Seismic SurveySeismic methods are the most important geophysical survey technique used in the oil and gas exploration industry. The first field demonstration of seismic for oil and mineral exploration took place in the 1920s at Oklahoma in the United States of America. The method is particularly suited for mapping sedimentary rocks and is popular also for engineering and environmental applications where mapping near surface sedimentary layers to determine:Depth to bedrockSoil and weathered layersWater tableOther subsurface horizons.In seismic surveying, seismic waves are created when an energy source is used to send a low frequency acoustic pulse into the Earth's crust to measure distances in terms of the time it takes an echo of the pulse to return to sensors. The energy is reflected or refracted at geological boundaries. By repeating the operation along a predetermined path the recorded data can be used to provide a continuous vertical profile of the geologic strata.

Marine Seismic

Figure 1. The basic application of seismic data acquisition in a marine setting.Marine seismic is used for a variety of investigations, including:Resources (petroleum and mineral) assessmentGeotechnical investigationsSubmarine hazard detectionHydrophone arrays called streamers are used as detectors and the hydrophones are grouped into so called channels which can vary from 1 to a thousand or more. For a petroleum seismic system, typical frequencies recorded are 8 to 250 Hz. In contrast systems using sparker sources instead of air guns operate at frequencies in the 100 Hz to 1 kHz range and are generally used on small vessels for intermediate depth and higher resolution purposes. They often use smaller receive arrays. Other sub-bottom profilers use even higher frequency piezo-electric transducers which operate in the 500 to 5 KHz range. These will typically penetrate between 10 and 100 meters but obtain decimeter resolution.The figure below shown a 2D high resolution data example of geo-hazard survey:

Figure 2. The data showed a possible shallow gas.