are best understood by comparing them to the corresponding direct (or forward) problems. In direct problems we have an input which we run through some known process to get an output. We know what the input is and we also know what the process is. From this information we need to produce the output. For inverse problems we reverse the order. Now we have given an output, we have some understanding about the process, and possibly are given a priori information about the input. Our task is to find an input which resulted in this output.
One might ask if the inverse problem is equivalent to a direct problem if we know the process exactly, so that we could then simply run the process backwards. Unfortunately this is rarely possible since most processes cannot be reversed or they are very sensitive for measurement errors. As a simple example one could consider the process which takes two numbers X and Y, and gives the sum X+Y. But if we are given some number Z and are asked to determine which X and Y sum to this Z, we would quickly find that there are many possible solutions without any other restrictions on the allowed inputs.
An other example is to find missing integers, of the grid given here, such that the sum of each row and column is the integer given in the picture. This toy problem is closely connected to medical imaging modality known the X-ray tomography. Each tissue type attenuates the X-ray beams differently and in principle the total attenuation of a beam, traveling trough a body, is the sum of attenuation coefficients multiplied by the depth of corresponding tissue. Thus to figure out the structure of the body we can try to solve an inverse problem of X-ray tomography, which is to recover the attenuation coefficient at each location, from the total attenuations of X-ray beams traveling through the body.
Geometric inverse problems in seismology
Consider Earth or some other solid body with a variable speed of wave propagation. The travel time of a wave between two points defines a natural non-Euclidean distance between the points. This is called the travel time metric and it corresponds to the distance function of a Riemannian / Finsler manifold.
A classical inverse problem is to determine the wave speed inside an object when the travel times between the boundary points is known. This problem is a mathematical analog of the geophysical problem where the structure of the Earth is to be found from the arrival times of seismic waves, initialized by earthquakes. Moreover, measuring the wavefronts of elastic waves scattered from discontinuities inside the Earth, one obtains information on the broken geodesics. Thus seismic measurements provide indirect information about the internal structure of our planet. Almost all of our knowledge of the deep earth is obtained using such data.