VERY LOW FREQUENCY METHOD    

 Introduction:-

         VLF measures perturbations in the magnetic field of radio waves. The perturbations are cause by geologic subsurface features. Thus by examining the nature of the magnetic field's perturbations we can identify anomalous areas in the subsurface.

 

VLF is used in several applications. Aquifers and groundwater and can be located. Leachate can be identified for environmental investigations. Geotechnical applications include fault and overburden mapping. Also, likely mineral deposits can be discovered for mining surveys.

 

  VLF techniques utilizes a plane-wave radio signal (15-30 kHz) emanating from one of several worldwide radio transmitters used for submarine communications. Military transmitters provide radio signal coverage for all of North America. The magnetic field of the military's radio signal (primary field) causes electric currents to be induced in conductive geologic units or structures. Those eddy currents in turn create a secondary magnetic field. This secondary magnetic field interacts with the primary magnetic field emanating from the military transmitters creating one perturbed magnetic field. The perturbed field may be phase shifted and oriented in a different direction than the primary field depending on the shape or geometry of the conductor, the orientation of the conductor, and the conductivity contrast with the surrounding material (e.g., the host rock).

 

This perturbed magnetic field is measured by the VLF receiver. Because certain information is known about original magnetic field created by the military transmitters, perturbations can be assessed and attributed to subsurface eddy currents. Because these perturbations are caused be subsurface features, the nature of the perturbations enable us to deduce things about the subsurface.

 

All VLF instruments measure two components of the magnetic field or equivalently the “tilt angle” and ellipticity of the field. Some instruments also measure the third magnetic component and/or the electric field. The electrical field is measured by inserting two probes in the ground, spaced about 5 meters apart, and measuring the potential difference at the transmitter frequency. The electric field provides additional information about the overburden thickness and conductivity.

 

The advantages of VLF techniques are that a large area can be covered quickly and only a one person crew is required. This makes VLF very economical. However, although VLF can identify anomalous areas, it is limited in its ability to provide detailed information about the nature of the geologic bodies causing the anomalies. For these reasons, VLF is generally used a as a reconnaissance tool for identifying areas requiring further investigation.  

     

    The VLF Method is one of a group of geophysical methods that make use of electromagnetic fields to reveal objects located far beneath the surface. The letters VLF stand for Very Low Frequency. A worldwide network of high power VLF Stations was planned for marine navigation. The sites are so that at least two stations can be detected anywhere on the earth’s surface.

 

 

 

 

Where the signal from these stations is too weak the
TX27 portable VLF transmitter may be used to provide the signal.

 

 

 

 

 

 

 

Physical principles:-

                                    When the field emitted by a transmitter strikes a body having low electrical Resistivity, secondary circuits are created in the body. This is called induction. These secondary currents in turn create a secondary magnetic field that is opposed to the original field emitted by the transmitter. Nature is quite conservative- it tries to counteract buildup of the external field by creating its own secondary fielding the opposite direction.

                                     The receiver for detecting VLF signals measures a tilt angle and a quadrature component by means of two mutual perpendicular coils wound on ferrite cores.

 

WADI instrument:-

                                                                                                 

                              In 1987 ABEM unveiled its WADI Instrument, designed especially for VLF measurement .Even though the WADI Instrument seems quite simple, it contains a computer and a built-in memory, and it incorporates advanced technology.

 

Working:-

                              A WADI VLF Instrument records the ratio of the strengths of the vertical and horizontal fields at the ground surface. Since the primary field emitted by the transmitter is horizontal, it is evident that a normal reading will be zero. A deviation from the normal reading is called anomaly indication. Anomaly indications can be caused by many things other than geological objects. Cables, metal pipes, electrical fences and the like can cause very strong anomaly indications.

                                The WADI Instrument is designed so that anomaly indications in the real part are always in the manner such that  a positive bulge (peak) appears to the left of the subsurface structure, and a negative bulge appears to the right of it. In a simple case it is easy to locate the fracture zone. In complicated case that involves a number of subsurface bodies, it can be very difficult to ascertain the position of individual bodies, even for professional geophysicists.

 

 

Filtering:

                  Processing the measured VLF data using numeric algorithm can make it easier to interpret results. The filter coefficients are moved along the profile station by station and the six multiplication and and additions are carried out for each station. For this type of filtering, the distance between stations H0, H1, H2, … can be selected as desired.

                  The filtered curve can be considered a representation of secondary currents in the ground. More precisely, however filtering comprises a numeric algorithm. After filtering it is much easier to locate a fracture zone since we have to look at the highest point of the curve. The peak of the curve appears directly over the fracture zone.

 

Location of the field:-

The field chosen for my VLF profiling was the JNV University campus. 3 Profiles of length 200m each were chosen in the E-W direction and they were separated by 20m. Readings were taken at an interval of 10m.The reading includes the ratio of the strengths of the vertical and the horizontal fields on the ground surface.

                                   

Parameters to determine:-

                                        The parameters to be determined were –

1)      Fracture

2)      Conductivity

3)      Depth

 

Field data:-

Profile no:- 2001

                                                     TABLE 1

Distance(m)	Ratio	Distance(m)	ratio
0	0	110	-1.1
10	-1.2	120	1.1
20	1.2	130	-0.3
30	0.8	140	4.5
40	-1.05	150	-2.6
50	0.3	160	0.3
60	-0.8	170	-2.1
70	0.3	180	1.9
80	0.7	190	0.5
90	-1.8	200	-0.9
100	-1

 

 

 

 

 

Profile no:-2021

                                               TABLE 2

Distance(m)	Ratio	Distance(m)	Ratio
200	0	90	0
190	0.1	80	-0.5
180	1.7	70	0.9
170	0.7	60	0.2
160	-0.3	50	-0.6
150	0.3	40	1.1
140	0.2	30	-2.1
130	0	20	-2.2
120	0	10	-0.9
110	0.3	0	0.3
100	-0.3

                  

 

 

 

 

Profile no:-2041

                                                  TABLE 3

Distance(m)	Ratio	Distance(m)	Ratio
0	0	110	2.2
10	-1	120	-2.5
20	-1	130	-0.3
30	-1.3	140	1.8
40	-0.1	150	0.7
50	1.1	160	-0.8
60	-1.7	170	-4.7
70	0.5	180	0.3
80	-2	190	-0.9
90	0.9	200	-7.1
100	-2.2

 

 

 

 

 

 

Interpretation:-

                       Since a filtered real-part curve is always displayed by default we shall concentrate on the simple models and calculated VLF anomaly indications for them. The models consist of one or more very long water-bearing fracture zones, and the profiles were taken perpendicularly across them.

 

                           Most recently, the software used to interpret the VLF data is named SECTOR.VLF Data is fed to the software .The data is at first filtered. From the filtered data,i.e., from the ratio of strengths of the vertical and horizontal fields the real and imaginary components are separated and graphically plotted. From the graph the fracture zones are located on the profile where the real and imaginary components are exactly the mirror images of each other. It also displays the vertical cross-section showing the depth of the anomalous feature.

 

 

After feeding the data to the computer and on filtering (filtered data is shown in fig.4) a fractured zone has been found at a distance of about 140m from the starting point i.e. 2001E at a depth of around 40m (fig.-5).

 

 Inference: Presence of groundwater is expected.

 

 

Advantages:

· VLF is very useful in hard rock areas where a normal electrical method fails.

• No extra transmitte3r is required.

• The field is tangent to the antenna mast hence it is less erratic.

• Direction of field as well as signal strength is known.

• The signal level is fairly uniform.

• The field instrument (WADI) is light and conveniently made and readings are taken    rapidly.

 

Disadvantages:

 

• Depth of penetration being small, used only for shallow exploration work.

• It is not always possible to use a transmitting station approximately on a geologic strike in the area.

• The high frequency of the source is also an inherent weakness.

• The reception is best in the morning, but adequate all day.