Geophysics

Geophysics is a discipline that uses different areas of knowledge, such as physics, mathematics, and geology to study the internal constitution and history of the Earth. One of the applications of Geophysics is in oil exploration.

Seismic exploration:

The purpose of seismic exploration is to create an image of the subsurface and to estimate the distribution of a range of properties - in particular, the fluid or gas content. https://www.geos.ed.ac.uk/seismic/acq-proc/

Seismic exploration is based on the same physical principles of an ultrasound Echo Sonogram. So the same way as we use sound waves to generate an image of the internal structures of the body. For example, in the case of a pregnant woman to see the baby inside her belly, we can use seismic waves to obtain an image of the subsurface (the internal structure of the earth).

So in the case of Seismic exploration, we generate seismic waves from an explosion that we detonated on the surface of the earth (using Dynamite - TNT, for example). So this explosion generates waves that propagate into the subsurface, reflex in the layers of the subsurface, and came back to the surface. On the surface, we place wave detectors (called geophones) that are able to measure ground movement. Using geophones we record ground motion as a function of time. These ground movements are related to the reflection of the waves in the layers of the subsurface. So, with these time series (called traces) we are able to build an image of the subsurface.

So, suppose that we are performing a Seismic acquisition survey where we want to cover an area of $100Km^{2}$ , we place geophones every $50m$ , and we performed 80 shots. In every shot, each geophone is going to measure ground motion for 5 seconds and we will record this ground motion every $5ms$ .

With these acquisition parameters, we have around 400 geophones. For each geophone, we will have one seismogram for each shot (in this case, we talk about a trace instead of a seismogram). So we will have $400*80=32,000$ seismograms. Now, each seismogram is a time series that contains ground motion values (which is called the amplitude and it is a value between -1 and 1). In this case, because we are recording ground motion values every $5ms$ during $5secons$ , we well have ${\frac {5000ms}{5ms}}=1000$ amplitude values for each of the $32,000$ traces in our data. So, as you can see we have a quiet lot of data here.