INTEGRATED GEOPHYSICAL APPROACH VIA JOINT INVERSION OF LONG-OFFSET AND CENTRALLOOP TEM DATA: CASE STUDY FROM THE PEREKISHKUL, AZERBAIJAN

Mud volcanism is commonly observed in Azerbaijan and the surrounding South Caspian Basin.
This natural phenomenon is very similar to magmatic volcanoes but differs in one considerable aspect:
Magmatic volcanoes are generally the result of ascending molten rock within the Earth’s crust,
whereas mud volcanoes are characterized by expelling mixtures of water, mud, and gas. The majority
of mud volcanoes have been observed on ocean floors or in deep sedimentary basins, such as those
found in Azerbaijan. Furthermore, their occurrences in Azerbaijan are generally closely associated with
hydrocarbon reservoirs and are therefore of immense economic and geological interest. The broadside
long-offset transient electromagnetic method and the central-loop transient electromagnetic method
were applied to study the inner structure of such mud volcanoes and to determine the depth of a
resistive geological formation that is predicted to contain the majority of the hydrocarbon reservoirs
in the survey area. One-dimensional joint inversion of central-loop and long-offset transient electromagnetic data was performed using the inversion schemes of Occam and Marquardt. By using the joint
inversion models, a subsurface resistivity structure ranging from the surface to a depth of approximately 7 km was determined. Along a profile running perpendicular to the assumed strike direction,
lateral resistivity variations could only be determined in the shallow depth range using the transient
electromagnetic data. An attempt to resolve further two-dimensional/three-dimensional resistivity
structures, representing possible mud migration paths at large depths using the long-offset transient
electromagnetic data, failed. Moreover, the joint inversion models led to ambiguous results regarding
the depth and resistivity of the hydrocarbon target formation due to poor resolution at great depths
(> 5 km). Thus, 1D/2D modelling studies were subsequently performed to investigate the influence of
the resistive terminating half-space on the measured long-offset transient electromagnetic data.