Study of Qater- Kazerun Fault by use of magnetometry and geoelectric method in Kazerun region in Fars province
||Firouz Jafari , Seyed Abolhasan Razavi
||13 January 2008
According to request of great deputy of GSI geophysical operation is done in order to geophysical study of Qatar- Kazerun in method of magnetometry and geoelectric in region of Kazerun- Fars province in order to geophysical study for Qatar- Kazerun Fault from location determination and determination of hydrous fault.
Study of Qater- Kazerun Fault by use of magnetometry and geoelectric method in Kazerun region in Fars province
Firouz jafari, Seyed Abolhassan Razavi, winter 1386
According to request of great deputy of GSI geophysical operation is done in order to geophysical study of Qatar- Kazerun in method of magnetometry and geoelectric in region of Kazerun- Fars province in order to geophysical study for Qatar- Kazerun Fault from location determination and determination of hydrous fault. In order to that, during rules No. 3946 and 4077 as a three people group by leadership of Mr. Engineer Seyed Abolhassan Razavi as group leadership and Faramarz Allah Verdi and Abbas Bagheri to Esfand Abadi as a technician in two launches for 20 Deys in Bahman and Esfand 85 for doing of geoelectric studies which are launched to the location.
Also during rules in No. 3942 and 4110, a two people groups is with leadership of Mr. Engineer Firouz Jafari and Hossein Iranshahi as technician in two 20 days launches and 12 days in Bahman and Esfand, 1385 are done for magnetic studies were launched to location of operation confirmation. Mr. Eng. Pedram Aftabi is as geologist in the region with geophysic group with cooperation to this mind and it is a part of Kazerun- Qatar fault zone is selected near to Komaraj village in shape of almost rectangular.
At first geoelectric surveys are done by electric sounding arrangement which is with longitudinal length up to 3000 m which is exceptional in its type in order to survey depth position of groundwater about 600m and magnetic surveys are done simultaneously in order to recognize contacts and fractures.
This part is a little than tributary of Kazerun – Boushehr- Ghaemieh (Komarej village) is situated. Cause of this selection is two reasons. One of them is, if above fault contain much amount in some parts that width of fault zone is decreased and we can survey and study that easier and then we proceed to its prevention.
Other matter is less alluvium sediments thickness and this matter provides needed studies for exploration of Asmari Karstic Limestone. Second area is to North West Qaemieh (Ghandil village for trail of studies is regarded). Purpose for doing of geophysics studies is determination of fault position in this area. In this region, at first, a polar – polar arrangement is used on a W-E profile and then with respect to information of this profile of two sounding on stations of 350 W and 450 W was surveyed. Also, contemporary area with an area with dimensions of 2 km in 200m (a network of 20*25m) which involves polar- polar profile too and it is under magnetometry survey.
Geographical position of studied area
Studied area is located in Kazeroun small province in Fars province. For doing of studies 2, the location is selected which is located on image No.1 and position of these two regions are shown with numbers of 1 and 2.
Region geology and results of fault study region geology
Kazerun line is a valley with approximate length of 200km which is oblique in Zagros anticlines with a trend of N-S trend. This extent has high seismic activities and some earthquake with medium magnitude and short reference time.
Zagros mount. Is a sequence from continental plateau sediments of Paleozoic to late tertiary age which has been deposited on salt formation with Infra Cambrian Hormuz (fig 1). Surveyed area is situated on geological – tectonical folded Zagros are that its western fault reaches to Dezful depression (fig 1).
This region is composed of numerous anticlines, synclines and defiles which formations of Aghajari, Gachsaran, Asmari and Bangestan group are main features of these structures. (Baker, 1993). States according to falcon that mentioned lineation does not cross anticline axis and dextral displacement of axis is only due to expected apparent imagination of this fault system which has oblique manner rather than dominant trend of Zagros simple fold belt.
Zare (1371) indicated by survey of fractures in Lordegan area and respect to main faults in this area that total system, dominant on this region is as right-hand transgression. An oblique convergence in orogenic zone with oblique vectors results in dextral strike-slip mechanism along Kazerun Fault (Authmayou et al., 2003).
Kazerun fault, separator of the region, containing abundant salt domes in the region has rare salt domes (Talbot and Alavy 1926), but there is a salt dome along this fault that its main part is from Gachsaran Fm. and salt is located in central part of dome.
Fault fragments of Kazerun and hydrous system in the region
Main topography of the region does not seem in Kazerun which are under influence of Kazerun Fault but Kazerun Fault has impression in Gachsaran Fm. Uplift its through and torsional traces are around this anticline (fig 2). As fracture systems has influenced vicinity anticlines.
The fault is in reality as a zone with total trend of N-S but trend of related faults are variable. Fault chain system and karstic and fissures are in the region are as it results in conductance of groundwater from a point to other point.
Folds and pores inside of these folds are accompanied with karstic dissolution of gypsum and calcareous formation and they are good locations for water conductance. These folding systems are related to Kazerun Fault and some of them are post-tectonical but they are situated as post tectonical under influence of fault (fig 3).
Water conductance is related to hydrous systems to folds, karstic pores and local topography and also faults in the region (fig 4).
As we have shown in map fig 4, most of surface and undersurface waters have special directions which are exited by influenced structures and they mainly from hydrous region of kazerun. Control of this exited source and finding a solution for further use from mater resources indicates that Kazerun Fault and folds and faults are very important ways to water transmissivity. Of course water suction in the region should be done scientifically because these under ground water movements results in feeding Parishan lake and exaggerating in hydrous system, feeding of lake results in its dryness, as lake has several maters of regression.
If figures of 2,3 and 4 are compared to each other so we will consider that hydrous system is seriously under influence of faults folds with movements and these present movements and topographic barriers are from limestone and gypsum resource have formed good water in the region which is sometimes as some exited springs and springs of water immigration and regions with high potential of groundwater in fig 5 are location of regions with hydrous potential based on tectonical studies of fault and joint and region topography which can be reached to 175m from water.(fig 5)
Water in these regions may be accompanied with gypsum but it is very important from agriculture aspect and it can be used for farmer people in the region. Locations with groundwater potential are shown by a star.
There are many karstic springs which can be used and also water of Shapour River is one of important resources which can be under further exploitation.
Depth of fault and width of fault zone
Related to find deep waters and use of them is surveyed in fault depth and width of fault zone. Based on fig 7, depth of Kazerun Fault is very high and its width is more than 1km in some regions. So far recognition of pores with water here, some deep geophysical works are suggested with a depth of more than 1km.
Although oil susceptibility of the region and presence of sulfur resources are in reality related to Kazerun Fault and they are polluted to poisonous materials like sulfur and arsenic and this point is considerable in suggestive plan.
Naturally, presence of Fahlian spring (Abgandou) that its water is poisonous and it is used for bath affairs for treatment of skin disease and it is related to Kazerun Fault and naturally all of waters in the region and especially waters which are discharged to Persian gulf and they are wastewater and they are not only related to Kazerun Fault and regions around Kazerun Fault needs to survey.
About water conductance, from Kazerun to Boushehr and around regions needs more precise works and it is mainly done by use of topographical maps, aerial photos and field operations. So it is suggested that mentioned basic information are suggested at the top.
It seems that salt dome, near to Kazerun, has not polluted water to salt due to embed by Gachsaran gypsum and it is in depth of 175m near to a blue pump station (water pump station) is a good place of or some more wells for finding underground conducted waters (fig 8).
Anyway, polluted waters to gypsum is effective for some deprived provinces especially Boushehr which is confronted to water shortage. Because it seems that salt has low influence on groundwater. It is suggested some more studies from tectonic aspect in the region and especially in Boushehr for finding water.
In method of resistance calculations artificial electrical currents are injected to ground by two electrode and resulted potential difference is calculated between two points in earth surface. Perturbation from shape of expected potential differences give some information about non-homogenous shape and electrical characteristics under ground. Specific resistance of a material is defined as specific resistance between opposite surfaces in 1m3 from an object in terms of ohm. Electrical specific resistance is one of physical characteristics of rocks which have much change.
Special minerals are such as natural metals and graphite, conduct electrical current by electron passages but most of composer minerals in rocks are impermeable and electrical current is transported mainly by water ions in rock pores.
So most of rocks conduit electricity by electrolyte rather than electronic method and that means porosity is major controlling factor for rock resistance and other than that, water inside of pores and water electrical resistance has main role in this direction too and it changes rock specific electrical resistance and hence there is considerable overlap between electrical specific resistance in various types of rocks and so characterizing of rock types is feasible only based on resistance data and we should consider above factors certainly.
In measurement of electrical specific resistance, electrical current is in type of direct with two current electrode (A,B) are submitted to the ground and resulted potential difference, between two potential electrode is measured in the ground (M and N). Electrical specific resistance is calculated from formula of P = K.V/I.
V is potential difference and I is intensity of injected current to the ground and K is geometric coefficient of used arrangement and even tough we can state its formula for each arrangement but total formula of calculation is as follow with this formula:
K = 2π.1/ C1/Am -1/ AN-1/BM+1/BN.
When ground type is homogenous, calculated specific electrical resistance is fixed based on this equation and it is independent from electrode distance but if underground heterogeneity is present, electrical specific resistance is changes with relative position of electrodes and each calculated amount will be named as apparently specific electrical resistance (a) are named and it depends on heterogeneity shape. In this survey, two arrays are used.
1. Pole- pole array
2. Vertical electrical sounding
Which are explained as summary.
- Pole-pole array
In theory of pole-pole array, only two electrodes are near to each other. A current electrode and a potential electrode and other two electrodes are situated in infinite distance which have less trace on the array.
In total manner, it is advised that distance of these two electrodes from electrodes be selected P1 and C1 is 10 and preferably 20 times their distance. Ins some of cases, that electrodes distance of P1 and C1 is very much, these minimums are not regarded for all of measurements. This matter can result in complexity in anomaly on resulted model from reversing during modeling.
This problem is caused by use of usual calculation method with coefficients of K in pole-pole array and if coefficient K is calculated by use of total method in terms of electrode distances, this problem will not be solved.
With declining in electrode distance of P2, C2 from surveying line, search depth is decreased. Anyway pole-pole arrangement has most ratio of searching depth rather than other arrangements. Other words, if ratio of searching depth in Bipole-Bipole array is 0.3 times apparent depth, in this method, searching depth is 0.9 times apparent depth.
Sounding Electrical vertical array or Electrical bore hole
Mainly this array is used for study of horizontal layers. In this method, after each measurement, current and potential electrodes are gained in some fined relative position and they are far from each other increasingly around a central point and so when current reaches to more depths, readings are done.
In Schlomberger arrangement, electrode distance is always bigger or equal to 5 times of potential electrodes (AB>5MN) and potential electrodes are located among current electrodes (fig 10).
After field measurements and calculation of a from K.V/I/ at first graph of specific change graph versus increase of distance in current electrode i.e., depth increase is shown. Some logarithmic papers are used for this affair, hence measured apparent specific resistance are regarded on vertical axis and distance of OA = AB/2 is regarded on horizontal axis and curve of electrical sounding is drawn that changes in this graph is indicative of changes in specific resistance in various depths.
Magnetometry method theory
Study of magnetism in the ground is oldest geophysics branch for first time, Gilbert show that magnetic field of the ground has a trend of N-S near to torsional axis of the ground. Since then, considerable progressions are caused in field of machines constructions and interpretations of measurements in this method.
In magnetometry methods, usually total field or vertical component is measured. Regarding magnetic field has two poles and also trend, so interpretation of related maps is more complicated than other methods.
On the other hand, in comparison to most of geophysical methods, field measurements are easy and in expensive and it do not need to operate complicated and long corrections in readings.
Magnetic field of the earth is related to geophysical
Exploration and it is composed of three parts:
1. Main field that even though it is not filed to time, it is relatively changed calmly and its source is internal and it forms about 90% of magnetic field of the earth.
2. External field is a small part of main in field that its origin is out of the earth and it is relatively changed rapidly, a change that a part of that is periodical and some of that is accidental (related to diurnal and annual changes of the sun and daily changes of the moon).
3. Changes in main field is usually smaller than main field and it is relatively fixed to time and place and it is caused due to magnetic anomaly near to earth crust surface. These changes form geophysical aims of explorations. If an object is located in earth magnetic field (F). in this case, a field in name of J (induced magnification) is induced to object internal parts, that we will have:
J = KF
Where K is magnetic coefficient (simultaneous magnetism), Objects are divided into three groups in terms of coefficient of K:
1- K0, paramagnetism. Some elements like nickel, calcium and … and this trace is decreased with temperature.