Well logging programs are usually designed to measure petrophysical properties within the reservoir and the immediate surrounding formations. Rarely, if ever, is much of the overburden logged. As more sophisticated surface-measured geophysical methods are used, properties of the overburden are becoming more important (fig. 1). One method that is becoming more prevalent, 4C OBC seismic, can benefit from logging formations from surface continuously down to the reservoir. In preparation for the processing and interpretation of a major acquisition of 4C surface seismic run to illuminate a carbonate reservoir within Idd El Shargi North Dome field located offshore Qatar (fig. 2), a specialized logging program was devised and acquired on a well in the field to obtain formation elastic properties of compressional and shear velocity, including HTI anisotropy information, continuously from the sea floor down to the reservoir.
Particularly challenging was the acquisition of well logs and borehole seismic in the interval just below sea bottom. A drilling methodology was devised to allow a relatively small diameter pilot hole to be drilled and logged prior to reaming out and setting surface conductor pipe. Logging tools were specially set up such that continuous valid measurements would be made to acquire this expected low velocity and density interval. Other logging intervals were in zones difficult to log in open hole so cased hole contingencies were implemented.
A "walkaround" borehole seismic survey was acquired in the reservoir section to obtain "reservoir scale" anisotropy information comparable to surface seismic and was complimented with cross-dipole sonic log information, which is a more localized anisotropy measurement.
Elastic wave properties of rock, specifically density plus compressional and shear velocities, provide useful information about mechanical and reservoir properties as well as drilling efficiency and production performance. Historically well log data has provided density and compressional information but not shear. Surface seismic and well log sonic data were almost exclusively confined to measuring compressional wave attributes. Shear waves were either ignored, not measured, or considered as noise and suppressed in acquisition and processing.
Borehole shear wave measurements became more common in the late 1980's and were more or less standard by the mid 1990's. Surface seismic shear wave measurements have been far less common due to the difficulties in acquisition, processing and interpretation of the data. Offshore, conventional surface seismic acquisition using air guns is not capable of generating direct shear waves at the source, nor will it record shear waves at the hydrophone receivers because both source and receiver are located in a liquid. To overcome this limitation, 3-component geophone-clusters (along with a hydrophone) are placed in a cable that is positioned on the seafloor (called a 4C OBC for 4 Component Ocean Bottom Cable). The 3-component geophone will then be able to record shear waves travelling up to the seafloor. Though the source generates only compressional waves, compressional to shear mode-conversions, either to upgoing shear at the reflectors or downgoing shear at shallower events and reflect as shear, are used (fig. 3). This method is called Converted Wave Seismic or CWS.
Processing CWS data is far less robust than conventional compressional data and there are several areas where additional information from borehole data can aid in acquisition, processing and interpretation.