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Geophysics - Vibration & Noise - Rock Mechanics

Providing Quality Geophysical Consulting Services since 1984

Marine Geophysical Methods

There are several geophysical techniques which are specifically designed for use over water. Other typically land-based geophysical techniques such as ground penetrating radar, seismic refraction, and electrical resistivity profiling can be adapted for use in water.  All of these techniques require the use of a boat and an accurate method for determining position during data acquisition. Differential global position systems (DGPS) are typically used for providing real time location data. Each of the techniques that are commonly used by Earth Dynamics are summarized below:

Acoustic Sub-bottom Profiling is a geophysical technique that can be used to determine sub-bottom stratigraphy. Sub-bottom profiling systems consist of an acoustic source, an array of hydrophones and a recording system. The source emits acoustic energy into the water. The energy is reflected off interfaces where the velocity of the energy changes. The reflected energy is detected by the hydrophones and the data are stored by the recording system.

A marine electrical resistivity survey is similar to a terrestrial survey except that a specially constructed marine multi-electrode cable is placed on the bottom of the water body. The cable consists of twenty-five electrodes. An Advanced Geosciences, Inc. control unit applies current and measures the potential from various pairs of electrodes to determine electrical resistivity as a function of depth along the electrode array. The data are processed using a two-dimensional inversion program. In the program, a non-linear least-squares optimization technique is used to automatically determine the best fit to the data. This method is useful in determining the geologic conditions below the mudline. For more information on this method, see our Electrical Resistivity page.

Ground penetrating radar (GPR) uses FM-frequency radio energy to echo-locate subsurface features. Additional details of the technique are described in the GPR section. The technique was originally developed to investigate subsurface features below ice sheets. The GPR method has been adapted for use over land or water. Over-water surveys are conducted by placing the radar antennae in rubber rafts. The rafts are towed behind or beside a moving vessel. Generally the GPR method is useful in fresh water at water depths ranging from a few feet to twenty feet. Beyond twenty feet there is insufficient signal strength from most antennae to penetrate the sub-bottom material.

Side scan sonar is useful for mapping underwater topography and for identifying features on the surface of river and lake bottoms. The technique is capable of providing data to over one thousand feet on both sides of a moving vessel. A side scan sonar survey consists of towing a transmitter/receiver (towfish) from a moving vessel. The towfish transmits high intensity, high frequency bursts of acoustic energy into the water. The acoustic energy is transmitted in fan-shaped beams which are narrow in the horizontal plane and wide in the vertical plane. Objects or topographic features on the river bottom produce echoes which are received by the towfish. The signals are processed and printed on a graphic recorder or stored in a digital format. Generally, hard materials provide high amplitude echoes and soft, fine grained materials provide weak signals. Side scan sonar produces a visual representation of the bottom of the body of water.