Spatial correlation between the Radon-222 and Radio-226 concentrations collected from Los Azufres Geothermal reservoir in fractured rocks.

Transcription

1 Spatial correlation between the Radon-222 and Radio-226 concentrations collected from Los Azufres Geothermal reservoir in fractured rocks. * Flores Ruiz J.H., ** Hernández Quintero E, ** Urrutia Fucugauchi J and * Martínez Angeles R. *Instituto Mexicano del Petróleo,IMP **Instituto de Geofísica de UNAM, IGEOF [México] The area in study is of 40 km 2 and it is located in the northeast part of the Mexican State of Michoacán and to the southeast of the geothermal field Los Azufres inside the geologic province of the Mexican Volcanic Belt (MVB). In the place exist a series of faults of orientation east - west and northeast - southwest. These faults give origin to grabens and horst produced by regional distensives stress with north - south preferential orientation. In these geologic structures it was carried out the exploration of gas Radon-222 ( 222 Rn) it was superficial and samples of volcanic rocks and soil, it being a bigger content of Radio-226 ( 226 Ra) in the floor samples. It was considered the maximum correlation distance between the data of 222 Rn and 226 Ra in the orthogonal directions (E-W, N-S, NE-SW and NE-SW). It is not necessary to forget that Radon gas has been used to measure dynamic processes, the resultant anomalies are considered the definition of the extension approximately in the underground of the thermal area for local structures that are continuous in the surface. This consideration is obvious, however, this is valid if the geologic structures are deep or near to the vertical one. It was also determined the space combined relationship between the emanations of the 222 Rn and the content of 226 Ra in the field samples for the method of Cokriging, it was estimated relationship between these two variables is respectively from 1 to 26. On the other hand it is significant the relationship between the permeable area and the density of radon measurements in the place. This factor would seem to be the most important in this area where is a fault that can be activated due to the flow of heat through its comparison with a broadly permeable area. But the success of the radon emanations has been had by means of the biggest quantity from traverse profiles to the fault trace appearances in the geothermal field. 1. Introduction The area in study is located in the geologic province of Mexican Volcanic Belt, (MVB) it has active vulcanism of calc-alkaline character which extends through of the central part of Mexico, it Includes stratovolcanoes, cinder cones, monogenetic cones and silicic centers. The MVB is not parallel to the trench, it follows an oblique direction with an angle between 15 and 20. Some authors try to explain this tectonic relationship (Molnar and Sykes, 1969; Mooser, 1972; Urrutia and Pal, 1977; Urrutia, 1981, 1984: Shurbet and Cebull, 1984; Bonel and Urrutia, 1988; Uribe and Urrutia, 1999). Many tectonic models have been proposed to 1

2 explain the tectonic origin and subsequent development of the MVB. rocks in this region are of ages from the half Cretacic to recent including continental sediments and marine limestones. The province has a width from 20 to 150 km, a longitude approximately of 1000 km and an altitude regional average of 2000 m.s.l. and it extends from Pacific Ocean to the Gulf of Mexico. The area in study has a crust thickness average of 45 km in Los Azufres (Campos and Garduño, 1995; Urrutia and Flores, 1996; Urrutia et al., 1999) and the biggest cortical thickness in the whole MVB is in the area of the big valleys of the State of Mexico, Puebla and Distrito Federal. The geothermal field of Los Azufres is located between the parallel ' 24 ' ' and ' north latitude and the meridians ' 19 ' ' and ' 37 ' ' of longitude west with an altitude average of 2700 m.s.l. This region crosses at level regional two systems of faults: one E-W it is more young (Chapala - Tuxtla), characteristic of the MVB, and another NNW - SSE that understands the area (Queretaro - Toluca - Taxco) it is older and it seems that is the continuation of the system of Basin and Ranges province (Urrutia, 1995). The faults that limit the field of Los Azufres are: to the north fault Agua Fría with a longitude of 7 km, at the center faults Chinapo of 5.5 km, both with direcction E-W, and at the south limits in lower part of the area; the topographical altitude is 2400 m.s.l, here exists the fault Presa and to the west the fault El Vampiro, with a longitude of 2 km both faults and common direcction NE-SW. Our radiometric survey carried out (irregularly sampled) on the geologic structures around 500 m for the sampling of gas Radon- 222 ( 222 Rn) in 20 places and for the Radio-226 ( 226 Ra) was carried out floor sampling and rocks in the same previous places. The fault planes are discontinuous and structures of steps faults are observed due to normal faults and the state of distensives regional stress with preferential orientation E-W (Uribe and Urrutia, 1999). In this work we applied the correlation space jointed to estimate the relationship between the emitions of particles alpha of gas radon and the radiation gamma due to the content of radioactive minerals in the volcanic rocks. 2. Spatial correlation The process of combined correlation of several variables it has estimated in situ. They give information of the space structure in two and three dimensions. The process is called Cokriging. The purpose of this analysis is to make an estimate of the interdependence that exists among the data of the variable one and the data of the variable two calculating the space variance between distributions. The cokriging is jointly an extension of the kriging of several variables that are using the estimator BLUE (best linear unbias estimator) (Carr et al., 1985). One of the applications of this method is asociated to the cemetery of low level radiation of Nevada site, where it has a database of the concentrations of Plutonium ( 239, 240 Pu) and Americious ( 241 Am) in the floor, vegetables and animal tissues. The goal of this radiometric exploration was forcasting the total quantity and space distribution of ( 239,240 Pu) and ( 241 Am) in the floor surface (Helterbrand and Cressie, 1994). 2

3 For our radiometric survey we took 20 samples of Radio ( 226 Ra) in the ground randomly and it has a quantitive minimum of sample 36 Bq / kg and a maximum of 197 Bq / kg, the variogram estimated in Lognormal given to the high variation of the data (Fleisher and Mogro, 1979) in the southeast area Los Azufres, the average is Bq / kg, standard deviation of Bq /kg and a variance of 1881 (Bq / kg) 2, the maximum distance of space correlation is 1000 m, with a variance associated of (Bq / kg) 2 and for the distance of 500 m the variance is of (Bq / kg) 2 see figure 1. The map of space distribution of ( 226 Ra) has a radiometric regional anomaly of 180 Bq/kg, near the fault El Viejo in direcction NE-SW, the second maximum is 120 Bq/kg and is located 200 m away from the fault Agua Fría, the minimum 90 Bq/kg is located 1125 m to the south of the fault Laguna Verde and the regional minimum of 60 Bq/kg is located in the fault El Vampiro see figure 2. For the Radon ( 222 Rn) we took the samples in the same places of the samples of the ( 226 Ra), and its variograms were calculated. The minimum value of the data for Radon is 235 (Bq/m 3 ), the maximum was 9599 (Bq/m 3 ), the average 3675 (Bq/m 3 ), standard deviation 3072 (Bq/m 3 ) and variance of (Bq/m 3 ). The maximum distance of correlation was 1000 m, the variogram gives an variance (Bq/m 3 ) 2 and at the distance of 500 m, the variance associated is of (Bq/m 3 ) 2 see figure 3. The distribution map of ( 222 Rn) has a radiometric regional anomaly of 9599 (Bq/m 3 ), to the southeast of the fault Laguna Verde, the intermediate values of 7760 (Bq/m 3 ) are located to the south of the vertex formed by the faults El Chinapo and Agua Ceniza and 7000 Bq/m3 to the southeast of the faults Agua Fría. The regional minimum anomaly is of 1750 Bq/m3 located in the hill El banco among El Chinapo, Laguna Verde and La Presa (see figure 4). It was also estimated in lognormal the crossvariogram starting from the variograms (figs 1 and 3) associated to the values of 226 Ra and 222 Rn of spherical model. 3. Results It is considered the space combined correlation between the random data of 226 Ra and 222 Rn located in the surface. This process will establish the correlation of both data, as if we had a mesh spaced regular each 500 m and it will interpolate the previous data, with their respective variograms and crossvariogram. The estimate value for regional average emanations of Radon-222 as a function of the content of Radio-224 in the floor and volcanic rocks in the same place gave a relationship lognormal from to 1 (see table 1). 4. Conclusions a). It was carried out the space combined correlation of the 20 random samples ( 226 Ra) of field and the ( 222 Rn) located in similar places by means of Cokriging method. In which we were considered both explorations to 500 m like if it was a single mesh sampled through their respective variograms and crossvariogram. It was calculated regional average value for the map of Radio-226 (Fig 2), with average of Bq / Kg. For the 3

4 map of Radon-222 (Figs 1 and 4) the value regional average is 3175 Bq/m3. The ratio estimated between the regional values average of ( 222 Rn) and the ( 226 Ra) is approximately of 26 to 1. b). The 20 random samples ( 226 Ra) of floor and volcanic rocks were gathered from surface, having the biggest regional values for radiation the floor samples. This is due to the change that suffers the rock mother, being this the generating source of radioactive minerals contribute for the composition of the local floor. On the other hand the geochemestry method try to link content of Radio in the 3 minerals of uranium with emanation coefficient for gas Radon. This is vastly variable and it gives us an idea of the great complexity for their modeling using geochemestry method. c). Concluding it is inferred that a great difficulty exists in the models estimated by geochemical methods to relate the content of ( 226 Ra) and the coefficient of emanation of gas ( 222 Rn). Due perhaps that the pattern geochemestry uses few minerals of uranium and a scarcity of data. The geostatistics offers an enormous help to evaluate this relationship by means of the combined estimate both variables in regional or single form in any place of interest through the multivariable Cokriging operator. 4

5 5. References 1. Campos-Enríquez, J.O. and Garduño-Monroy V.H. 1995, Los Azufres silicic center (Mexico): inference of caldera structural element from gravity, aeromagnetic, and geoelectric data. Journal of volcanology and Geothermal Research V67, p Carr J.R., Myers D.E. and Glass C., 1985., Cokriging-A computer program., Comp. And Geos. V.11., N 2., p Fleisher R.L. and Mogro C.A., 1979., Integrated radon mapping in the earth assessment of the Rn-222 signal it s Exclution. Geophys. V.44, N 9., p Helterbrand J.D and Cressie N., 1994, Universal cokriging under intrinsic coregionalization., Mathematical Geology. V 26., N 2., p Molnar P and Sykes L.R., 1969., Tectonics of the Caribbean and Middle America regions from focal mecanism and seismicity. Geol. Soc. Am. Bull. V.80, p Mooser F., 1972., The Mexican Volcanic Belt. Structure and Tectonics. Geog.Inter.V.12, p Shurbet D.H. and Cebull S.E., 1984., Tectonics interpretation of the Trans-Mexican volcanic belt. Tecto. V. 101, p Urrutia Fucugauchi J and Pal S., 1977., Paleomegnetic data from Tertiary igneuos rocks, notheast Jalisco, Mexico., Earth Planet. Sci. Lett., V.306., p Urrutia Fucugauchi J., 1981., Palemagnetism of the Miocene Jaltetelco graniodiorites and Tepexco volcanic group and infered of the block rotations in central Mexico.Tecto. V.76, p Urrutia Fucugauchi J., 1984., On the tectonic evolution of Mexico: paleomagnetic constrains: Am. Geophys. Union, Geodyn. V.12, p Urrutia Fucugauchi J and Harald Bohnel., 1988., Tectonics along the Trans-Mexican volcanic belt according to paleomagnetic data., Phys. Earth Planet. Inter., V. 52., p Urrutia Fucugauchi J, Soler Arechalde A.M and Flores Ruiz J.H., 1995, Tectonics and vulcanism in the Central Mexico influence of crustal structure and Pre-neogene tectonics in the subduction magmatic arcsystem. Geol.Soc. Ame. (GSA)., p

6 13. Urrutia Fucugauchi J. and Flores Ruiz J.H., 1996.,Boguer gravity anomalies and regional crustal structure in Central Mexico., Inter. Geol., V. 38., p Uribe Cifuentes R.M. and Urrutia Fucugauchi J., 1999., Paleomagnetic study of the Valle de Santiago volcanics, Michoacan-Guanahuato volcanic flied, Mexico., Geof. Inter., V. 38.,N 4, p Urrutia Fucugauchi J, Flores Ruiz J.H., Bandy W.L., and Mortera Gutierrez C.A., 1999., Crustal structure of the Colima rift, western Mexico: Gravity models revised., Geof. Inter. V.38, N 4.,p

7 FIGURE 1 FIGURE 2 7

8 FIGURE 3 FIGURE 4 8

9 TABLE 1. 1 ******************** CO-KRIGING PROGRAM **************** NO OF ROWS IN KRIGED ARRAY = 1 NO OF COLS IN KRIGED ARRAY = 20 MAXIMUN Y COORDINATE = MAXIMUN X COORDINATE = INCREMENTE ON X = INCREMENT ON Y = A TOTAL OF 2 VARIABLES WILL BE ESTIMATED ****** VARIOGRAM AND CROSSVARIOGRAM PARAMETERS ******* SINGLE VARIABLE(VARIOGRAM)PARAMETERS VARIABLE NUGGET SILL RANGE ANGLE RATIO INFLUENCE MODEL INTER-VARIABLE (CROSS-VARIOGRAM)PARAMETERS VARIABLE NUGGET SILL RANGE ANGLE RATIO INFLUENCE MODEL ********** CO-KRIGING *********** ROW COL NORTH WEST DATA ESTIMATES VARIANCE RA-226 RN-222 Ln(X)