MOTIONAL NOISE IN TDEM DATA: EM61-MK2 EXAMPLES AND IMPLICATIONS

Detection and discrimination of low amplitude targets during time domain electromagnetic (TDEM) surveys is frequently dependent on noise levels resulting from site-specific and<br>implementation-specific factors which may not be apparent until you are on-site. Motional noises, which are generated by the movement of the instrumentation and the operator during data collection, are an underappreciated source of noise. These noises can be difficult to recognize and difficult to compensate for by processing, but can be mitigated by careful data acquisition procedures, and realistic assessment of the field conditions and achievable data quality. The Geonics EM61-MK2 (EM61-MK2) is the most commonly fielded TDEM system. Test and<br>survey data showing various motional noises and signal degradations in EM61-MK2 data are presented and discussed. The measured response of induced fields from anomalies varies by the pitch/roll/yaw of the coils, often resulting in significant understatement of the actual/optimal response and distortion of anomaly geometry. Motional noise comparable to target responses can be generated by abrupt changes in the position/velocity of the coils. Even in well-functioning instruments, equipment noise can be generated by metal imbedded in the wheels and by cable movement. Lastly, field personnel may generate noises by their actions and choices during data collection. Site features which affect motional noise include terrain roughness, slope, and obstacles. These may not be addressable by site preparation. Many motional noises can be mitigated by the form factor of the instrumentation, regular instrumentation inspections and testing, modification to survey procedures, minimizing tow speeds, and by increasing operator awareness.