Enhancing the Low-Frequency Content of Vibroseis Acquisitions with Maximum Displacement Sweeps: A Case History from Kuwait
Claudio Bagaini1*, Adel El-Emam2, and Ayman Shabrawi3
Several applications of the seismic method to hydrocarbon exploration and reservoir characterization would greatly benefit from the presence of energetic low frequencies in the acquired data. Some of these applications are deep imaging, imaging beneath high-velocity and highly absorbing formations, inversion of surface waves to characterize the near-surface elastic properties, velocity model estimation in geophysically complex areas and acoustic impedance inversion. Unfortunately, the generation and the accurate recording of low frequencies was and, to some extent, remains challenging in both marine and land seismic. In the case of land seismic acquisition, the growing use of accelerometers in surface and borehole seismic enables the acquisition, with an high signal-to-noise ratio of frequencies lower than 2 Hertz. On the source side, hydraulic vibrators, which are the most widely used onshore seismic sources, have their output energy limited at low frequencies by mechanical constraints such as the maximum reaction-mass displacement. Only low actuator forces can be used to drive the vibrators at these low frequencies, which in turn can yield extremely long sweeps in the absence of a design criterion or if a too-conservative one is used. In this presentation, we first analyse the reasons why the above-mentioned seismic applications do benefit from the presence of energetic low frequencies. We then focus on two of these applications, namely: (1) imaging beneath high-velocity formations, and (2) acoustic impedance inversion. We show the enhancements obtained during a recent point-receiver surface seismic survey over the Minagish field in Kuwait acquired using a sweep design method (maximum displacement sweep) developed to enhance the low-frequency content of land seismic acquisition.
AAPG Search and Discovery Article #90077©2008 GEO 2008 Middle East Conference and Exhibition, Manama, Bahrain