High-Resolution Seismic Imaging of the Geologically (and Politically) Complex Coastal and Nearshore Zone [or What Can You Get With 2-kJ or Less?]
The region between the modern and glacial lowstand shorelines is one of the most complex geological environments on Earth. Changing sea level between glacial and interglacial epochs create complex depositional environments with alternating erosional and depositional cycles. In California, plate margin tectonic activity deforms these sedimentary sequences and may elevate or submerge the coastal platform through geologic time. Critical resources including hydrocarbons and freshwater (or saltwater for desalination) lie beneath the continental shelf and coastal plain. Deep water exploration offshore and land-based exploration onshore have proven successful in finding such resources around the globe, yet the coastal zone between these two environments remains as one of the biggest exploration data gaps. Historically, seismic exploration in the coastal area often involved single-channel analog systems using various energy sources including airguns, waterguns, sparkers, boomers and subbottom profilers. Development of the common-depth-point (cdp) method with multichannel streamers helped to obtain deeper imaging in shallow water by attenuation of the water bottom multiple. Political pressure from environmental concerns limit source energy for seismic exploration within the California State Lands (3-mile limit) to maximum of 2-kilojoules; no airgun systems are allowed. Consequently, new seismic data acquisition in the coastal zone requires innovative designs to maximize the retrieval of sufficient seismic wavefield energy to image deeper targets. Several examples of high-resolution seismic imaging projects performed in coastal and nearshore California areas will be described to demonstrate the capabilities of low-energy source systems with digital hydrophone streamer and geophone receiver arrays. Imaging targets range from active faulting and deformation to nearshore alluvial basins and paleochannels. In some areas, subbottom imaging below 1.5 sec two-way travel time (5000 ft) is achieved in 2-D profiles. Steep dips associated with complex geological structure requires fine spatial sampling for migration. State-of-the-art high-resolution 3-D acquisition and processing provides a powerful tool for accurate imaging of complex geological systems in the coastal and nearshore zone.
AAPG Search and Discovery Article #90162©2013 Pacific Section AAPG, SPE and SEPM Joint Technical Conference, Monterey, California, April 19-25, 2013