Development of 3-D Gravity-Magnetic Models of the Earth ’ s Crust of Azerbaijan and Adjacent Areas : an Overview

Azerbaijan is located in the Alpine Himalayan collisional zone and is characterized by its complex and variable geological structure. To study Azerbaijan’s deep structure, twelve main regional interpreting profiles were selected for comprehensive 3D combined gravity-magnetic modeling. The development of the initial physical-geological models (PGMs) was based on known surface geology, drilling data, previous seismic, magnetotelluric and thermal data analysis, examination of the richest petrophysical data, as well as quantitative and qualitative gravity/magnetic data examination. The PGMs thus reflect the key structural-formational specifics of Azerbaijan’s geological structure, beginning from the subsurface (hundreds of meters) up to the Moho discontinuity (40 60 km). The PGMs revealed common factors controlling oreand hydrocarbon bearing formations, primarily the boundaries of tectonic blocks, masked faults, and buried uplifts of magmatic rocks. Many of these factors can be used to investigate long-term geodynamic activity at a depth. The article summarizes many years of investigation by exemplifying the most typical PGMs for the Greater and Lesser Caucasus, the Kura depression, and central and northern Azerbaijan.


Introduction
Investigating the South Caspian basin to localize commercial oil-and-gas deposits is of great interest to geoscientists [e.g., [1][2][3][4][5][6].At the same time, Azerbaijan is known to be rich in economic minerals.Besides oil and gas there are iron, copper, gold, mercury, zinc, molybdenum, cobalt, aluminum and other deposits [7].Rich geological-geophysical data (including physical properties) are available and have been analyzed in numerous gravity-magnetic surveys at different scales with different structural-geological implications for Azerbaijan.The first models of the Earth's crust of Azerbaijan were put forward in the mid-1960s [8][9][10][11].These models were subsequently evaluated in the works of Tzimelzon [12], Azizbekov et al. [13], Shikhalibeyli [14], Khesin [15], Gugunava [16], Alexeyev et al. [17], and later by Khesin et al. [18,19].Some recent works have studied the deep structure of the Caucasus and Azerbaijan and have revealed the important generalized regional peculiarities of this region [e.g., [20][21][22][23][24][25][26][27].This overview presents a detailed evaluation of the variable deep structure of Azerbaijan primarily by using 3D combined gravity-magnetic data analysis.This study was preceded by a complex qualitative and advanced quantitative gravity/magnetic data analysis supported by detailed examination of available geological, seismic, magnetotelluric and thermal data, and utilization of numerous magnetic, paleomagnetic and density properties of geological samples from Azerbaijan.This series of physical-geological models (PGMs) can be used not only for substantiation of various types of prospective economic deposits, but also to delineate the tectonic-structural factors affecting long-term seismological prognosis.

Azerbaijan: a Brief Geological Outline
The complexity of Azerbaijan's geological structure stems from its location in the Alpine Himalayan collisional zone.The NE part of Azerbaijan is part of the Pre-Caucasian foreland filled by Cenozoic terrigenous sediments.A heterogenic Nakhichevan folding system is located in the SW part, where carbonate Paleozoic strata and Cenozoic magmatic formations are mixed (Figure 1).At the mega-anticlinorium of the Greater Caucasus, stratified Cenozoic and Mesozoic thick (predominantly, sedimentary) strata are present.The prevalence of Mesozoic magmatic formations is typical of the megaanticlinorium of the Lesser Caucasus.The Kura megasynclinorium, dividing the Greater and Lesser Caucasus, is characterized by an accumulation of thick (up to several kilometers) Cenozoic terrigenous sediments.The Talysh anticlinorium is located on the SE flank of the Kura depression, where Paleogene magmatic associations are widely distributed [28][29].
According to Khain [29], the most ancient Pre-Baikalian 1 structural complex is characterized by a sub-meridian strike.A less metamorphosed Baikalian complex is rumpled to latitudinal folds in separate areas.The Caledonian complex is practically unknown.The Hercynian complex is characterized by a Caucasian strike identical to the overlying Mesozoic rocks.
The Alpine tectono-magmatic cycle is characterized by more complete geological data.As a whole, the Azerbaijan territory is typical of frequently changing geological associations on the vertical and lateral axes, the presence of multifarious fold and fault structures of different orders, and regional and local metamorphism.All these factors make the development of reliable models of these media highly complex.

Creation of Space Indicators and Petrophysical Basis for Modeling
Gravity-magnetic data processing is generally intended to reduce and eliminate noise factors of different origins and intensities.The main problem faced by qualitative interpretation is to single out a desired target, whereas quantitative interpretation needs to determine and refine the target parameters.Thus, geological problems need to be resolved in terms of: (1) the capabilities of the geophysical method selected for measurements of the field containing the information required, (2) the properties of the medium under study, its capability to generate detectable signals (anomalies), (3) the methods for data processing and interpretation; namely, their ability to extract information from the field revealing the effects of the geological objects.Figure 2 presents a general flow chart for analysis and synthesis of geophysical data for complex regions.Each step in this flow chart is divided into sub-steps [19].
The complex geological structure of Azerbaijan determines the highly intricate nature of the gravity and magnetic fields, which reflect the superimposed effects of outcropped bodies and structures and those buried at various depths.Therefore to identify anomalies in the different classes of geological sources, not only observed geophysical fields need to be used but also their transformations.Such a classification can be carried out by the use of spatial features (regional and local anomalies), morphological features (isometric anomalies, elongated anomalies or ledges), or the sign and intensity of geophysical fields.
To study the gravity anomalies in Azerbaijan, the results of Bouguer gravity analytical continuation to levels of 4, 8 and 20 km were examined.on the basis of these transformations, difference g B(0-4) , g B(4-10) and g B(8-20) maps were computed.It was found that the residual field g B(0-4) in orogens reflects the influence of the near-surface Alpine structures composed of volcanogenic and volcanogenic-sedimentary associations and intrusions; in the Kura depression this field reflects the distribution of dense heterogeneities in the upper part of the To examine the regional magnetic anomalies the following magnetic data were used: field T (in the Middle Kura Depression -field Z a ) at the levels of 6 and 10 km, and a field of T (Z a ) horizontal gradient at the level of 2 km.Attempts to reveal local magnetic anomalies as a residual field of T (0-6) or T (0-10) were unsuccessful because of sharp differentiation of the observed magnetic field in the Lesser Caucasus and in some other areas.
The potential field characterization procedure was based on the richest data from petrophysical studies (more than 80 000 d lity and more than 25 000 determinations of density) conducted in different areas of Azerbaijan [17].Some average density values were obtained from seismic velocities using the known relationship [e.g., 30].A study of magnetic and palaeomagnetic characteristics, which are vectors and substantially more variable parameters, was mainly based on direct measurements of rock samples.

Common Characteristics of The Petrophysical Boundaries and Geological Associations
T e specificity sh h ared by all these sequences is the s petrodensity boundaries etween the Quaternary (s s and (II) a boundary between the Cenozoic and Mesozoic availability of two geostructure typical of all (Figure 5): (I) b ometimes Neogene-Quaternary) deposits and underlying more dense as ociations ( aver = 0.2 -0.3 g/cm 3 ), associations (the contrast  aver consists of 0.14 -0.44 g/cm 3 ).However, in the latter, the boundary is no fferentiated cle t di arly in the sequence of the Lesser Caucasu ge The density of the Late Alpine intrusives 0 to 2.85 g/cm 3 ; the density of hibits a clear tendency to deth 5 The rocks composing effusive associations mainly belong to the class of increased ( aver = 2.70 -2.85 g/cm 3 ) and intermediate ( aver = 2.50 -2.70 g/cm 3 ) density.This increased density is typical basically of effusive rocks of the Early Alpine sub-stage.The density of the later sub-stages decreases gradually, and rocks of the orogenic sub-stage almost completely belong to the class of low density ( aver = 2.45 -2.60 g/cm 3 ).
Among the sedimentary deposits, the sandy-shales of the Greater Caucasus, as well as carbonates of the Late Jurassic-Early Cretaceous are the densest ( aver = 2.60 -2.75 g/cm 3 ).The tendency for density to decrease from the early to late stages is also found here.The main petrodensity discontinuities show similar peculiarities in the central and western regions of the Caucasus, namely in Armenia [31] and Georgia [32].
As a first approximation, the geomagnetic sequence can be divided into three floors corresponding to the main tectonic stages.The lower floor (Pre-Jurassic), represented mainly by terrigeneous-carbonate formations penetrated by acid intrusions, is practically non-magnetic.Some exclusions can be found in the volcanogenic associations revealed in the Shamkhor anticlinorium of the Lesser Caucasus.The middle floor (Mesozoic-Eocene), within which thick volcanites of basic and intermediate composition have developed, is differentiated by its increased magnetization.The upper floor (Post-Eocene) represented mainly by the terrigeneous deposits of the sedimentary cover is characterized by a lower magnetization (although in some areas Neogene-Quaternary lavas with high magnetization are known).e usu-Cenozoic volcanogenic associations of the Talysh as au la at ued magnetic anomalies (associated mag tion composition.The magnetization of ulconcluded that these floors are correlated in terms of age, but the average values of their physical characteristics and relations differ.Obviously, this is caused by the different history of geological evolution of these geostructures.Nevertheless, these physical parameters served as the basis for creating 3D physical-geological models (PGMs) of the Earth's crust for the 3D combined gravity-magnetic modeling presented below.

Advanced Inverse Problem Solution for Magnetic and Gravity Anomalies
To conduct a quantitative interpretation of magnetic and gravity anomalies, advanced methods have specially been developed for complex environments [19] including oblique magnetization (inclined magnetization is typical of the regions located in tropical and temperate latitudes; in Azerbaijan the normal value of magnetic inclination is about 58˚), rugged topography (or uneven surface of measurements) and an unknown level of the normal field.These methods (modified versions of the characteristic point method, tangent method and method of characteristic areas) have been tested in numerous models (with an accuracy of 1 -3%) and in real situations (with an accuracy of 3 -20% for magnetic anomalies and 5 -25%for gravity anomalies).
One example of such an interpretation is the Guton magnetic anomaly situated in NW Azerbaijan, near the border with Russia (Figure 6).Examination of this anomaly was conducted along fifteen profiles crossing this anomaly.The results along one of these profiles (an anomalous body was approximated y a model of a thick interpretation can be found in Khesin et al. [19]).The s:  aver reaches only 0.05 g/cm 3 .Here the reference horizon of lower density and magnetization are liparites and plagio-liparites of the Upper Bajocian.
As a whole, the densest rocks are typical of intrusive bodies.The density of intrusive rocks of the Early Alpine sub-sta ranges approximately from 2.80 to 2.95 g/cm 3- (excluding Bajocian plagiogranites with a density of 2.50 -2.60 g/cm 3 ).ranges on average from 2.7 pre-orogenic intrusives ex crease to the class of intermediate density ( aver = 2.65 -2.75 g/cm 3 ).The intrusives of e orogenic sub-stage have the lowest density ( aver = 2.5 -2.60 g/cm 3 ).ally typical of the zones of inverse magnetization.The In the intermediate floor volcanogenic associations of the Middle Jurassic have the highest average magnetic susceptibility -in the Lesser Caucasus they are approximately 4 km thick.The remanent magnetization of these rocks may be parallel or antiparallel to the induced magetization; relatively low-magnetic associations ar inclined bed) are presented in  abovementioned characteristics point to the basic-acid composition of the anomalous body (intrusion).The conclusion as to the significant vertical thickness of this body concurs with the geothermic data for the depth of the Curie discontinuity in this area (about 30 km) [34].The upper edge of this intrusion is at a depth of 2.6 km (from the Earth's surface) in its middle part and at depths spectively.This magmatic intrusion is associated with the rich pyrite-polymetallic deposits of the Belokan-Zakataly ore area [35] and possibly other areas in the Greater Caucasus [17].The first s (A) A geological Section is trusive, effusive and other associations, as well as faults and the surface of folded foundation are compiled on the basis of geological data within a strip 15 -20 km wide.The interpreting Section is located in the middle of this strip.

Greate
asus mega-anticlinorium.Profile 6, pre-is developed.Here all t ty and magnetization values according to the preceding petrophysical data and results of geophysical field interpretations.When no data are available on the magnetization direction, it is assumed to be parallel to the normal geomagnetic field.Further, the magnetization direction is refined in the course of physical-geological modeling.The petrophysical model includes deep-seated layers of the Earth's crust: (1) the "basaltic", (2) the intermediate between the crust and the upper mantle, and (3) the upper mantle.Their surfaces are constructed and physical properties are associated with them according to the data from previous seismic, magnetotelluric and other deep geophysical studies.
(C) The initial (preliminary) petrophysical model includes hidden bodies pth, density and magnetization are obtained from a quantitative analysis of magnetic and gravity anomalies as well as seismic data.
The second step selects the gravity and magnetic fields along the interpret ace Field Computing (GSFC) program [19,35].Each time the fields are from different bodies, groups of bodies and the total computed model are displayed and compared to the observed gravity and magnetic fields.Using the results of this comparison, the changes that match the gravity and magnetic effects are introduced into the model of the medium.Computations, comparisons of fields and model modifications are repeated until the desired fit between the computed and observed fields is obtained.
Then, a regional gravity (and sometimes magnetic) field is roug ep-seated complexes are not changed; the modi-fications only affect the shape of their roof.Next, fields of local bodies are selected.If necessary, this is followed by a verification of the regional field and the field of the local bodies.
At each computational step, a separate analysis of gravity and m riations are verified in the subsequent steps, and then introduced into the model.This procedure leads to an integrated quantitative interpretation for anomalous gravity and magnetic fields.The selection ends when the computed gravity and magnetic fields coincide accurately with the observed fields.
The third step involves a detailed geological interpretation of these models.A 3-D vestigation is developed based on the qualitative and quantitative data.This yields the final geological maps, and the models are characterized by a more complete rendering of the geological targets, mineral controls and mineral deposits, including deep-seated ones.
The geological interpretation of the complexes and local bodies of the selected (final) petrophys es not usually present any difficulties, since in the implementation of the interactive selection system almost all the bodies in the model acquire some specific geological content.The geological nature of new sources introduced into the model during the selection and reflected either in the initial geological section, or in the initial PGM, is determined according to the similarity of their physical properties, dimensions, and depth of occurrence with respect to the known targets.The age of the bodies is determined according to their interrelations with the host rocks.

3-D Integrated Gravity-Magnet
Along Profiles Crossing the Less sus s 1 and 2 crossing the Lesser Caucasus (Figures 8 illust this region.The Late-Alpine effusives in these PGMs compose an ophiolite zone (which is a relic of the ocean crust).It is thought that the same rocks occur in the NE immersion of the Lesser Caucasus.Pre-orogenic and orogenic intrusive and effusive rocks are fixed in the southern parts of these PGMs.Thick sedimentary deposits are found in northern parts of these profiles.A smooth high of the Moho discontinuity is observed from south to north from a depth of 52 -54 km up to 42 km.Classes of disturbing objects were revealed here such as acid intrusions of lower density and magnetization, basic magmatic rocks of increased density and magnetization and fault zones.It was determined that the clearest density boundaries were associated with the base of the Cenozoic sedimentary strata and to a lesser degree with the base of the Alpine complexes.According to the modeling, geomagnetic boundaries were associated mainly with the roof and bottom of the Mesozoic floor of heightened magnetization.
In the southern part of profile 2, a zone of strong decompaction is obs trusive at depths from 4 to 17 km, and by a horizontal thickness of 30 km.This rock decompaction zone produces the greatest Kelbadzhar-Dalidag gravity minimum.This large "granite room" apparently was a source of pre-orogenic and orogenic granitoid magmatism.

3-D Integrated Gravity-Magnetic Model
Along the Profile Across the Greater C casus profiles (No. 4, 5 and 6) were studied across the r Cauc sented in Figure 10, points to the sharp increase in the depth of the Moho discontinuity from the south to the middle part of this profile (from 47 to 60 km).The behavior of the gravity field depends primarily on the Moho depth.
In this PGM the Early-Alpine cores of the Greater Caucasus are clearly linked to the development of Jurassic associations present in the Lower-Middle-Jurassic sandy-shale deposits.A surface of the Pre-Alpine foundation occupies the highest position in this core.In the PGM (the Greater Caucasus) their wide distribution was predicted given the results of 3D combined gravitymagnetic modeling of the Early-Alpine Lower-Middle-Jurassic sandy-shale deposits of the Greater Caucasus (occupying this position as a result of overthrust), as well as the sedimentary cover of the Kura Depression.In the northern parts of this profile, rocks of significant magnetization were identified at a depth of 9 -10 km.These rocks are presumably related to tuff-terrigeneous deposits of the Middle-Late-Paleozoic age.

3-D Integrated Gravity-Magnetic Model
Along the Lesser Caucasus -Kura Depression -Foothills of the Greater Caucasus Profile Figure 11 presents results of 3D combined interactive modeling of gravity and magnetic fields along stakes 700 -1350 of profiles 3-4 (see Figure 1).A distinct peculi-arity of this profile is that it crosses three significant regions of Azerbaijan: the Lesser Caucasus, the Middle Kura Depression and abuts on a sub-mountain zone of the Greater Caucasus.As can be seen in Figure 11, the magnetic field pattern is more complex in the southern (Lesser Caucasian) part of this profile.The gravity field behavior has a generally negative correlation with the depth of the Moho discontinuity; this can be accounted for by the predominant gravity effect of eclogites at depths of 9 -20 km.

Combined PGM of the Saatly Super-Deep Borehole Area (Central Azerbaijan)
For many years the prevailing view was that in the Kura depression separating mega-anticlinoria of the Greater and the Lesser Caucasus, thick sedimentary deposits occur in the crystalline Pre-Alpine basement, and subvertical deep faults divide these structures.On a projection of the buried uplift of the basement to the Earth's surface, which is assumed to exist on the basis of high densities and velocities of elastic waves, the Saatly Super-Deep borehole (SD-1) was drilled in 1965.However, analysis of the magnetic properties of rocks and magnetic survey results showed that the basement was not magnetized, and the Mesozoic magmatic associations of basic and intermediate composition (of high magnetization) occuied most of the geological section in the Middle Kura p depression [15].The abovementioned (mainly Jurassic) associations are widely distributed in the NE part of the Lesser Caucasus.These associations have a deep-seated gently sloping underthrust under a thick sand-shale series of the Greater Caucasus Jurassic.The validity of this interpretation was confirmed by the results of SD-1 drilling: the borehole exposed Mesozoic volcanogenic rocks at a depth of 3.6 km and did not come from the rocks at its bottom at 8.2 km [17].Advanced interpretation methods (improved modifications of tangents, characteristic point methods and areal method) were applied to study gravity and magnetic anomalies along all profiles surrounding SD-1.A fragment of this interpretation along profile 18 is shown in Figure 12.First of all, note that the behavior of the magnetic Z curve and graph B g x   which testifies to the fact that these anomalies are due to the same geological objects.A quantitative analysis of the magnetic curve showed two magnetic targets.The main target apparently is a source of the Talysh-Vandam gravity anomaly (its upper edge coincides with the data obtained by SD-1 drilling).are very simular, An integrated PGM (gravity-magnetic-seismic with elements of thermal field analysis) of the Earth's crust along profile No. 9 is shown in Figure 13.This PGM clearly accounts for the main sources of gravity and magnetic anomalies in this area.
The analysis indicates that the area of the Talysh-Vandam gravity maximum (TVGM) has a highly inhomogeneous geological structure.Separate elements of the TVGM -anomalies of the second order -reflect differently, showing that they originated independently in different areas of the Earth's crust.Taking into account the location of the lower edge of the magnetized masses and similarities between this geological section and the Lesser Caucasian, it was presumed that magmatic rocks should be found down to 10 km in depth (comparable to the Lower Bajocian rocks of the Lesser Caucasus).Obviously, the main source of the TVGM is associated with the underlying highly dense, strongly metamorphosized (initially chiefly sedimentary associations) nonmagnetic or low-magnetic Pre-Baikalian floor (this type of floor has a submeridional strike in Russian and African platforms [29,36]).The depth of the upper edge of these highly dense rocks was estimated at 9.5 km.
Thus, SD-1 has yet to discover (it is unlikely that these drilling operations will be pursued in the coming years) the source of the TVGM, but clarified the origin of the Gandja magnetic maximum.This is crucial not only for analysis of the tectonic-magmatic evolution of the Caucasus region, but also for prospecting of its ore-and oil and gas-bearing deposits.For example, the geomag- netic models makes it possible to extend prospecting of oil deposits in the Middle Kura depression, which are associated with the zones of protrusions of the Mesozoic magnetoiactive associations.Many geophysical methods have mapped the dense Mesozoic associations, but only magnetic prospecting has been able to reveal the geological associations of basic and middle consistency [18].

Conclusion
The complex geological structure of Azerbaijan determines the highly intricate nature of its gravity and magnetic fields, which reflect the superimposed effects of outcropped bodies and structures and those buried at various depths.It was shown that a combined gravity-magnetic investigation (including the field behaviors at different levels, qualitative and modern quantitative analyses, and 3D combined modeling of potential fields) is powerful tool for studying such complex geological regions as the land of Azerbaijan.A significant role is played by the analysis of petrophysical characteristics, as well as an integrated examination of seismic, magnetotelluric and thermal data.On the basis of a comprehend-sive combined analysis of gravity and magnetic fields, a series of 3-D regional PGMs (including the area of the Saatly Super-Deep borehole), covering Azerbaijan and some adjacent land areas, were developed.The key examples of PGMs presented here highlight the significant formational and tectonic peculiarities of Azerbaijan's deep structure and could be used for various types of geological-geophysical zonation.The next steps should include a more detailed physical-geological examination of these PGMs.

Figure 2 .B
Figure 2. Interpretation of geophysical fields under complex environments: A general scheme.

Figure 7 [
33].The data indicate that the anomalous body is characterized by comparatively low magnetization (250 mA/m), large vertical thickness (about 30 km) and a steep dip in the lateral contacts (detailed methodology supporting this n well as lava covering the central part of the Lesser C casus have other characteristics.Here paleomagnetic zones of different po rities alternate.Analyses indicate th upward contin with the gently sloping Cenozoic associations) are rapidly attenuated.The lavas are characterized by significant netization values compared to pyroclastic associas of a similar trabasites is approximately proportional to their degree of serpentinization (at the same time the density of the ultrabasites decreases as a function of their serpentinization).By analyzing the sequence of the main Caucasian petrodensity and petromagnetic floors (Figure 5), it can be b Copyright © 2011 SciRes.

Figure 6 .
Figure 6.Guton anomaly: map of the total magnetic field T (isolines are given in mA/m) and location of profile A -B. location of this area is shown in Figure 1.

of 1 .Figure 7 .
Figure 7. Example of tangent plotting and results of interpretation of T graphs on two levels along the profile A -B through b Earth's crust, e Earth's surface to the Baikalian basement.Deeper parts of the intrusive bodies and certain faults are formed by extrapolation of the available constructions, general geological considerations and the results of previous geophysical analyses.(B)A preliminary petrophysical model of the Section he geological bodies acquire densi as well.Their location, thickness, de ing profiles using the Geological Sp hly selected.As a rule, the densities of de agnetic fields is carried out.rate the very complex geological structure of erved in the root parts of the Dalidag in au-

Figure 12 .Figure 13 .
Figure 12.Fragment of gravity and magnetic field analysis along profile 18 (location of this profile is shown in Figure 1.(1) Bouguer gravity field g B ; (2) magnetic field Z; (3) first horizontal derivative of gravity field   B g x ; (4) contour of magnetized body and position of magnetization vector; (5) contour of body determined by analysis of   B g x .