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Application of gravimetry and electric tomography methods to obtain stratigrafic profiles: case study at University of Quindio and Puerto Espejo area, Armenia-Colombia.



ABSTRACT

The methodology used to develop resistivity resistivity

Electrical resistance of a conductor of unit cross-sectional area and unit length. The resistivity of a conductor depends on its composition and its temperature.
 and gravimetry gra·vim·e·ter  
n.
1. An instrument used to measure specific gravity.

2. An instrument used to measure variations in a gravitational field.
 geophysical surveying applied to the sub-surface is described. These are totally indirect methods whose purpose is to determine the density and resistivity contrasts in the sub-surface, in order to relate them with the different types of soils in the areas of study, located South and North In Armenia-Quindio (Puerto espejo and University of Quindio respectively), to determine the geometry of longitudinal profiles based on existing geotechnical boreholes which only provide unidimensional information. The importance of sub-surface is analyzed, using electric resistivity and gravimetry methods in Civil Engineering projects. The most important factors influencing the changes in electric resistivity and gravimetry values for differerent geological materials are also analyzed, as well as the ranges of variation of these values, depending on climate conditions and formation of layers in each area of study.

For the electric tomography, the electrodic array Wenner-Schlumberger is used due to its high vertical and horizontal resolution The number of elements, dots or columns from left to right on a printed page, display screen or fixed area such as one inch. Contrast with "vertical resolution," which is the number of rows, dots or lines from top to bottom.

. For the gravimetric method, the microgravimetry technic is used because this allows better detail in shallow layers of the sub-surface.

Results obtained are presented in electric and gravimetric profiles in Puerto Espejo and University of Quindio, which are useful as calibration parameters of resistivity and gravity ranges for the types of soils found in these areas in Colombia, depending also on the humidity and temperature factors. The analysis and discussion of results is what finally permits to obtain conclusions, important for choosing the methods of geophysical exploration as an alternative to get familiar with the structures located in the sub-surface.

Key words: Gravimetry, electric tomography, stratigrafic profiles, volcaniclastic fan, scarp scarp: see escarpment. , Armenia (Colombia).

RESUMEN

Se describe la metodologia usada para desarrollar campanas geofisicas de resistividad y gravimetria aplicadas al subsuelo. Estos son metodos totalmente indirectos cuyo proposito es determinar los contrastes de densidad y resistividad en el subsuelo, para relacionarlas con los diferentes tipos de suelos en las areas de estudio, localizadas al sur y norte de la ciudad de Armenia-Quindio (Puerto Espejo y Universidad del Quindio respectivamente), para determinar la geometria de perfiles longitudinales basados en pozos geotecnicos existentes los cuales solo proveen informacion unidimensional. La importancia del subsuelo es analizada usando metodos electricos de resistividad y gravimetria en proyectos de ingenieria civil. Los factores mas importantes que influencian los cambios en la resistividad electrica y valores gravimetricos para diferentes materiales geologicos tambien son analizados, asi como los rangos de variacion de estos valores, dependiendo de las condiciones climaticas y la formacion de capas en cada area de estudio.

Para la tomografia electrica, se usa el arreglo electrodico Wenner-Schlumberger debido a su alta resolucion vertical y horizontal. Para el metodo gravimetrico, se usa la tecnica microgravimetrica porque permite mayor detalle en capas superficiales del subsuelo.

Los resultados obtenidos son presentados en perfiles electricos y gravimetricos en Puerto Espejo y la Universidad del Quindio, los cuales fueron utiles como parametros de calibracion de los rangos de resistividad y gravimetria para los tipos de suelos encontrados en estas areas de Colombia, dependiendo tambien de los factores de humedad y temperatura. El analisis y discusion de los resultados es lo que finalmente permite obtener conclusiones, importante para la escogencia de metodos de exploracion geofisica como una alternativa para familiarizarse con las estructuras localizadas en el subsuelo.

Palabras clave clave 1  
v. Archaic
A past tense of cleave1.



clave 2  
v. Archaic
A past tense of cleave2.
: Gravimetria, tomografia electrica, perfiles estratigraficos, abanico volcaniclastico, escarpe, Armenia (Colombia).

INTRODUCTION

The city of Armenia, Colombia
This article is about the city in Colombia. For other uses, see Armenia (disambiguation).


Armenia is the capital of Quindío, a department in Colombia. The city is located at coordinates 4.5170° north, 75.6830° west, 290 kilometers west of Bogotá.
 has been the location of several studies (INGEOMINAS, 1999, Espinosa, 2000, Monsalve y Vargas, 2002) as a result of the most recent earhquake in Quindio departament (25-01-1999, Mw=6.2). For this reason, scientists and investigators have tried to improve general knowledge in geophysical and geological aspects in Quindio departament. The most recent interest for the understanding of the sub-surface in Armenia has been the Geophysical And Structural Modeling Of Urban Area and Surroundings in Armenia project, which includes the present study, where the physical methods of gravimetry and electric tomograpy are used, to obtain stratigraphic stra·tig·ra·phy  
n.
The study of rock strata, especially the distribution, deposition, and age of sedimentary rocks.



strat
 profiles.

The geophysical surveying methods, among which we can mention the electric resistivity and gravimetry, measure a series of physical properties of objects or structures in the sub-surface, that make them different from the surrounding site.

The exploration of these contrasts between the properties of diverse materials and the encasing rocks is common in the exploitation of natural resources Exploitation of natural resources is an essential condition of the human existence.

This refers primarily to food production, but minerals, timber, and a whole raft of other entities from the natural environment also have been extracted.
 (petroleum, underground water, minerals etc) and in prospection of support for Civil Engineering, just like the surveying for the study of conditions for dams or buildings foundations, or the level of soil saturation. The objective of geolectric and gravimetric prospection is to know the dimensions and depth of structures in the sub-surface, taking as reference the resistivity and gravity data, obtained in the site using different techniques. When geoelectric and gravimetric data are correctly analyzed with modern methods and using the appropriate mathematical and computational tools, they provide a cartography cartography: see map.
cartography
 or mapmaking

Art and science of representing a geographic area graphically, usually by means of a map or chart. Political, cultural, or other nongeographic features may be superimposed.
 of gravity of the sub-surface, which permits to have a clearer idea of geometric characteristics of every layer below.

The methodoly used to obtain geophysical imaging Geophysical imaging (also known as geophysical tomography) is geophysical technique that investigates the subsurface. There are many different kinds of imaging techniques, all which are based on applied physics.  of sub-surface using gravimetry and electric tomography applied to University of Quindio and Puerto Espejo, is described in general. In the same way, we present the results obtained from the the analyses and discussions are presented regarding these results.

The present study finally considers the electric tomography and gravimetry as tools, having a high potential in the survey of sub-surface, allowing to obtain stratigraphic profiles in two dimensions that represent in a more exact way the real geometry of the layers. This is done in a better form than with currently available geotechnical methods.

LOCATION

The area under study is specifically located at University of Quindio and Puerto Espejo, located Northwest and Southwest in Armenia-Quindio--Colombia respectively, areas that have relatively flat topographies. Inside each of these areas there is a geotechnical borehole bore·hole  
n.
A hole that is drilled into the earth, as in exploratory well drilling or in building construction.
, drilled during the seismic microzonification of Armenia-Quindio project (AIS, 1999).

Figure 1 shows the location of the two zones under study and the soils that support them. Figure 2 shows the geotechnical boreholes in them.

[FIGURES 1-2 OMITTED]

The geographycal coordinates of gravimetric and tomographic profiles and the geotechnical boreholes are shown in chart 1.

GEOLOGICAL SETTING

The outcrops in the study area are geological formations belonging to Cretaceous period Cretaceous period (krĭtā`shəs), third and last period of the Mesozoic era of geologic time (see Geologic Timescale, table), lasting from approximately 144 to 65 million years ago.  (Arquia complex), Terciary period (fluvio volcanic and volcanic deposits) and Quaternary period Quaternary period (kwətûr`nərē), younger of the two geologic periods of the Cenozoic era of geologic time (see Geologic Timescale, table) from 2 millon years ago to the present.  (Quindio fan). These are, residual and saprofit deposits, coluvial deposits, pyroclastic fall A pyroclastic fall is a uniform deposit of material which has been ejected from an eruption or plume such as an ash fall or tuff. Pyroclastic flows occur relatively spontaneously in the geological time scale and are great indicators of time.  deposits, alluvial al·lu·vi·al  
adj.
Of, relating to, or found in alluvium: alluvial soil; alluvial gold.


alluvial
Adjective

of or relating to alluvium

Noun
 sediments, and recent fluviotorrential sediments of Quindio river.

The area of interest is located on a large volcano-sedimentary deposit, formed by alternation alternation /al·ter·na·tion/ (awl?ter-na´shun) the regular succession of two opposing or different events in turn.

alternation of generations  metagenesis.
 of pyroclastic flow A pyroclastic flow (also known as a pyroclastic density current) is a common and devastating result of some volcanic eruptions. The flows are fast-moving currents of hot gas, ash, and rock (collectively known as tephra), which can travel away from the volcano at up to  deposits, laharic and fluviotorrential, alluvial and glacial materials in smaller proportion, mostly covered by volcanic tuffs (ash, , lapilli la·pil·lus  
n. pl. la·pil·li
A small, solidified fragment of lava.



[Latin, diminutive of lapis, stone.]
 and volcanic dust), that covered a paleo-topography, generating round and low grade hills, which are the geographical shapes that characterize the region. The materials forming the big volcanic-sedimentary deposit come from the Central Cordillera cor·dil·le·ra  
n.
An extensive chain of mountains or mountain ranges, especially the principal mountain system of a continent.



[Spanish, from cordilla, diminutive of cuerda, cord
, located East and north- East of Quindio. These deposits are mostly products of volcanic activity originated at Nevado del Quindio, Santa Isabel, and Cerro Santa Rosa volcanoes (Gonzalez and Nunez, 1991).

Espinosa (in press), assigns this unit the name of Quindio fan and makes a morphological difference with others, concluding that its thickness is not constant, reffering to an increase from the apex, associated with distance to volcanic centers. Espinosa's work defines the terracing (see Figure 3) of Quindio and Pereira fans as a series of descending levels from the apex to the distal part, for this reason, the Quindio fan is divided in three sub-groups named as proximal, intermediate and distal fans, so that the closest fan to the apex is encased en·case  
tr.v. en·cased, en·cas·ing, en·cas·es
To enclose in or as if in a case.



en·casement n.
 in an older unit. In the same form, the degree of incision incision /in·ci·sion/ (in-sizh´un)
1. a cut or a wound made by cutting with a sharp instrument.incis´ional

2. the act of cutting.


in·ci·sion
n.
1.
 in the oldest units (closest to the apex) is much bigger than that of recent units due to its bigger exposition to alluvial erosion.

[FIGURE 3 OMITTED]

METHODOLOGY

Figure 4 shows the methodology used in gravimetric surveying. The objective of the study was to determine the contrast of densities and geometry (orientation and aproximate thickness) of sub-surface layers for the zones of Puerto Espejo and University of Quindio. Its reach permitted to classify the study as Local Type, for this reason the technique used was micro-gravimetry

Relative gravity testing was done every 10m along profiles of about 500m in longitude, using a Worden gravimeter gra·vim·e·ter
n.
1. See hydrometer.

2. An instrument used to measure variations in a gravitational field.



gra·vim
. The cycles were done in periods of less than 2 hours and by the same surveyor, seeking to minimize the level of human error.

[FIGURE 4 OMITTED]

The Bouguer anomaly Bouguer anomaly  

The difference between the expected value of gravity at a given location (taking into account factors such as latitude, longitude, altitude, and the rotation of the Earth) and its actual value.
 data obtained, along with known geologic information of the region, were processed using a computer program for inversion of gravimetric data (GM-SYS V.3.6), from which a geological model of the zones was obtained.

Figure 5 shows the methodology used in tomographic exploration. The objective of the study was to determine the contrast in resistivity in the sub-surface layers in two dimensions, confirming the presence of materials associated to those found in geotechnical and geological studies for the zone.

[FIGURE 5 OMITTED]

The method used to determine the stratigraphic profiles was the electric resistivity, since the objective was to find the geometry of the layers trough the contrast in their resistivities. The method of electric tomography was applied, using the Wenner-Schlumberguer array due to the fact that bi-dimensional profiles were needed and for the necessity of obtaining complementation Complementation (genetics)

The complementary action of different genetic factors. The term usually implies two homologous chromosomes or chromosome sets, each defective because of mutation and unable by itself to promote the normal development or metabolism of
 parameters to regional geotechnical and geological studies which are uni-dimensional. This array provides a good resolution in both vertical and horizontal directions.

For the area of University of Quindio, a 180m profile was made (see Figure 1), with electrode spacing of 5 meters, reaching a 38.7 meter deep research, while in Puerto Espejo the profile reached 144 meters, with a spacing between electrodes of 4 meters and a 31 meter depth. The data of apparent resistivity obtained in the processing of the information were used to find the values of real resistivity using a program specialized in inversion (RES2DINV, V 3.48).

RESULTS

GRAVIMETRY

Measured Bouguer anomaly values in the range of -33,23 to -22,29 mgal in Puerto Espejo and 57,79 a 65,27 mgal at University of Quindio, resulted in the following models:.

Puerto espejo zone

The profile shown in Figure 3 is extended along a gravimetric line with a longitude of 543,68 meters, heights between 1327,52 and 1349,16 meters above sea level Meters Above Sea Level is a standard metric measurement of the elevation of a location in reference to mean sea level. Uses
Meters above sea level is the standard measurement of the elevation or altitude of:
 and the values for Bouguer anomalies increase to the North of the profile as shown in Figure 6.

[FIGURE 6 OMITTED]

The geometry of the layers was stablished, obtainig values in density between 1.5 and 1.75 g/[cm.sup.3], increasing with depth. Four layers relatively horizontal for the profile were obtained (see Figure 6.), which show an increase in thickness of the fan to the South, presenting a lightly dipping structure. The deposits found in this regions were associated to La Florida La Florida (Spanish for "flowery") is a commune of Chile located in Santiago Province, Santiago Metropolitan Region. Stats
  • Area: 70.8 km²
  • Population: 365,674 (2002 census)
  • Per capita income: US$6,147 (PPP, 2003)
  • Population living in poverty: 9.
 fan, which belongs to the group of distals fans.

The first layer, with a density of 1.5 g/[cm.sup.3] shows uniformity in thickness (17 m. aproximately) as shown in geotechnical borehole (see Figure 2) associated to volcanic ash See under Ashes.

See also: Ash
. For this profile, the presence of Arquia Complex is observed below 100m depth approximately, in the North direction of the profile and shows a tendency to deepen in the South direction, due to accumulation of Quaternary quaternary /qua·ter·nary/ (kwah´ter-nar?e)
1. fourth in order.

2. containing four elements or groups.


qua·ter·nar·y
adj.
1. Consisting of four; in fours.
 deposits.

University of Quindio

The profile shown in Figure 5 is extended along a gravimetric line with a longitude of 581,06 meters, heights between 1511.67 and 1544,725 meters above sea level, presenting Bouguer anomalies in the range of 57,79 to 65,27 mGal, from North to South respectively, as shown in Figure 7.

[FIGURE 7 OMITTED]

The values for density obtained are in the range 1.5 to 1.75 g/cm3 increasing as depth increases. Five relatively horizontal layers were also obtained (see Figure 7), these show an increase in the thickness of the layers toward south, showing a slightly dipping surface. The deposits found in this sector were associated to Circasia fan, which belongs to the intermediate fan group.

The first layer presenting a density of 1.5 g/[cm.sup.3] shows a variation in depth from 11 to 24 m. Confirming the thickness of volcanic ashes (21 m.) indicated in geotechnical boreholes P6 (see Figure 2), increasing thickness toward North. The presence of Arquia Complex is after the 200m deep. Aproximately in the North side, with tendency to deepen to the South direction of the profile, due to accumulation of Quaternary deposits.

ELECTRIC TOMOGRAPHY

A model with real distribution of resistivity in the layers of the sub-surface was obtained for every zone under study, which is represented by the variation in colors. After analyzing tomographic profiles with the corresponding mechanical boreholes, the resistivity ranges for every type of soil were computed. The variables involved were: Natural humidity, unitary weight, bearing capacity, S- wave speed and RQD RQD Rock Quality Designation/Designator
RQD Reactor Quality Department (SRL) 
 (R=?, Q=?, D=?).

Puerto Espejo

The correlation of data obtained in mechanical boreholes P4 and the resistivity values found, allowed to obtain a definitive model for Puerto Espejo, which reflects a great influence of natural humidity in the values of resistivity in the upper layers. This can be observed in the graph obtained during the analysis (Figure 8), while in lower layers, the dominant factor is the degree of cementation cementation

In geology, the hardening and welding of clastic sediments (those formed from preexisting rock fragments) by the precipitation of mineral matter in the pore spaces. It is the last stage in the formation of sedimentary rock.
 and meteorization.

[FIGURE 8 OMITTED]

Although the variation of resistivity with natural humidity is not linear, there is a tendency in the resistivity, to decrease while humidity increases. A model was obtained in which 7 areas were found (see Figure 9) every one with special characteristics as described:

[FIGURE 9 OMITTED]

--Area 1

Presents the lowest humidity condition, being reflected in the largest resistivity range, which is, between 4500 and 9500 Ohm-m and is associated to lime sandstones.

--Area 2

Exhibits an intermediate humidity condition and resistivity values between 2200 and 4500 [ohm ohm (ōm) [for G. S. Ohm], unit of electrical resistance, defined as the resistance in a circuit in which a potential difference of one volt creates a current of one ampere; hence, 1 ohm equals 1 volt/ampere. ].m., which is associated to sandy limestone and lime clays.

--Area 3

Exhibits the highest humidity conditions, as reflected by the smallest resistivity range, this is, between 335 and 2200 [ohm].m. and is associated to Sandy limestone and lime clays.

--Area 4

Shows a high humidity condition and certain degree of cementation. Due to the position of this area inside the layer of the volcanic ashes, cementation is dominant over humidity, which is reflected in an increase in the resistivity range (4500 a 9500 [ohm].m.), associated to cemented sandy limestone.

Below the volcanic ashes layer, there is a lithological material stratum which shows a constrast with the alter(?) and that corresponds to residual soil Noun 1. residual soil - the soil that is remaining after the soluble elements have been dissolved
residual clay

dirt, soil - the part of the earth's surface consisting of humus and disintegrated rock
 with resistivity values ranging between 9500 and 26300 [ohm].m., the aproximate thickness is 8 m. (see Figure 2). Finally, a layer presenting the highest resistivity values in the whole tomographic profile (from 26300 [ohm].m. and aheah), which is associated to volcanic flow after 26 m in depth (see Figure 2).

University of Quindio

The correlation of data obtained in borehole P6 along with the resistivity values found, determined the definitive model for University of Quindio, where a high influence of natural humidity is observed in resistivity values for upper layers, from which we produced the graph represented in Fig. 10. on the other hand, it was possible to note that the dominant factor in lower layers is the degree of cementation and meteorization, same as in the model found for Puerto Espejo.

[FIGURE 10 OMITTED]

For this sector there is a linear variation of resistivity with the humidity, presenting an inverse variation of resistivity values with natural humidity. We can see here that the atypical values belonging to residual soil and volcanic flow do not make part of the chart, since these deposits are deeper, and there is no significant influence of natural humidity, but there is a high influence of cementation and metheorization.

This way, a definitive model was found, with seven defined areas (see Figure 11) as described:

[FIGURE 11 OMITTED]

--Area 1

Presents the lowest humidity conditions, being this reflected in the highest resistivity values, with a wide variation range (from 12000 to 47030 [ohm].m.), the superior and inferior limits of resistivity values for this area are high, due to the high percentage of compactation, since it is located below a parking lot. This material is associated to lime sandstone.

--Zona 2

Presents a midterm mid·term  
n.
1. The middle of an academic term or a political term of office.

2.
a. An examination given at the middle of a school or college term.

b. midterms A series of such examinations.
 to high humidity condition and resistivity values between 2000 and 7315 Ohm-m, wich is associated to sandy limestone and lime clays.

--Area 3

Presents the highest humidity condition, being this reflected in the lowest resistivity range (from 449 to 2000 [ohm].m.). It is associated to sandy limestone and lime clays.

--Area 4

Presents a midterm humidity condition and certain degree of cementation. Due to the position of this area inside the volcanic ashes layer, there is a marked tendency of cementation upon the humidity condition, which is reflected in an increase in the resistivity range (7,315 to 18,548 [ohm].m.), associated to cemented lime clays.

Below the volcanic ashes layer there is a stratum of lithic lith·ic 1  
adj.
Consisting of or relating to stone or rock.

Adj. 1. lithic - of or containing lithium
2. lithic - relating to or composed of stone; "lithic sandstone"
 material which contrasts with the latter, and corresponds to residual soil. Here, values of resistivity ranging between 18,548 and 47,030 [ohm].m. are observed. The aproximate thickness is 8 m. (see Figure 2).

Finally, a layer with the highest resistivity values in the tomographic profile is observed. (after the 47030 [ohm].m.), which is associated to volcanic flow and is presented aproximately after 26 m in depth. (see Figure 2).

ANALYSIS AND RESULTS DISCUSSION

GRAVIMETRY

Taking into account the terraced topography of the area, the gravimetric models of Puerto Espejo and the University of Quindio show the tendency of terracing of Quaternary deposits like the Arquia Complex, reflecting a lightly dipping structure.

It is noticeable that Arquia Complex at University of Quindio is presented at a bigger depth (? dificil entender el significado de la expresion) than in Puerto Espejo, possibly due to the distribution of quaternary deposits. In Puerto Espejo, these deposits are less antique in comparisson to those at University of Quindio, allowing to find the Arquia Complex closer to the surface. For both sectors there is a tendency in the layers to increase their thickness toward the South direction of the profiles, reflecting a slightly dipping surface, indincating the form how the deposits have been accumulated.

The micro gravimetry method allowed to analyze the layers of the sub-surface in both sectors in a more detailed way, although superficial. The comparison between the different profiles found however, with the existing ones, was only established to a maximum depth of 40 meters aproximately, since this was the biggest depth found in geotechnical boreholes so as in the tomographic survey.

ELECTRIC TOMOGRAPHY

It is observed that the range of values in resistivity of the sub-surface at University of Quindio is bigger with respect to that of Puerto Espejo, possibly due to soil characteristics and age of the deposits, those at University of Quindio are older, which is reflected in the cementation range, one of the most influencing factors in the increase of resistivity. The age of shallow deposits analyzed at University of Quindio is relflected in a higher degreee of compactation, which has a direct influence in the increase of resistivity ranges.

Both models show a dominant tendency of natural humidity in the variation of resistivity values in the volcanic ashes layer, allowing to diffrenciate permeable permeable /per·me·a·ble/ (per´me-ah-b'l) not impassable; pervious; permitting passage of a substance.

per·me·a·ble
adj.
That can be permeated or penetrated, especially by liquids or gases.
 soil from those non permeable, while in the lower layers (residual soil and volcanic flow) the dominant factor is the degree of cementation and meteorization.

The electric tomography method allowed to view the transition between the different layers of the soil. (volcanic ashes, residual soil and volcanic flow), this is evidenced by the strong contrast in the resistivity values mapped.

The anomalies found in every profile demonstrates the effectivity of the electric resistivity method in the survey of underground structures since it is possible to observe a good resolution of geometry and depth, providing Civil engineers with an reliable alternative in the analysis of foundations, piping, ducts, or underground structures.

A limitation in the electric tomography works applied to Civil Engineering is the necessity to be carried out on a natural surface (soil) (??)

CONCLUSIONS

--The ranges of apparent resistivity for certain types of geologycal material and minerals found reported by various authors differ with the values found in the present study, we suggest this is because in those refferences the conditions under which the resistivity values were obtained are not considered. Moreover, these studies have been made in other countries were the composition of the layers of the sub-surface may be different to those of this geographycal region.

--Analyzing the tomographic profile, according to according to
prep.
1. As stated or indicated by; on the authority of: according to historians.

2. In keeping with: according to instructions.

3.
 the geotechnical data from the area under study, its is observed that in the layers of volcanic ashes, (upper layers) the dominant factor in the variation of the resistivity is the Natural Humidity, while in the lower layers like residual soil and volcanic flow the metheorization degree is dominant.

--The Electric Tomography method is useful as a tool of sub-surface surveying at shallow levels and the construction of stratigraphic profiles, an appropriate calibration of the ranges of resistivity is necessary however for the soils of the municipality and for different seasons of the year in order to consider climate factors.

--It is useful to know the tendencies of the sub-surface structures using the electric tomography in Civil Enginnering, and more specifically in Geotechnics Geotechnics (synonymous: Geotechnique) is the application of scientific methods and engineering principles to the acquisition, interpretation, and use of knowledge of materials of the Earth's crust and earth materials for the solution of engineering problems. , because the level of uncertainty in excavations is reduced for the foundation. With this, the number of tests is also reduced as well as the cost of construction projects.

--The gravimetric method is of great help for geological characterization studies as well as for the seismic microzoning, because it permits to identify the more dense layers of the sub-surface associated to rock formation such as Arquia complex and diffrenciate them from the less dense like Quindio fan.

--Due to the difference in characteristics between soils at University of Quindio and Puerto Espejo, there is a variation in the ranges of resistivity, being reduced in Puerto Espejo. At University of Quindio, the resistivity ranges obtained for the same type of soil in Puerto Espejo were wider.

--It can be observed that the thickness of the volcanic-sedimentary deposits, corresponding to densities between 1.5 and 1.75 g/[cm.sup.3], is strongly related to the location of each area under study in respect to volcanic centers that originated them. For this reason, the gravimetric profile of Puerto Espejo presents smaller thickness for those deposits, since this area is located at a distal fan (La Florida), while the thickness determined for deposits below University of Quindio is larger because they are located in a midterm fan (Circasia), being consequently older.

(Manuscript received May 2005, Paper accepted October 2005)

REFERENCES

* AIS, Asociacion Colombiana de Ingenieria Sismica. 1999. Estudio de Microzonificacion Sismica para Orientar la Reconstruccion de Armenia--Quindio. Informe Tecnico-Cientifico. Bogota.

* Burger H. R., 1992. Exploration geophysics Exploration geophysics is the applied branch of geophysics which uses surface methods to probe or detect properties of the subsurface Earth.

Exploration geophysics is the practical application of physical methods (such as seismic, gravitational, magnetic, electrical and
 of shallow subsurface, Prentice-Hall. 489p. Englewood Cliffs-New Jersey.

* Chica A., 1998. Geofisica Aplicada -Magnetometria y Gravimetria-. Cuadernos de Ingenieria geologica. No. 5. 125 p. Medellin, Colombia.

* Duyoner, M. y Aguirre, Y (19997) Identificacion y caracterizacion macroscopica de la secuencia de tefras de la ciudad de Armenia (CRQ CRQ Call Request
CRQ Common Recall Queue (IBM mainframes, dataset management)
CRQ Contrôle routier Québec
CRQ Claim Reunderwriting Query (insurance)
CRQ Command Response Queue
CRQ Continuous Repeat Request
).

* Espinosa, A., 2000. Mapa Preliminar de Fallas Activas del Quindio, Zona Central. Informe Final Proyecto 152, Universidad del Quindio, Armenia, 70 p., inedito.

* Espinosa, A. (En prensa). Un modelo del Cuaternario del Quindio y Risaralda. Revista de la Academia La Academia (The Academy) is a Mexican reality television show shown on TV Azteca. Although the show itself is not affiliated with the Endemol franchise, which includes the "Star Academy" shows, it does share the competition format of many of the variants of the  Colombiana de Ciencias Exactas, Fisicas y Naturales. Bogota.

* Espitia, M. y Caro, E Capitulo 1 Geologia local y tectonica en Ingeominas, (1999) Vol II.

* GM-SYS, Gravity and magnetics Modeling System. Version No. 3.60. Mike Gemperle, Gerry Connard, Milt Sagen, and Myungmun Bae Northwest Geophysical Associates, Inc.

* Loke, M. H. 1999. Electrical Imaging Survey for Enviromental and Engineering Studies: A Practical Guide to 2-D and 3-D Surveys. Malasia. p 58.

* Monsalve-Jaramillo, H. y Vargas-Jimenez, C.A. 2002. El Sismo de Armenia (25-01-99, Mw=6.2): Analisis telesismico y caracteristicas de las ondas de cuerpo. Revista IPGH IPGH Incontinence, Pelvic Floor and Prolapse, General Factors and Handicap (pelvic floor disease assessment) , No 57, 21-56.

* Nunez, A. y Gonzalez, H. 1991. Memorias mapa geologico generalizado del departamento del Quindio. INGEOMINAS, Bogota.

Erika Cristina Jaramillo Giraldol (1), Ximena Vega Manchola (1) and Hugo Monsalve Jaramillo (1)

(1) Centro de Estudios e Investigaciones de la Facultad de Ingenieria (Research Center at University of Quindio)--CEIFI, Quimbaya Group e-mail: hugom@uniquindio.edu.co--e-mail: manchol@hotmail.com.
CHART 1. GEOGRAPHICAL COORDINATES OF FINALAND INITIAL STATIONS FOR
EVERY GRAVIMETRIC PROFILE OF GEOTECHNICAL BOREHOLES (P4: PUERTO
ESPEJO AND P6: UNIVERSITY OF QUINDIO) AND THE MIDPOINTS OF EVERY
TOMOGRAPHIC PROFILE

               Location         Puerto Espejo

               N (m)            4[degrees] 31' 5.65" - 4[degrees]
                                  31'10.02"
Gravimetry

               E (m)            72[degrees] 42'44.36" - 72[degrees]
                                  42'29.40"

                            N   4[degrees]31.296'
Electric
Tomography     Pto. medio
                            W   75[degrees] 42.849'

               N (m)            4[degrees] 31'7.22"
Geotechnical
boreholes
               E (m)            72[degrees] 42'43.19"

               Location         University of Quindio

               N (m)            4[degrees] 33'14.93" - 4[degrees]
                                  33'23.12"
Gravimetry

               E (m)            72[degrees] 39'43.75" - 72[degrees]
                                  39'30.85"

                            N   4[degrees] 33.466'
Electric
Tomography     Pto. medio
                            W   75[degrees] 39.903'

               N (m)            4[degrees] 33'22.25"
Geotechnical
boreholes
               E (m)            72[degrees] 39'36.04"
COPYRIGHT 2005 Universidad Nacional de Colombia, Departamento de Geociencias
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Author:Giraldol, Erika Cristina Jaramillo; Manchola, Ximena Vega; Jaramillo, Hugo Monsalve
Publication:Earth Sciences Research Journal
Geographic Code:3COLO
Date:Dec 1, 2005
Words:4185
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