Improving the Low-Frequency Content of Land VSP Data Acquired using a Vibroseis Source

The targets for VSP surveys are becoming more challenging, being deeper and overlain by more complex sub-surface formations. Coupled with this is a move to more complex acquisition methodologies, 3D VSPs in particular, and a desire for wider bandwidth data, in particular enhanced low-frequency content. As the preferred source for land VSP surveys is the seismic vibrator there is a requirement for vibrators to be able to emit lower frequencies than has previously been possible. Emitting such frequencies requires a sweep design methodology that takes into account the mechanical, hydraulic and control electronics limitations of the vibrator. Through measurements made both at the vibrator and downhole we show that Maximum Displacement Sweeps can be successfully employed to considerably increase bandwidth.