**4.2 Ground Magnetometry**

*Magnetometers - Fundamentals and Applications of Magnetism*

general shows preferential aeromagnetic alignments in an N-S direction, also exist-

*Map where the magnetic alignments are observed based on the isovalues contour of the first vertical Derivativ* 

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

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

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

ing in the NE–SW direction, with few showing NW-SE direction.

*upwards continuation 250 m from the reduced to the pole magnetic field.*

of La Dulcita outside the ejido boundaries.

**34**

**Figure 7.**

**Figure 6.**

*Luis Potosí, Mexico.*

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 a zero-degree polynomial from the TMF.

#### **Figure 8.**

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

### **Figure 9.**

*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.*

Two of the sections had NW-SE orientation and two NE–SW (**Figure 9**) with the population of La Dulcita being in the central part of these profiles.

The magnetic section 1 **(Figure 10**) displays four terrestrial magnetic domains (TMD): first station 0 to 54 was characterized by a series of magnetic anomalies related to short wavelengths (20–40 m), high frequencies and amplitudes of 160 nT. It was geologically correlated with a highly fractured zone, while the horizontal gradients give values of up to 11 nT/m. Second, TMD 2 is located between stations 55 and 78, and it is defined by presenting a normal magnetic field, where no abnormal areas are observed. Third TMD 3 is located between stations 79 and 87 and shows an anomalous zone limited by two magnetic anomalies that have amplitudes of 33 and 65 nT and horizontal gradients of 2.6 and 6 nT/m, respectively. Geologically, it is correlated with an area of medium fracture possibilities. The last, TMD 4 is limited between stations 88 and 135, in general it shows a discretely disturbed magnetic field where it is not considered with the possibility of associating at depth with permeability.

The magnetic section 2 is located towards the E portion of La Dulcita (**Figure 9**), it presents five TMD's (**Figure 11**), the first one limited between stations 0 and 32 shows a normal behavior of the RMF, where magnetic anomalies are distinguished.

#### **Figure 10.**

*Ground magnetic profile 1, with a NW-SE orientation. At the upper part, the residual magnetic field (RMF) is plotted (red); the horizontal gradient of the RMF is plotted at the lower part (blue), and at the bottom a qualitative interpretation of the percentage of probabilities of association with fracturing in the underground is shown. NF, not fractured; F, fractured.*

#### **Figure 11.**

*Ground magnetic profile 2, with a NE–SW orientation. At the upper part the residual magnetic field (RMF) is plotted (red); the horizontal gradient of the RMF is plotted at the lower part (blue), and at the bottom a qualitative interpretation of the percentage of probabilities of association with fracturing in the underground is shown. HF, highly fractured; LF, light fractured; NF , not fractured.*

**37**

**Figure 12.**

*shown. F, fractured; NF, not fractured.*

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

The TMF 2 is located between stations 33 and 45 and does not show areas of high frequencies that can be correlated with fracturing effects at depth. The TMF 3 is located between stations 46 and 84, it is identified by presenting a magnetic response characterized by anomalies with short wavelengths (60–100 m), high frequencies and amplitudes of the order of 28–41 nT and horizontals gradients from 2.7 to 2.3 nT/m, respectively. It correlates an area with average possibilities that associating with the existence of secondary permeability. The TMD 4 is identified between stations 85 and 113 and has short wavelengths (20–80 m) high frequencies and amplitudes of 18 at 29 nT and horizontal gradients of 0.4–1.5 nT/m, respectively. They are geologically associated to an area with average possibilities of correlation with permeability in the underground. The TMD 5 is delimited between stations 114–133, characterized by showing short wavelengths (20–40 m), high frequencies and magnetization amplitudes of 54 nT up to 160 nT, with horizontal gradients of 6.5 nT/m up to 14.7 nT/m, is geologically correlated with an area of

The magnetic section 3 is located in the NW of La Dulcita (**Figure 9**) shows two TMD (**Figure 12**). The first domain is located between stations 1 and 16 is identified by presenting a series of magnetic anomalies. These are characterized by short wavelengths (20–100 m), high frequencies and amplitudes from 32 to 107 nT and horizontal gradients from 2.9 to 7.6 nT/m, which correlates with average possibilities of being associated in the underground with fracturing. The second TMD is located from station 17 to 75 and shows a normal magnetic field where the possibil-

The magnetic section 4 located outside La Dulcita, the NW portion (**Figure 9**),

Two electrical sections (or profiles) of apparent resistivity, induced polarization and self-potential were made with the Schlumberger type electrode array (**Figure 14**), using two electrode spacings AB/2 = 100 and 200 m and a Syscal R-2 resistivity instrumental (**Figure 15**). The sections were made in the same directions as the magnetic profiles 1 and 2, which were showed more possibilities of associat-

*Ground magnetic profile 3, with a NE–SW orientation. At the upper part the residual magnetic field (RMF) is plotted (red); the horizontal gradient of the RMF is plotted in the lower part (blue), and at the bottom qualitative interpretation of the percentage of probabilities of association with fracturing in the underground is* 

shows two TMD (**Figure 13**), neither of them of interest to be associated with

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

strong fracture and permeability.

ity to correlate with fracturing at depth zero.

fractured zones in the underground.

ing with fracturing in the underground.

**4.3 Electrical methods**

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

The TMF 2 is located between stations 33 and 45 and does not show areas of high frequencies that can be correlated with fracturing effects at depth. The TMF 3 is located between stations 46 and 84, it is identified by presenting a magnetic response characterized by anomalies with short wavelengths (60–100 m), high frequencies and amplitudes of the order of 28–41 nT and horizontals gradients from 2.7 to 2.3 nT/m, respectively. It correlates an area with average possibilities that associating with the existence of secondary permeability. The TMD 4 is identified between stations 85 and 113 and has short wavelengths (20–80 m) high frequencies and amplitudes of 18 at 29 nT and horizontal gradients of 0.4–1.5 nT/m, respectively. They are geologically associated to an area with average possibilities of correlation with permeability in the underground. The TMD 5 is delimited between stations 114–133, characterized by showing short wavelengths (20–40 m), high frequencies and magnetization amplitudes of 54 nT up to 160 nT, with horizontal gradients of 6.5 nT/m up to 14.7 nT/m, is geologically correlated with an area of strong fracture and permeability.

The magnetic section 3 is located in the NW of La Dulcita (**Figure 9**) shows two TMD (**Figure 12**). The first domain is located between stations 1 and 16 is identified by presenting a series of magnetic anomalies. These are characterized by short wavelengths (20–100 m), high frequencies and amplitudes from 32 to 107 nT and horizontal gradients from 2.9 to 7.6 nT/m, which correlates with average possibilities of being associated in the underground with fracturing. The second TMD is located from station 17 to 75 and shows a normal magnetic field where the possibility to correlate with fracturing at depth zero.

The magnetic section 4 located outside La Dulcita, the NW portion (**Figure 9**), shows two TMD (**Figure 13**), neither of them of interest to be associated with fractured zones in the underground.
