(kup’ l∂ng) Interaction between systems. 1. A device for fastening together, as a plug for connecting electrical cables. 2. Aspects that affect energy transfer. Thus the ‘‘coupling of a geophone to the ground’’ involves the quality of the plant (how firmly the two are in contact) and also considerations of the geophone’s weight and base area, because the geophone-ground coupling system has natural resonances and introduces a filtering action. 3. The type of mutual electrical relationship between two closely related circuits. An ac-coupling would exclude dc voltages by employing a series capacitive element. A dc or direct coupling may allow all components to pass, or it may exclude higher frequency signals by using a capacitive element shunted across the inputs. Capacative coupling may occur because of mutual capacitive impedance, as between the wires in IP circuits or between a wire and ground. Inductive coupling occurs because of mutual inductive impedance, such as between grounded IP transmitter and receiver circuits, especially at higher frequencies, greater distances, or lower earth resistivity. This may give rise to false IP anomalies. Also called electromagnetic or EM coupling. Resistive coupling in IP surveying is caused by leakage between wires, between a wire and ground, or through the resistance of the ground itself between two grounded circuits.