BASE GEOPHYSICS

It is obvious that after the Normal Move Out (NMO) correction and stacking, only a zero-offset section is synthesized, and the fact that the offset dependence of the receiver position with respect to the source has been removed. That means, we have a section as if we did a seismic survey with source and receiver at the same place, thus zero-offset. Migration deals with a further removal of wave phenomena via focussing in order to arrive at a section which is a true representation of the subsurface . Migration is the focussing process which results in a true image of the subsurface from primary-reflection data, assuming the velocity model is correct. Migration is the process of reconstructing a seismic section so that reflection events are repositioned under their correct surface location and at a correct vertical reflection travel time. Equivalently, this mean that migration obtains the true image in (x, y, z) from seismic data that are obtained in (x, y, t), where x, y and z sta...

There are many different geophysical investigation techniques, However when there is a need in exploring detailed information about the subsurface, we have to drill the hole and make a detailed collection of subsurface data, this is termed Borehole geophysics . This procedure involves the depth wise sequential record of physical parameters such as resistivity , density, temperature. It is sometimes known as Geophysical Logging or Geophysical well logging. There are many different geophysical logging however this post will let you know about electrical logging which is commonly includes the resistivity log and self-potential (SP) log . geologic log vs geophysical log G eological Log: This displays variation of subsurface geology ( lithology/rock types ) with depth. Such as from dry sand (top) to sand (middle) to clay (bottom). Geophysical Log: This displays variation of subsurface physical parameters with depth. The term well log is more general that includes both geological and ge...

Most surface geophysical techniques provide vast information about subsurfaces. However when it comes to trace detailed information about subsurface in a certain cross section- area, it is highly recommended to deploy these techniques in a drilled hole. This is termed as Borehole geophysics .  What is Borehole Geophysics? Borehole geophysics involves the depth wise sequential record of physical parameters such as resistivity, density, temperature, porosity, etc. It may sometimes be known as Geophysical Logging . Borehole : Is the narrow hole drilled into ground for the purpose either of obtaining subsurface information or extracting fluids such as water, gas, oil, contaminants. It is the same as a well, but they may differ due to the fact that the borehole is drilled at great depth and has small diameter compared to traditional wells. The basic geophysical borehole system is made up of these parts,  Downhole probe : It consists of sensing instruments/equipment that are lower...

MAGNETO-TELLURIC METHOD What is the Magnetotelluric (MT) method? Magnetotelluric (MT) method involves measuring the response at the surface of the Earth to time-varying electromagnetic waves in the subsurface generated by natural and man made (artificial) sources. We can consider the Magnetotelluric (MT) method as the electromagnetic method, since it comprises two components such as magnetic and electric fields, with the electric field in Magnetotelluric (MT) method is associated with telluric currents .  However electromagnetic and magnetotelluric methods have proved slightly some similarity and clear differences. If you want to know these comparisons in clear and simplified way check and visit to Comparative Geophysics eBook . Magnetotelluric (MT) signals Based on the type of sources of Magnetotelluric (MT) fields, Magnetotelluric (MT) signals can be categorized into two (2) forms.  1.  Natural Magnetotelluric (MT) signals These originated naturally from external fie...