Geophysical survey, or reflection seismology, is a method of exploratory geophysics that uses the principles of seismology to estimate the properties of the earth's subsurface from reflected seismic waves. The method requires a controlled seismic source of energy, and by noting the time it takes for a reflection to arrive at a receiver it is possible to estimate the depth of the feature that generated the reflection. Some of the acoustic signal will penetrate the seabed and be reflected when it encounters a boundary between two layers that have different acoustical properties, known as acoustic impedance.

Sub-bottom profile record showing rockhead profile, boulder clay & water column.

The system uses this reflected energy to provide information on sediment layers beneath the seabed, thus creating a profile of the sub-bottom sediments.

Several factors (output power, signal frequency and pulse length) affect the instrument performance. An increase in output power gives better penetration into the sub-bottom layers as this will usually provide deeper penetration into the sub-bottom layers. However, if the seabed is very hard or the water column too shallow, the increase in power will cause more of the signal to be reflected back off the seafloor. The signal might then be reflected off the sea surface, leading to multiple reflections and "noise" in the data.

Signal frequency also has an effect on system performance. Higher frequency systems, such as a Chirp, will produce high definition data of the upper seafloor sediment layers. These higher frequency signals have shorter wavelengths and are able to discriminate between layers that are close together.

Lower frequency systems, such as a Boomer, will give greater penetration but at a lower resolution. Longer sound pulse lengths transmit more energy and yields deeper seabed penetration. However, a long pulse length may decrease the ability to discriminate between adjacent reflectors, thus decreasing the system resolution.

Reporting

With a full geophysical interpretation, analysis and reporting function offered as standard we are uniquely positioned to offer our clients a comprehensive report outlining the sub-strata conditions not only within the required survey area but also the geological conditions within the survey locale, often supported by supplementary seismic data, existing maps, data from our offshore sample database and onshore single borehole data index to aid interpretation.