Developments in exploration geophysics, 1975-1980
Developments in exploration geophysics, 1975-1980
Geophysics (August 1981) 46 (8): 1088-1099
Advances in exploration geophysics have continued apace during the last six years. We have entered a new era of exploration maturity which will be characterized by the extension of our technologies to their ultimate limits of precision.In gravity and magnetics, new inertial navigation systems permit the very rapid helicopter-supported land acquisition of precise surface gravity data which is cost-effective in regions of severe topography. Considerable effort is being expended to obtain airborne gravity data via helicopter which is of exploration quality. Significant progress has also been made in processing and interpreting potential field data.The goal of deriving the maximum amount of accurate subsurface information from seismic data has led to much more densely sampled and precise 2- and 3-D land data acquisition techniques. Land surveying accuracy has been greatly improved. The number of individually recorded detector channels has been increased dramatically (up to 1024) in order to approximate much more accurately a point-source, point-detector system. Much more powerful compressional-wave vibrators can now maintain full force while sweeping up or down from 5 Hz to over 200 Hz. In marine surveying, new streamer cables and shipboard instrumentation permit the recording and limited processing of 96 to 480 channels. Improvements have also been made in marine sources and arrays.The most important developments in seismic data processing--wave-equation based imaging and inversion methods--may be the forerunners of a totally new processing methodology. Wave-equation methods have been formulated for migration before and after stack, multiples suppression, datum and replacement statics, velocity estimation, and seismic inversion. Inversion techniques which provide detailed acoustic-impedance or velocity estimates have found widespread commercial application. Wavelet processing has greatly expanded our stratigraphic analysis capabilities. Much more sophisticated 1-, 2-, and 3-D modeling techniques are being used effectively to guide data acquisition and processing, as direct interpretation aids, and to teach basic interpretation concepts. Some systems can now handle vertical and lateral velocity changes, inelastic attenuation, curved reflection horizons, transitional boundaries, time-variant waveforms, ghosting, multiples, and array-response effects. Improved seismic display formats and the extensive use of color have been valuable in data processing, modeling, and interpretation.Stratigraphic interpretation has evolved into three major categories: (1) macrostratigraphy, where regional and basinal depositional patterns are analyzed to describe the broad geologic depositional environment; (2) qualitative stratigraphy, where specific rock units and their properties are analyzed qualitatively to delineate lithology, porosity, structural setting, and areal extent and shape; and (3) quantitative stratigraphy, where anomalies are mapped at a specific facies level to define net porosity-feet distribution, gas-fluid contacts, and probable pore fill. In essence, what began as direct hydrocarbon-indicator technology applicable primarily to Upper Tertiary clastics has now matured to utility in virtually every geologic province.Considerable effort has been expended on the direct generation and recording of shear waves in an attempt to obtain more information about stratigraphy, porosity, and oil and gas saturation. Seismic service companies now offer shear-wave prospecting using vibrator, horizontal-impact, or explosive sources.Well logging has seen the acceleration of computerization. Wellsite tape recorders and minicomputers with relatively simple interpretation algorithms are routinely available. More sophisticated computerized interpretation methods are offered as a service at data processing centers. A new 'slimhole' borehole gravimeter was developed and put in use in 1978. The instrument yields 20-microgal accuracy routinely.Petroleum industry interest in electrical methods has increased, particularly in magnetotellurics (MT) and in active electromagnetic (EM) sounding. The improved MT method is still used primarily in reconnaissance. Renewed interest in EM 'reflection' sounding stems from the success in direct hydrocarbon detection claimed by some service companies. Direct detection of shallow commercial deposits is possible, but any deep-target successes are attributed to measurable secondary effects in the overburden caused by upward-migrating hydrocarbons.In minerals exploration, deeper and more costly drilling and generally higher exploration costs for more difficult targets have required the acquisition of increasingly larger amounts of independent geophysical (and geochemical) data. More expensive techniques such as reflection seismology, spectral induced polarization, and borehole logging with large suites of tools are becoming economically acceptable. More and better data acquisition has become possible with the rapid development of microelectronics. Minicomputers and microprocessors are employed both to control sensors and to record their outputs digitally in many airborne, surface, and borehole systems. Some contractors provide valuable initial digital data processing in the field. A trend away from anomaly detection and toward the synthesis of large suites of airborne, surface, and borehole measurements is evident.