A method for monitoring a drilling operation in a subterranean formation, wherein the drilling operation is performed using a drill bit on a drillstring and the drill bit generates seismic and electromagnetic noise as it penetrates the earth to form a wellbore, the method comprising: while performing a drilling operation, receiving, from a first sensor array (26, 28), one or more seismic signals caused by the drilling operation in the subterranean formation, wherein the first sensor array (26, 28) comprises one or more electromagnetic sensors and one or more seismic sensors, wherein the one or more seismic signals are received by the one or more seismic sensors of the first array (26, 28); receiving, from the first sensor array (26, 28), one or more electromagnetic signals generated by an electroseismic or seismoelectric conversion of the one or more seismic signals caused by the drilling operation in the subterranean formation, wherein the one or more electromagnetic signals are received by the electromagnetic sensors of the first sensor array (26, 28); determining a property of the drillstring and, optionally, of the subterranean formation based, at least in part, on the seismic signals and the corresponding electromagnetic signals received from the first sensor array (26, 28); characterized in that the first sensor array (26, 28) is located on or near the surface of the earth and is arranged to monitor the drilling operation and in that the step of determining a property of one or more of the drillstring and the subterranean formation based, at least in part, on the seismic signals and the corresponding electromagnetic signals received from the first sensor array (26, 28) further comprises: determining a drill bit location based, at least in part, on the seismic signals and the corresponding electromagnetic signals received from the first sensor array (26, 28). while performing a drilling operation, receiving, from a first sensor array (26, 28), one or more seismic signals caused by the drilling operation in the subterranean formation, wherein the first sensor array (26, 28) comprises one or more electromagnetic sensors and one or more seismic sensors, wherein the one or more seismic signals are received by the one or more seismic sensors of the first array (26, 28); receiving, from the first sensor array (26, 28), one or more electromagnetic signals generated by an electroseismic or seismoelectric conversion of the one or more seismic signals caused by the drilling operation in the subterranean formation, wherein the one or more electromagnetic signals are received by the electromagnetic sensors of the first sensor array (26, 28); determining a property of the drillstring and, optionally, of the subterranean formation based, at least in part, on the seismic signals and the corresponding electromagnetic signals received from the first sensor array (26, 28); characterized in that the first sensor array (26, 28) is located on or near the surface of the earth and is arranged to monitor the drilling operation and in that the step of determining a property of one or more of the drillstring and the subterranean formation based, at least in part, on the seismic signals and the corresponding electromagnetic signals received from the first sensor array (26, 28) further comprises: determining a drill bit location based, at least in part, on the seismic signals and the corresponding electromagnetic signals received from the first sensor array (26, 28).