Question: Comparing two solutions to dsolve

I am exploring the onset of chaos in the driven damped pendulum. I have set up the non-linear initial value problem that describes the situation in terms of phi(t), the angle of the pendulum from the vertical. I can solve the differential equation numerically for certain initial conditions using dsolve, and then graph that solution using odeplot. I can then explore what happens when I change the initial conditions ever so slightly. Again I can solve this new initial value problem numerically with dsolve, and then graph it with odeplot, Suppose the solution to the first problem is phi1(t), and the solution to the second initial value problem phi2(t). I would like to plot log | phi2(t) - phi1(t) | versus t, that is, the logarithm of the absolute value of the difference of the two solutions versus t. I’m interested in how sensitive the system is to initial conditions.

So, how can I disentangle the two solutions, phi1(t) and phi2(t), from dsolve and then plot the graph I want?