Description of VECS method
The method of vertical electric current sounding (VECS) is taken to mean such electrical investigations in performance of which a circular electric dipole (CED) is used as a source of electromagnetic field. The CED configuration is formed by several radial grounded horizontal lines (usually 8) which are simultaneously energized by pulse current of the same form and amplitude. Measurements are carried out over an arbitrary network around the field source (for the long CED radius and inside a source) with the aim of recording a dense areal transient response (data cube).
A magnetic induction velocity is usually measured by means of compact induction receiver. Recently, measurements of an electric field by grounded receiving lines are also of great interest.
CED excites transient process of a field of the ТМ-type in the ground. As opposed to this, a loop excites pure TE process and a horizontal line excites process of the TE-type. The magnetic field being excited from CED on the Earth’s surface is absent above a horizontally-layered medium. This feature is most important for practical application in electrical prospecting. Inasmuch as a response from a host medium is absent, then the presence of a signal itself is indicative of the presence of 3D disruption in the host medium and characteristics of this signal are governed by features of the target and host medium. This response from an 3D body is well localized, i.e. the measured signal is mainly governed by targets beneath the measurement point of a magnetic field. The consequence of this is obvious in the recorded response that shows significant suppression of the effect from other heterogeneities located between the source and receiver of the field. It should be noted that using the 1D approach in interpreting the results of magnetic components is meaningless; in this case the only 3D approach is needed. By the way, in view of the character of the response related only to heterogeneity, areal pictures of VECS signals possess a high imaging capability and sometimes they are undoubtedly valued by customers. The other interesting feature of the CED field is that the local target anomalies of conductivity in a response from vertical magnetic induction velocity (we call this as dBz/dt component) and in the dBfi/dt component manifest themselves in different ways. In the areal representation of a signal, the location of the target center coincides with the boundary of sign change of the dBz/dt component. In this case, the maximum of the dBfi/dt component coincides with the center of 3D target. That is, we can determine well the center of the local target via the dBz/dt signal, and target boundaries are well defined via the dBfi/dt signal. The great support gets measurement of the electric gradient Er.The Er component is normal, i.e., it is the response from an one-dimensional host medium in working with CED as a source. Nevertheless, the character of a signal is sharply changed when a local target is situated in a horizontally-layered medium. As a rule, sharp change in the character of coincides with the target boundary nearest to the CED.
Next note one more important feature of the CED field as opposed to the fields of conventional loop and horizontal line: exploration with CED makes sense since that supposes a dense survey network. When works are carried out using the VECS method, a signal varies rather faster in passing point-to-point; this is because the signal characterizes mainly a medium under the measurement point rather than an averaged medium between the field source and receiver.
Procedure of field works
Prior to start of measurements, the power supply setup CED is installed. The source radius corresponds to the depth and area of investigations. In performing the most of works, the radius was stipulated by the area being studied. Qualitative measurements are usually carried out at a distance up to five CED radii, although in our practice we met localities where we successfully performed measurements at the distance 6-7 radii from the CED center. It is implied that the geometry of a source is valid and currents in lines are equalized, the currents flowing in the pulse regime. Automatic system for maintenance of equal currents in lines is just the specific VECS equipment. Depending on a posed problem, every measuring group is equipped with one or more compact induction sensors, one or more measuring lines, and measuring devices for a transient signal. All receiving elements such as induction sensors, measuring lines, and measuring devices for a transient signal are standard elements applied in the method of transient processes (or in time-domain electromagnetic sounding (TDEM)). Each of groups with its own measuring system freely moves over the area using space-time satellite navigation to locate the field source. Thus, if the standard distance equal to five radii from the center of CED source is taken as a maximum distance, then the area of ~25 km2 may be studied rapidly when the fixed source with the radius of 0.5 km2 is used.
In the processing program, a measured the and components are normalized by some function of the distance from a measurement point to the source center, and the time is transformed with respect to the depth via the formula for a skin layer. Such constructions allow one to detect 3D target being studied directly during field measurements practically without any processing. Display of a target in an areal field measurements of the and components is somewhat displaced from a real target in the plan, but nevertheless, the display visualizes the target being studied very well.
We also execute comprehensive three-dimensional interpretation of signals using three-dimensional direct problems. If the target is complicated or weakly contrasting, then, using even direct problem of three-dimensional simulation, it is not always possible to fit qualitatively a local inclusion having data only for one component. For this reason, we perform measurements for complicated targets up to three components of a electromagnetic field such as , , and . This data set makes it possible to accurately reveal and identify a local conductivity target via field data by comparison and on the strength of all the evidence.