OpaCitiZen and Tridus have some good points, and while my answer includes theirs in some ways I feel that the semantics change the meaning entirely, so here goes:
You almost need to think of distance for tremorsense like a character governing (otherwise mundane) walking distance with Spider Climb. Sometimes when people measure distance they'll say "as a crow flies" or something meaning if you look at a map and draw a straight line then that's the distance. However, 20 miles as a crow flies can easily be 50 miles driving distance - for example if you're navigating a mountain and thus have to traverse the winding paths going around the peaks you'll easily go a long distance than a direct flight would bring you. So where vision is almost always a direct line of sight (like flight), tremorsense should be calculated by the distance the vibration needs to travel to reach the character.
Usually people ignore ceilings and the like because in a cubic room it's X feet to a wall, Y feet up the wall, and Z feet to the actual source of vibration across the ceiling and with something limited like 20ft that gets used up fairly quickly. Therefore, most pits/chasms/gaps will absorb your tremorsense because a 10' deep pit is 20' without factoring width. Thus to reach the opposite side, the depth on both sides and the breadth of the gap could not be more than 20'. This small a range is best served in buildings because just like our intestines they wind about to create segments and paths in a confined space. So while the walking distance gets immense, and visual distance becomes quite limited, connected walls and ground lets you feel a lot farther than other sense can clearly detect.
In your specific example, if a character is standing on the edge of a chasm (on ground connected to the chasm), their tremorsense is wasted unless they hop the gorge, in which case it is cut off from their prior side but get twenty new feet of this extra sensory ability. At ten feet across and any arbitrary distance greater than 20' wider than the character's position at the ledge on either side, the chasm can be no deeper than 5' on either side just to see the other lip let alone anything else.