Extending Geobandwidth Using the Multipulse Configuration

Abstract

Measurement bandwidth is an important feature of a geophysical system. Bandwidth allows detection of very resistive features (such as some kimberlites) and very conductive targets (like massive sulphides). In electromagnetic systems, bandwidth is not simply the sample rate of the data acquisition system or the earliest time channel, but also depends on transmitter spectrum, distance to target and processing. Optimising a system to detect a feature or measure a specific signal requires design considerations and trade-offs for different targets. The choice of excitation waveform in electromagnetic systems is one such trade-off. A square-pulse allows high-frequency energy to be excited, but, because of electronic limitations, has only limited dipole moment. A half-sine waveform efficiently generates energy at the base frequency and first few odd harmonics (low-frequency energy) and less high-frequency energy. Here, we describe the Multipulse configuration, an option on the Helitem system which employs both a half-sine and a trapezoid waveform to efficiently generate high- and low-frequency energy. Using survey data, we show the resolution power of the combined system compared to a single waveform. The combined data is better able to resolve near-surface features and deep structure than data from either waveform alone.