Carmel anomaly. Its peak to peak reaches 210 nT. This anomaly, extending over 20 km, is bounded to the south and north by steep hori- zontal gradients. The peak to peak of the M3 anomaly reaches 120 nT. Its marginal gradients are more moderate than those of the other anomalies mentioned above. The magnetic anomaly M4, shown in the northeastern corner of the studied area, is only a small part of a high magnetic anomaly located in the southern off- Ž . shore of Lebanon Fig. 1 . No magnetic anoma- lies are present in the southwestern part of the studied area. High frequency magnetic anomalies, located offshore, have been mapped by the marine mag- netic survey. We speculate that these anomalies were caused by shallow basic volcanics. The location is marked on the interpretation map as Ž . M5 Fig. 6 . These volcanic rocks probably belong to the Cretaceous volcanic formation that outcropped in Mount Carmel. The deep- Ž . seated magmatics M3 are marked in the north- west part of the area using the pseudo-gravity transformation. The results obtained from potential field data are inherently non-unique. The present interpre- tation should also be considered a member of the class of possible solutions that could pro- duce magnetic and gravity patterns matching the observations. The appraisal values of the den- sity, magnetic susceptibility and the calculated depths should be regarded as rough estimates. In the first stage of the quantitative interpre- tation scheme, magnetic data were interpreted Ž using the Werner deconvolution technique the USGS potential fields software package, Cordell . et al., 1992 . This 2-D program computes depths associated with magnetic basement dikes or faults using the input magnetic profile for the Ž . depth solutions dike model and the horizontal derivative of the input profile for depth solu- Ž . tions fault model . In spite of the sprays of Ž solutions are widely recognized features Fig. . 7 , these data appeared to be useful to compile an initial iteration for the 3-D magnetic model- ing. The modeling was carried out using the Ž . MAGPOLY program Godson, 1983b which calculates the magnetic effect of polygonal bod- ies bounded by two horizontal planes and a Ž number of intersecting vertical planes Talwani, . 1965; Plouff, 1976 . The magnetic effect of an assemblage of polygonal prisms is calculated for 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 with the vector of the Earth’s total field. 2. Magnetic susceptibility is the same as the sample measurements. 3. The anomalies examined are caused by the magmatic bodies, which are similar to the Ž Asher shield volcano Gvirtzman et al., . 1990 . The MAGPOLY program was combined with interactive PC programs that permit digitization of polygon corners as well as imaging of the observed and calculated fields. The resulting 3-D model was obtained after some experimen- tation. The modeling iterative process was stopped when the main features of the calcu- lated anomaly had been adjusted to the observed one. Based on the 3-D modeling, it appears that concealed magnetic bodies located inside as well as outside the investigated area cause the magnetic anomalies. The plane view of these bodies is shown on Fig. 6. The best fit for the magnetic anomalies that were observed in the area was obtained using the parameters shown in Figs. 6 and 7. It is important to estimate the reliability of the 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 the 2-D depth solutions obtained by using the Werner deconvolution technique. model give rise to the small anomalies and, hence, limit the resolution. The plane projections of the magnetic body are more reliably defined than the upper and lower limits. The relatively simple geometry obtained can be altered using a more complicated Ž assemblage of polygonal prisms as in a pyra- . mid which could lead to bodies with smoother slopes. The difference between the anomalies of the simple and complicated models is negli- gible. The top of the simple model was determined, after some experimentation, with an accuracy of about 0.5 km for the Carmel magmatics and about 1 km for the western bodies. The Asher volcanics were penetrated by the Atlit-1 and Ž Yagur-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 using realistic limitations for the initial parameters of the causative body, we could average the misfit between the observed and calculated anomalies to less than about 25. The magnetic data for the deepest body has been interpreted by using Ž . 2-D magnetic modeling Rybakov, 1991 . The magnetic effects of a large magmatic body with thick 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 intercalated within the sedimentary rocks and a solid mag- Ž matic body gabbro intrusion with a similar . geometry and depth extension is small. The assumption that a magnetic body is located below the Phanerozoic strata at a depth of about 8-km was also checked. Modeling showed that a magmatic body could produce a magnetic anomaly of the same magnitude as the measured one, if an unrealistic magnetic susceptibility of about 0.15–0.2 SI units is used. However, even in this case, the gradients of the observed and calculated anomalies cannot be adjusted; there- fore, we suggest that a crystalline basement does not cause the magnetic anomalies in the studied area. The magnetic effect of the Early Cretaceous Tayassir volcanics and the Late Cretaceous vol- canics, which outcrop on Mount Carmel, was calculated for various levels using 2-D model- ing. The magnitude of the calculated anomalies was 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