John Richard Tulett

The Relationship between the Signal-to-Noise Ratio of Downhole Data and Vibroseis Source Parameters

The signal-to-noise ratio of vibroseis data is theoretically proportional to the force of the vibrator multiplied by the square root of the product of: the sweep length and the number of sweeps. These relationships are confirmed using both simple synthetic and real downhole data. Other noise sources are, however, likely to be much stronger than the ambient noise and the ability to overcome or remove their effect may not be reflected by the theoretical relationship. Using this relationship we compared different source parameters that could be combined to produce equivalent signal-to-noise ratios. These comparisons showed that the most effective way to improve vertical seismic profile (VSP) productivity, and therefore reduce survey cost, is to increase the drive-level or force of the vibrator because that is the only factor that lies outside the square root sign in the relationship.

New Truck Vibrator for VSP Surveys, Extending the Frequency Bandwidth

On land, the preferred seismic energy source for Vertical Seismic Profile (VSP) surveys is the seismic vibrator, a special vehicle mounted, mobile energy source. Seismic vibrators come in a variety of shapes and sizes, with truck-mounted vibrators the most common vehicle type during the early years of vibroseis surveys. Truck-mounted vibrators are particularly useful for VSP surveys because the vehicle can be driven directly to the well site, but over the years the surface seismic industry has converted to buggy vibrators due to the buggys excellent mobility in off-road terrains. This has left a gap in development of truck-mounted vibrators. With petroleum exploration occurring in deeper wells, more complex sub-surface formations, and with steadily higher expectations for the results of VSP operations, there is a need for higher amplitude, and wider bandwidth seismic vibrators to provide critical geophysical information of petroleum reservoirs. In oilfield operations a key objective is reducing the safety risk during vibrator transport to and from the wellsite. To properly acquire a VSP survey the critical geophysical objectives are:  Improving seismic data quality  Widening the frequency bandwidth  Delivering consistent and repeatable sweeps (essential for 4D surveys) In this paper we describe how we met these challenges by developing two new VSP truck vibrators that offer a new level of safety, data quality and efficiency. The new truck vibrators for VSP surveys provide high output, wide bandwidth with extended low frequencies, low distortion data, together with purpose built carriers which improve safety and logistics for wellsite operations. The paper will provide actual borehole seismic data examples comparing a conventional VSP vibrator vs. the new extended bandwidth VSP vibrators.