Results of Mathematical Simulation of Transient Processes for the Sea Shelf Conditions

摘要

Electrical exploration measurements in water areas and on land are carried out to study the electromagnetic (EM) properties of geological formations. A distinctive feature of aquatic electrical exploration is associated with the specific influence of a water column. Numerical calculations of the EM signal of the transient process for electrical lines in the axial region of the source under the conditions of marine waters with a depth of 50 to 250 m are presented in order to demonstrate how an induced polarization (IP) signal manifests itself in a transient process signal on different setups, to identify differences in the manifestation of galvanic induced polarization (GIP) and induction-induced polarization (IIP) in a transient process signal, and to substantiate these differences. The influence of the setup dimensions on the manifestation of IP during a transient process is studied by analyzing a change in the transient signal (Delta U(t)), the final difference of the transient process signal (Delta U-2(t) and transform P1(t) (ratio of these values) for a horizontal electrical setup with a source (AB) 50 to 2000 m in length, a three-electrode measuring line (M1M2M3) 50 to 2000 m in length, a distance between the source centers and the measuring line M1M3 (spacing, r) from 100 to 4000 m. Some of these parameters arc used in differential-normalized electrical prospecting (DNME). The comparison of Delta U(t) and Delta U-2(t) and their transforms in conducting and conducting-polarizable models under the same conditions is performed. The setup is placed on the surface and inside a conducting medium (a sea shelf water column) with a conducting polarizable base (geologic medium (ground) covered with a water layer). The polarizability of the base is taken into account by introducing a frequency-dependent resistivity using the Cole-Cole equation. It is shown by the calculations performed that the transient process components associated with the formation of an FM field and with GIP and IIP manifest themselves in dissimilar ways on differently sized setups at various depths. In a water area, IP manifests itself in two ways, being associated with both galvanic and eddy currents. In previous practical measurements, IIP was considered to be associated with interference, but this signal is simulated and can be regarded as information about IP. The factor influencing the IP manifestation in a transient process signal is a reduced setup height (h(Delta)), i.e., a distance between a setup and a sea bottom (polarizable base of the model) attributed to the AB line. Depending on the reduced setup height, the IP signal in transform P1(t) can manifest itself as an ascending branch at later times or appear as a descending branch passing into the negative values of P1. The pulse impact duration and the transient process measurements affect the contrasting manifestation of the polarizable base in the signal, but the measurements performed when the setup is being towed impose certain restrictions. The optimal parameters of the EM survey for IP studies should ensure a sufficient polarization range and the proper quality of measurements. The software used in the calculations was developed by OOO Sibirskaya Geofizicheslcaya Nauchno-Proizvodstvennaya Kompaniya.