Geophysical Surveys play a key role in any development activities and are almost invariably used as a tool for investigation prior to commencement of any major engineering work.
Sometimes geophysical surveys are supported by ground truthing such as Geotechnical Investigations which involves drilling of borehole giving an insight into the underlying strata. At times, an integrated approach combining geophysical and geotechnical investigations may be the best solution for arriving at the desired results.
Geophysical Surveys for a particular project can be carried out at various stages of activities beginning from initial planning of a project till the completion of engineering/exploration activities.
Various geophysical methods such as electrical, electro-magnetic, gravity, magnetic, magneto-telluric, seismic are being currently used for mapping subsurface geology. Each of the above methods is aimed for fulfilling different objectives.
Out of the above methods, seismic methods are the most widely used of all the geophysical methods in mainly geological surveys, mineral exploration, various engineering activities and petroleum exploration activities. The main advantage is that it provides the most accurate rendition of the geometry of subsurface layers.
In marine applications, often seismic surveys are combined with other surveys such as bathymetric, side scan sonar and magnetometer surveys to get complete information of the seabed and sub-seabed in the area of interest.
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Acoustic (pressure pulse), which transmits sound energy in the form of a short pulse towards the seabed. This sound energy is reflected from the seabed and the subsequent layers of soil. The reflected energy intensity depends on the different densities of the seabed, the denser (harder) the seabed, the stronger the reflected signal. The reflected signal travels back through the water to a surface towed hydrophone (an underwater).
The sequence is described thus:-
A transmission pulse from the boomer travels through the water at the same time as recording sequence is started. As the signals travel in time to the seabed and subsequent layers of soil and returns, the synchronized record displays a line of signal on a paper or digital recorder. The line contains signals proportional in intensity (darkness) to the strength of the reflected data. Another transmission pulse starts the sequence again to produce another line on the record. This sequence continues several times per second as the equipment is towed over the seabed, resulting in a record showing cross section through the seabed.
Caution needs to be exercised while interpreting seismic records. The following is just a brief guide for carrying out seismic interpretation judiciously.
Challenges come from acquisition and interpretation of the seismic reflection data from the shallow marine environment including the masking, attenuation, delay, and scattering of seismic energy by various kinds of environmental noise, entrained gas, and coarse lithologies.
The seismic records show layer thickness as a function of time, thus the true thickness may only be determined if the speed of sound through the material is known. Accurate sound wave velocity data are seldom available, therefore layer thicknesses must be considered approximate.
The reflecting surface does not always represent a change in the sediment type, but may only be a change in the physical character of the soil such as grain size, porosity, density, or hardness.
At greater water depths, the area of sea floor receiving sound increases. Thus, records in deep water will tend to show average conditions over an area rather than a specific profile directly below the ship.
Interpretation can also be difficult where submerged objects, out-of-line obstructions, and seismic multiples complicate the seismic records. Entrained gas can produce bright reflectivity in otherwise unreflective horizons, and because of its very slow seismic velocity, delays the arrival of other reflected energy.
Deconvolution, band pass filtering, f-k filtering, NMO, DMO, stacking are also applied to the data and serve as an aid to interpretation.
Marine Refraction Surveys
The principle difference between the geometry of refraction and that of the reflection methods is in the interaction that takes place between the seismic waves and the litohological boundaries they encounter in the course of their propagation. In refraction work, the energy arrives at the receiver after travelling through the rock layers; whereas in reflection work, the energy is received after it has reflected from the subsurface layers.
Interpretation of refraction is normally done on the initial arrival of seismic energy (first break picking). Seismic Profiles are generated with compression wave velocity information; and assumptions based on relationships between measured velocities and lithologies according to experience, and to project data available at processing and reporting phase of the survey.
Side Scan Sonar Surveys
Side scan sonar; also referred to as side scan sonar, side-imaging sonar, and side-looking sonar; is often towed from a survey vessel and has the ability to capture hundreds of meters of seafloor on each side of the moving vessel. The near photographic quality images produced by side scan sonar along with its ability to map large areas of seafloor quickly make it an essential piece of kit for anyone requiring high definition images of the seabed.
Side scan sonars transmit a narrow fan-shaped acoustic pulse (ping) perpendicular to its direction of travel. As the acoustic pulse travels outward from the side scan sonar, the seabed and other objects reflect some of the sound energy back in the direction of the sonar (known as backscatter). The travel time of the returned pulse is recorded together with its amplitude as a time series and sent to a topside console for interpretation and display. The topside console stitches together data from successive pulses, creating a long continuous image of the seafloor as the side scan sonar is towed from a survey vessel.
Interpretation of side scan sonar data develops with experience. Side scan sonar reflections of isolated small objects give no indication of shape or attitude. Man made structures, such as platforms or rock walls tend to have regular patterns that are easier to identify. Increasing the scan range of the sonar severely limits the resolution in the far field of the acquired data and as a consequence interpretation becomes difficult.
Magnetic surveys are carried out on land a well as marine environment and measure variations in the earth's magnetic field. The magnetic field observed by a magnetometer is a local disturbance is termed as 'anomaly' Magnetic field and is generally expressed in 'gammas'. The observed anomaly expresses only the net effect of the induced and remnant magnetizations which usually have different directions and intensities of magnetization.
Magnetometer surveys are well suited for the detection and mapping of all sizes of ferrous objects. This includes anchors, chains, cables, pipelines, ballast stone and other scattered shipwreck debris, munitions of all sizes, aircraft, engines and any other object with magnetic expression
Interpretation of magnetometer can be quantitative as well as qualitative. The raw magnetometer data have to be reduced for the anomalies by applying corrections for earth normal magnetic field, diurnal variations, regional -residual separation , upward continuation etc. The identified anomalies have to be interpreted for the causative sources.
We at Geosense Surveys professionally understand our client's requirement by giving the best possible geo-approach to suite his specific needs.
Some of the common work areas where geophysical services needed:
Sand/Cap Rock surveys for land reclamation/beach replenishment
Prefeasibility for Port& Harbours/Power Plants/ Desalination Plants
Post dredging /channel access survey
Seabed/Pre-engineering Surveys for pipelines/jetty constructions/bridge construction
Prelay/Pre Construction Surveys for offshore establishments
Anchor Clearance Surveys for offshore establishments
Debris/Salvage Surveys for lost objects at sea such as dredge cutter, anchor chains, ship wrecks, rudders, and pipelines.