I was unclear about how they determined that the internal cavities were large. I get the same density if I have a rock with 30% of it's interior carved out by

I was unclear about how they determined that the internal cavities were large. I get the same density if I have a rock with 30% of it's interior carved out by phasers, or if I have volcanic style (porous) rock. Are their instruments sensitive enough to get a line graph of gravitational attraction as their sensors pass near the moon? If that were the case, you might be able to determine large voids vs porosity, but if they only get a single data point, than you wouldn't be able to.