Тhe task of searching for kimberlite pipes in covered areas of the Yakutia kimberlite province is very difficult due to the significant heterogeneity of the rocks overlying kimberlite pipes. The overlying strata of terrigenous sediments contain rocks of the trap complex (dolerite sills, tuff bodies). We consider the results of the controlled source radiomagnetotelluric (CSRMT) soundings in Yakutia/Siberia. Due to the great thickness of the overlying rocks (near 100 m) and the relatively small horizontal sizes of kimberlite pipes (80–200 m), they cannot confidently be detected directly. An additional difficulty in identifying pipe anomalies is the presence of a layer of conductive carbonaceous siltstones in the overlying strata. Therefore, the main aim of the CSRMT surveys was the study of overlying rocks and the search for indirect indications of the presence of pipes. Possibilities to study the structure of dolerite sills located within overlying sediments and to map the top edge of hosting carbonate rocks are demonstrated using the CSRMT data. The pinching out of dolerite sills above pipes («windows in traps») and the lowering of the top edge of hosting rocks at pipes can be considered as indirect indications of the presence of pipes.
The common approach to explore sub‐seafloor electrically resistive targets (e.g. hydrocarbons, gas hydrates, fresh groundwater, etc.) is based on using galvanic transmitter‐receiver arrays either in frequency or in time domain. Such arrays utilize vertical electric currents and provide better resolving capability than inductive arrays in case of sub‐horizontal relatively thin targets located within roughly 1‐D environment. If, however, a sub‐seafloor resistive target is located in the transition zone at distances up to a few kilometers from a shoreline, the 2D sea‐coast resistivity contrast significantly affects the resolving capability of the measurements. An extensive multi‐dimensional modeling supported by numerous offshore measurements carried out in time domain showed that short‐offset broadside AB‐Bz array (horizontal electric dipole transmitter — vertical magnetic dipole receiver) demonstrates much better resolving power compared to all other arrays. This effect only takes place at short offsets and if the receiver coil is located between the transmitter dipole and the coast. If the coil is located at the other side of the transmitter dipole, the signal lacks the resolving capability almost entirely. Although the measurements were only conducted to explore a shallow target (fresh sub‐seafloor groundwater body), calculations show that the same phenomenon also exists in case of deep targets (e.g. hydrocarbons).
Galvanic transmitter-receiver arrays commonly are used in marine controlled-source electromagnetic (CSEM) exploration of electrically resistive targets such as hydrocarbons, gas hydrates, etc. These arrays utilize vertical electric currents and, as a result, are expected to provide better resolving capability for exploring subhorizontal resistive structures than arrays including horizontal coils. If, however, a subseafloor resistive target is located within a transition zone at distances of up to a few kilometers from the shoreline, the 2D sea-coast resistivity contrast significantly affects the resolving capability of the measurements. An extensive multidimensional modeling supported by numerous offshore measurements showed that the inductive array consisting of a horizontal electric dipole transmitter and a broadside vertical magnetic dipole (horizontal coil) receiver exhibits much better resolving power in time domain compared to all other arrays but those with a vertical electric dipole. This effect takes place only if a short offset receiver coil is located between the transmitter dipole and the coast. If the coil is located at the seaside of the transmitter dipole, the signal lacks the resolving capability almost entirely. At large offsets, the resolving capability of the measurements is relatively low at both sides of the transmitter dipole. Although actual field measurements were conducted only to explore a shallow target (fresh subseafloor groundwater body), calculations show that the same phenomenon exists in case of deep targets (e.g., hydrocarbons).
