Investigation of targets of the pipe type near Mirny city (Sakha (Yakutia) Republic, Russia ) by the VECS method.
The works were conducted under the contract with NIGP AC “ALROSA”. Within the framework of these investigations over the dense network 50*50 m, the components dBz/dt, the horizontal component dBfi/dt, and electric component Er were measured. As a result of these works, we were able to localize a pipe, which is weakly contrasting target that is not practically distinguished by other methods of electrical prospecting.
Before fitting a geological heterogeneity configuration, we have distinguished two zones with the maximum signals. The first zone of maximum signals was distinguished at times 0.545 ms, 0.780 ms, and 1.058 ms that corresponded to the target situated nearer to the Earth’s surface. The second zone of maximum signals was distinguished at 1.965 ms and 2.429 ms and it corresponded to the target situated deeper. We also have taken into account that with increasing time, the signal maximum would not be above the target center, but some farther from the CED center. We supposed that a signal at times 1.431 ms corresponded to the intermediate position between the first and second zones. In accordance with these zones, we preset two targets corresponding to the first and second zones. After primary calculations had been conducted, we located the first and second targets at depths from 10 to 60 m and from 60 to 500 m, respectively. The distinguished targets are given in Fig. 1.
Fig. 1. Areal (normalized) measurements of dBfi/dt component with account for the local topography. Times 0.545 ms, 0.780 ms, 1.058 мс, 1.431ms, 1.965ms, and 2.429 ms. Outlines of the first and second targets selected as initial approximation are shown with green color in the plan.
Given in Fig.2 is the distribution of a signal for the fitted target model and the dBfi/dt component at various times. Target planes from which the model is composed are also shown in Fig. 2. The targets marked with light-blue, yellow, red, and blue colors are located at the depths: from 10 m to 30 m, from 30 m to 60 m, from 60 м to 140 m, and from 140 m to 500 m, respectively. The calculations demonstrate that we have data on a medium up to the depth about 300 m. As we can see, depths of targets in the fitted model are somewhat changed, the arrangements of the targets being also changed. Nevertheless, the original arrangement of the targets assigned by the field data before model fitting turns out to be good as an initial approximation.
Fig 2. Areal (normalized) calculations results for the fitted model of the dBfi/dt component at times 0.531 ms, 0.795 ms, 1.073 ms, 1.444 ms, 1.943 ms, and 2.365 ms. The targets marked with light-blue, yellow, red, and blue colors are located at the depths; from 10 to 30m, from30 to 60 m, from 60 to 140 m, and from 140 to 500 m, respectively
After a model had been fitted by the dBfi/dt component, we checked the result by the dBz/dt component. Given in Fig. 3 are areal (normalized) dBz/dt components with account for corrections for the local topography. Fig. 4 demonstrates areal (normalized) calculation results for the fitted model of the dBz/dt component at various times. The targets marked with light-blue, yellow, red, and blue colors are located at the depths: from 10 m to 30 m, from 30 m to 60 m, from 60 м to 140 m, and from 140 m to 500 m, respectively. No disagreements in observed signals were recognized. Signals of the component completely substantiated our inference that the fitted model is consistent with field signals.
Fig 3. Areal (normalized) measurements of the dBz/dt with account for local topography. Times 0.545 ms, 0.780 ms, 1.058 ms, 1.431 ms, 1.965 ms, and 2.429 ms.
Fig 4. Areal (normalized) calculations results for the fitted model of the dBz/dt at times 0.531 ms, 0.795 ms, 1.073ms, 1.444 ms, 1.943 ms, and 2.365 ms. The targets marked with light-blue, yellow, red, and blue colors are located at the depths; from 10 t0 30m, from30 to 60 m, from 60 to 140 m, and from 240 to 500m, respectively.
Conditions complicating our work were as follows:
1) Weak contrast between resistivity of the host medium and a pipe being investigated. According to the TEM (loop-loop configuration) method, the host medium resistivity was about 70 ohm*m and the pipe resistivity was about 40 ohm*m. Our signal was sensitive to the pipe. According to the results of one-dimensional inversion of Er signals, we determined the host medium resistivity to be 150 ohm*m that is more than twofold as compared with the host resistivity acquired with the help of transient electromagnetic method. The increase in the host resistivity acquired by the VECS method is explained by the fact that when working with vertical currents, the vertical resistivity plays significant role, whereas when working with a source of the loop type, only the horizontal resistivity plays a role.
2) The presence of topography. There was an elevated locality of 40 m in height in the area. If it considered that the target begins from depths of 10 m, then this circumstance could essentially complicate the picture. We managed to take into account the effect of topography. Given in Fig.1 are field signals with account for the topography effect.
Fig. 5 presents 3D visualization of the dBfi/dt signal acquired under field conditions and calculated for the fitted model. The field dBfi/dt signal with account for correction for topography is represented with light-blue color, and the dBfi/dt signal calculated for the fitted model is shown with yellow color, whereas the fitted model is represented as orange dots.
Fig. 5. Three-dimensional visualization of a signal dBfi/dt acquired under field conditions and calculated for the fitted model. The field dBfi/dt signal with account for the topography is represented with light-blue color and the dBfi/dt signal calculated for the fitted model is shown with yellow color, whereas the fitted model is represented as a set as orange dots.
Conclusion.
1) Making use of dBfi/dt and dBz/dt components, we managed to distinguish two targets, which were located in the immediate vicinity from one another (the distance between the target centers was about 300 m ) and even they are partly overlapped.
2) We have executed comprehensive three-dimensional inversion making use of direct problems of three-dimensional simulation.
3) The weak contrast between resistivities in the host medium and in the studied target makes it impossible to distinguish qualitatively a target when working with classical sources. When exploration is performed with the VECS method, the target was fully manifested also due to the stronger contrast between vertical resistivities in the target and host medium.
Booklets
Electrical prospecting by the method of vertical electric current sounding (VECS) as applied to ore targets. 3.0 Мb, pdf-file