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**WHAT IS GEOPHYSICS?**

Before answering this question, let us do a premise.

Doing geophysics is fun. And recently, with the evolution of technology and software, it is also the simplest thing in the world. Anyone can do it, really!

What are the steps to do geophysics? Let’s see:

- having an instrument, better the cheapest one, with preset configurations based on the type of survey to be carried out;
- depending on the technique, we must also have some sensors, just a few;
- pressing the “acquire” button on the instrument;
- having a software, preferably the easiest to use, with all the preset settings;
- pressing the “process” button to process the acquired data.

The images that come out of the processing are very nice, they are colored most of the time and make a good impression in any report.

Is it all clear? Good! So let’s get back to our question: what is geophysics?

The transition from the subsoil model, defined on the basis of a set of parameters, to the acquired data is regulated by the physical laws: propagation of acoustic and electromagnetic waves, Snell’s law, Fresnel’s laws, Maxwell’s laws, Lenz’s law, Faraday’s law, potential fields, near and far field concepts. Knowing the physics behind a problem means understanding the problem, the approximations, and the limits of the methods.

Data are numbers, their processing is done with algorithms, and the results are still numbers. If physics regulates the process that leads from the model of the subsoil to the geophysical datum, mathematics regulates the inverse process, which is called inversion. The inversion is nothing but a mathematical process; it is the transformation of a numerical matrix into another, with its problems and limits. And the inversion processes are not all the same, but they depend on the type of problem, whether linear or non-linear, whether over or under-dimensioned, if searching for a global or local minimum. Knowing the mathematical processes behind all the stages of processing means choosing the most suitable algorithms for solving a problem, and understanding the robustness or the weakness of a result.

Every acquired data is a signal, and to correctly acquire a signal it is necessary to know the sampling problems in time and space, the windowing, the aliasing phenomena. And in data processing we often do apply filters, switch from time domain to frequency domain or wave number, and vice versa, re-window the signal. Knowing the signal theory helps in appropriately designing the acquisition and avoiding artifacts due to the processing phases.

Geophysics is a complex, multisectoral subject that must necessarily integrate engineering, physical and mathematical skills.

There is a close analogy between geophysics and image diagnostics in medical field: in both cases the objective is to “see” within the matter, be it the subsoil or a human body, without taking samples. In medical field, the radiology technician must provide a clear image to a doctor who will have to assess the extent of the problem and to provide possible solutions; so the geophysicist must produce a clear and reliable result that, as a rule, a geologist must interpret to provide an exhaustive geological model of the investigated area. The collaboration between geophysicists, geologists and engineers or architects in works designing is fundamental for the final result and the preservation of structures and human lives.

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