ELECTROMAGNETIC SOUNDING OF THE KAPURDI LIGNITE DEPOSIT IN WESTERN RAJASTHAN, INDIA*

In the Kapurdi block of the Barmer lignite field in western Rajasthan, India, lignite occurs as a number of discontinuous seams of varying thickness up to a few m. The country rock is a succession of clay, sand and various members of the clay‐sand family. The overburden, comprising sand, Fuller's earth, bentonitic clay, clayey sand/sandy clay and clay, is thick and, electrically, highly conductive. Both its thickness (40–90 m) and conductivity (0.5–1.0 S/m) are highly variable. The lignite seams may occur anywhere in the clay‐sand sequence below this overburden.

A ten‐layered electrical model of the Kapurdi area was parameterized on the basis of galvanic resistivity sounding and core drilling results. This model was then validated by electromagnetic sounding measurements made with a 200 × 200 m square loop transmitter and energizing current of variable frequency (80 Hz to 11 kHz), using central induction sounding techniques. With a loop of this size, only formations in the overburden could be mapped. Subsequent computer modeling has shown that a larger loop of size, say, 800 m and a frequency band of 30–500 Hz are required to map formations below–as well as those within–the overburden. Even then only various clay‐sand interfaces can be mapped, i.e., only over‐burden thickness and the likely depth range of lignite seams (if they occur) can be estimated. Direct mapping of lignite seams is not feasible in practice. This is true of loop‐dipole, as well as central induction sounding. It is interesting that skin depth need not always be a factor limiting depth of exploration in electromagnetic prospecting.