**4. Results**

*Magnetometers - Fundamentals and Applications of Magnetism*

In this case, the resistivity (Eq. 6) is given by:

a = \_

ration, its units is ohm per meter (Ω/m).

"cluth" during the measurement of the SEV [1].

electrical resistivity in a horizontal direction.

apparent.

In geoelectrical exploration, the resistivity of the underground is normally measured with an electrode arrangement of four electrodes, with the electrodes AB

> V I

where K is the geometric factor (1/AM-1/BM-1/AN+1/BN) of the electrode array, the subscript "a" in the resistivity indicates that the calculated value is

The resistivity is an inverse property of the electrical conductivity and in explo-

In geoelectric exploration, the variation of the resistivity is studied horizontally

A quick way to know the electrical behavior of the underground in a given area is to make profiles of electrical resistivity of two electrode openings, for example, at 200 and 400 m opening of the current electrodes with the Schlumberger electrode array (AB/2 at 100 and 200 m). In this way, we have knowledge of the variation of

If the resistivity behaves similar to both electrode array separations, it will imply that the entire electrically scanned area is the same. If the resistivity of the profile generated with a larger electrode aperture is higher than that generated at a smaller aperture, it will indicate that at depth the possibility of detecting humidity is zero. On the other hand, if the resistivity is lower at larger electrode aperture, it will have greater chances of detecting humidity. If the profiles of apparent resistivity show an irregular behavior, it will have greater possibilities of detecting a resistive resistance where the resistivity at the largest aperture changes form more resistive to less resistive and indicates that the area under study shows in the underground, where the current circulates more easily and will be an area where the variation of resistivity with depth must be studied, which is done with vertical electric sound-

The VES' must be interpreted qualitatively and quantitatively. Firstly, the morphology of the VES curve must be defined [21] which in order to be associated with humidity, must necessarily have a correlation with the H-type curves (ρ1 > ρ2 < ρ3), which indicates that there is a lower resistivity contrast between the central layer and those that enclose it. The VES curves can also be KH (ρ1 < ρ2 > ρ3 < ρ4), QH

The quantitative interpretation is carried out using commercial software that allows an inversion of the resistivity data [22]. It is convenient to perform a VES in wells where its stratigraphic column is known, in a way that the VES can be

Once the previous stages have been carried out, the zones that are chosen for drilling must have a magnetic response which correlates with a fractured zone (permeability) and electrical methods (resistivity) with an area that has a relation with a humid area, represented by a resistive contrast that contains a minimum between

(ρ1 > ρ2 > ρ3 < ρ4) or some of the curves that show a portion of type H.

by means of profiles in which the electrode array is moved as a whole to the different stations. Conversely, equispaced or the vertical variation of the resistivity can be studied by means of vertical electric soundings (VES). At a certain point, for this, the current electrodes (AB) are increasingly opened and the measuring or potential electrodes (MN) are opened only when the measured values are very small (Schlumberger electrode array). In such manner in which data exist in one or two points with different MN opening for the same values of AB, there is an overlap or

× 2K (6)

being the emission electrodes (current) and MN the potential electrodes.

**32**

ing (SEV, [20]).

calibrated.

two resistivity maxima.

The procedure described above has been applied to an area, which is located in the Mesa Central, Mexico, specifically to a rural population called La Dulcita, municipality of Villa de Ramos, San Luis Potosí.

## **4.1 Air Magnetometry**

The area under study was flown by the Mexican Geological Service, using an Islander aircraft BN2-A21, equipped with a Geometrics G-822 magnetometer, of cesium vapor optical pump, with a sensitivity of 0.25 nT, and an acquisition system of P-101 Picodas data, Automax video camera, 35 mm. A Geometrics G-826A magnetometer was used, with a sensitivity of 1 nT as the base station. Also, Sperry altimeter radar was also used.

The course of the flight lines was N-S, with a distance between flight lines of 1000 m and a height above ground level of 300 m, the navigation was controlled with an Ashtech GG24 GPS system and the data was subtracted from the IGRF 1990 reference.

The total magnetic field intensity in the central portion was 44,858 nT, with an inclination of 50°43′ and declination of 8°13′ for July 1995.

The magnetic field behavior analysis began with the generation of the RMF map (**Figure 5**), which as mentioned in previous paragraphs, is obtained by subtracting the IGRF from the total magnetic field. Based on the RMF, the RMPF was calculated (**Figure 6**). In the W portion of the RMPF, there is a "trend" of magnetic highs (red) that represent the W limit of an area of the graben that exists with a general direction N-S and is characterized on the map with anomalies associated with magnetic lows (blue color). Towards the central portion, two "trends" of magnetic anomalies with direction NE–SW and NNW–SSE are shown that are possibly associated with the geologically multiple intrusive "El Socorro" [7]. The Dulcita area is located on the first step of the graben and alignments (**Figure 7**) with direction N-S and E-W towards its portion W is observed, which can be geologically associated with zones of faults and/or fracturing and/or contacts. The area investigated in

#### **Figure 5.**

*Map showing the isovalues contour of the residual magnetic field of the Dulcita area, Villa de Ramos, San Luis Potosí, Mexico.*

#### **Figure 6.**

*Map showing the isovalues contour of the reduced to the pole magnetic field of La Dulcita, Villa de Ramos, San Luis Potosí, Mexico.*

#### **Figure 7.**

*Map where the magnetic alignments are observed based on the isovalues contour of the first vertical Derivativ upwards continuation 250 m from the reduced to the pole magnetic field.*

general shows preferential aeromagnetic alignments in an N-S direction, also existing in the NE–SW direction, with few showing NW-SE direction.

The analyzed area in general shows the existence of up to 10 AMD's, each characterized by different amplitudes and wavelengths. The area where the water is extracted for the population of La Dulcita, is correlated with the AMD II that is associated with a tectonic pit area, characterized by low values of magnetism. The graben is limited by AMD I to W and by AMD's IIII and IV to the E. In AMD I, a highly productive well was located for the area (16 L/s) at a distance of 2.3 km SW of La Dulcita outside the ejido boundaries.

La Dulcita area is located in the aeromagnetic domain map (AMD), zone that show similar magnetic susceptibility (**Figure 8**) and is situated between the limits

**35**

**Figure 9.**

**Figure 8.**

*field.*

*The Magnetometry—A Primary Tool in the Prospection of Underground Water*

of AMD's I, II and IX, which allows us to interpret possibilities of the existence of

From above interpretation of the aeromagnetic information, four ground magnetic sections were programmed with reading stations of the total magnetic field (TMF), during every 20 m, by using two magnetometers, one GEM-GSM-19 and another Geometrics G-856 A, to perform the measurements, in which they were corrected by daily and hourly drift and a residual was obtained by subtracting

*Map of the aeromagnetic domains (AMDs) interpreted in the isovalues contour of the magnetic reduced pole* 

*Map showing the location of the ground magnetic sections. The water well that appears to the north of the map* 

*where the population of La Dulcita is supplied with a yield of less than 1 L/s.*

*DOI: http://dx.doi.org/10.5772/intechopen.84322*

permeability in the zones of the contacts.

a zero-degree polynomial from the TMF.

**4.2 Ground Magnetometry**

*The Magnetometry—A Primary Tool in the Prospection of Underground Water DOI: http://dx.doi.org/10.5772/intechopen.84322*

of AMD's I, II and IX, which allows us to interpret possibilities of the existence of permeability in the zones of the contacts.
