Darrell Coles

Electromagnetic cave-to-surface mapping system

This paper presents the principle, design, construction, and methodology for an electromagnetic (EM) system to be used in the detection/location and mapping of underground cavities using surface measurements. The EM instrument consists of a loop-loop transmitter/receiver system with the transmitter placed inside the cavity. The transmitters position and depth are determined by analyzing the shape and distribution of the transmitted field on the surface. From the perspective of a cylindrical coordinate system, the vertical component of the through rock transmitted magnetic field peaks at the point where the transmitter and receiver are vertically collinear. On the other hand, the horizontal component reaches a minimum at this point. Based on these observations, a procedure is presented and tested that efficiently locates the position as well as the depth of the transmitter. A physical model for the system was developed and compared to the results of calibration experiments, with very good agreement. The model allows the study of different responses for EM waves/fields propagating through a homogenous Earth of different electrical characteristics and therefore enables several type-curves to be generated that aid in the development of an optimal system.

Minimization of self-potential survey mis-ties acquired with multiple reference locations

Self-potential (SP) surveys often involve many interconnected lines of data along available roads or trails, with the ultimate goal of producing a unique map of electric potentials at each station relative to a single reference point. Multiple survey lines can be tied together by collecting data along intersecting transects and enforcing Kirchhoff’s voltage law, which requires that the total potential drop around any closed loop equals zero. In practice, however, there is often a nonzero loop-closure error caused by noisy data; traditional SP processing methods redistribute this error evenly over the measurements that form each loop. The task of distributing errors and tying lines together becomes nontrivial when many lines of data form multiple interconnected loops because the loop-closure errors are not independent, and a unique potential field cannot be determined by processing lines sequentially. We present a survey-consistent processing method that produces a unique potential field by minimizing the ...