Can a sonic boom produced at 60,000' be heard on the ground?
Yes, a sonic boom produced at 60,000' can be heard on the ground.

First of all, a sonic boom consist in a steep increase of air pressure, followed by a slow, linear decrease below the nominal ambient pressure, and again a steep increase back to the nominal pressure. It is therefore called N-shaped wave or just N-wave. Here is a graph with several sonic booms, measures by NASA and found on Wikipedia:



The same wiki page cites a document from NASA 

Aircraft        speed   altitude        pressure
               [mach]   [ft (m)]        [lb/ft² (Pa)]

SR-71           3      80,000 (24,000)  0.9  (43)
Concorde SST    2      52,000 (16,000)  1.94 (93)
F-104           1.93   48,000 (15,000)  0.8  (38)
Space Shuttle   1.5    60,000 (18,000)  1.25 (60)


These measurements are from aircraft with different altitude. As sound pressure depends from distance like  $p\sim\frac{1}{r}$, one may calculate the pressure for a certain altitude as in the following table. However, this assumes that the aircraft causes the same "boom intensity" at that altitude, which you should doubt at least for the SR-71.

Aircraft        speed   altitude        pressure
               [mach]   [ft (m)]        [lb/ft² (Pa)]

SR-71           3      48,000 (15,000)  1.44 (69)
Concorde SST    2      48,000 (15,000)  2.07 (99)
F-104           1.93   48,000 (15,000)  0.8  (38)
Space Shuttle   1.5    48,000 (15,000)  1.5  (72)


As you can see, the Concorde is the loudest aircraft, the most comparable F-104 (speed, altitude) produces just 40% of its sound pressure. But the F-104 is also much much smaller than the Concorde.

The Space Shuttle is very loud for its low speed. However, it looks more like a flying brick than an elegant, streamlined aircraft, so no wonder that it creates strong shock waves.

The SR71 seems to be very quiet for its speed, but as said, I don't think you can pull it down to 48000ft and expect the same behaviour. However, a sonic boom starts to disperse in some (larger) distance, may be this already happened for the SR-71. (Also, it's a spy plane with a very special shape - may be, this reduces the boom, too).



It has to be said that it is hard to translate the numbers into a comparable sound level, because what makes up the loudness is not only the maximum pressure, but also the rise time.
As counter-example, the air pressure changes by 130Pa every meter of altitude (at ground), and you don't hear the change of pressure when standing up from your chair (~100kPa)

Also, a sudden noise appears to be much louder than a noise slowly increasing to the same level - and the bang has the most sudden wave form possible. (Listen loud music with earphones - when the player clips a second, the first moment after really hurts)

However, there is a long table on Wikipedia listing noise sources and their levels. For example:

Trumpet at 0.5m:                63Pa
Jack hammer at 1m:               2Pa  (I'd say: a little quite...)
Loudest human voice at 1 inch: 110Pa


These levels are effective sound pressure. To translate a perfect N-wave with given maximum pressure to effective pressure, you have to multiply the value by $\frac{1}{\sqrt{3}}\approx0.577$.

Thus, the sonic boom of the Concorde has an effective value of 57Pa, which is comparable to the trumpet in 0.5m distance.

(As said, there is still a difference, because the boom is a sharp, short noise, but it gives an idea.)



By the way, I just stumbled on another document, about primary and secondary sonic booms.



It states that you hear a primary sonic boom coming from the aircraft within a lateral distance of ~20nm (solid lines) as well as a secondary boom within a distance of 60-85nm (dashed lines). This secondary waves are a reflection of the upwards travelling shockwaves from the aircraft at the atmosphere. 
The secondary boom is not a short, loud bang, but a longer and less intense rumbling, making it difficult to identify the source.