How to hide this underground city from seismologists, oil prospectors, and ground surveying projects?

Question

In my previous question Would this underground city be noticed by seismologists?, it was established that a Goblin city in one of my settings would probably not go unnoticed do to the meddling of seismologists, oil prospectors, and ground surveying projects. So, I want to see if there is anything they could intentionally do, or somewhere I could move the city too, to keep their city hidden from the humans given the following details.

Sorry about the excessive setting outline, but... who they are and what they can do may have a lot to do with how they could/could not stay hidden.

Beneath a major US coastal city. There is a goblin city that exists beneath it. It's chambers range from about about 500-2000ft underground and has a population of about 5-10 thousand. The layout of the city comprises of several fairly large caverns interconnected by tunnels. The largest caverns are about 30-50ft high and 200-300 hundred feet across in either direction, but most caverns are much smaller and more tunnel like.

At the entrance to the city, there is a small waterfall of fresh water where the goblins dug through the water table to reach the surface. This then feeds down a bunch of streams that function kind of like an aqueduct system for the city, and ends with an underground lake that is also a portal to a pocket dimension ruled by a Sídhe Lord who uses the water to build his ever expanding kingdom of ice in the faerie world.

What are Goblins in this setting?

The goblin city is made up of goblins and their descendants who were once slaves of the Sídhe Lord on the other side of the portal who for one reason or another have gained their freedom. Returning to the faerie world or asking the Sídhe Lord for help is out of the question, as it would mean returning to a life of slavery.

All goblins are able to glamour themselves to look like a human or animal of comparable size, but the glamour can be broken by direct sunlight; so, they only go out at night.

To break an agreement or fail to repay a debt makes you another person's property under goblin law; so, the smartest goblins tend to rule society by getting other goblins into their debt while avoiding being indebted to others. While this law does not necessarily extend to humans, goblins disguised as humans tend to carry some odd seaming behaviours into human society like refusing to go through a door a person has opened for your or refusing offerings of free food as accepting such gifts would be seen as a debt in goblin society.

These cultural differences and the limitations of glamours make long term, direct interacts with humans risky; so, goblins try to avoid it as much as they can.

The goblins also do not have any proper industrial infrastructure of their own; so, a lot of what the goblins have comes from things they steal from humans at night: radios, power tools, babies, and whatever other odds and ins they find lying around. Stealing does not create a debt under goblin law since it was never offered. Thus many goblin homes are adorned much like shops with all of their possessions labeled with obscenely high price tags to make sure no one takes anything. Those pens at the bank, yes you are allowed to take them, but they cost $100 each.

There are 6 distinct breeds of Goblin in the city:

Grimilans: Average about 1 ft tall and have unnaturally good hearing. By disguising themselves as rodents and taking up vantage points in attics, under homes, and inside walls they tend to know about what the humans are up to giving the goblins time to react to what humans are planning. They are also known to participate in sabotage, when the humans get too close to the truth, but this is not 100% reliable since too much sabotage can create more suspicion than it alleviates.

Duende: Average about 2-3 ft tall and are by far the most intelligent of the goblins. They have an average intellect somewhat higher than a human. They are too few in number to really progress goblin technology beyond that of what the humans are coming up with, but they are very quick about adapting to and understanding human technology. They rarely leave the city themselves instead preferring to use other breeds of goblins to do their bidding.

Uruks: Average about 4-6 ft tall and moderately intelligent, they are the most passable for human. When goblins need to interact with the human world (while passing for an adult), it's usually a Uruk.

Bugbears: Average about 6-8 ft tall and weighing in at several hundred pounds. They have temperaments and let's face it, raw strength comparable to that of a horse. Bugbears are used mostly as chattel slaves by the smaller breeds of goblins.

Kobolds: Average about 3-4ft tall. These goblins have a bit more potential for magic than the other goblin races, but their magic often comes with unexpected consequences. When the price of spell is small, they tend to be able to control the equivalent exchange well, (like transmuting a silver coin into a loaf of bread), but trying to do something like hide the whole city from ever expanding human technology would almost certainly end disastrously.

Red Caps: Average about 3-5ft tall. Rarely seen in the goblin city, and usually killed on sight due to their uncontrollable impulsiveness.

Answer 1

Anywhere there are natural caves, AKA IN PLAIN SIGHT:

I would think that anywhere there are natural caves, geologists would see the caves, but not think twice about them. There are HUGE cave complexes around the world, and humans can tell they are there, but not do a thing about them.

"Say, students, did you know the ground half a mile under the city is riddled with caves? It's true. I know some of these look repetitive and manufactured, but similar structures are seen around the world an the product of natural forces."

You might have trouble with the occasional spelunker, but it's a dangerous sport, and if a magical goblin showed up looking like a fellow cave explorer and warning you this way was too dangerous, would you ignore them? If they went on, might they not meet with a terrible accident?

Answer 2

"The Goblin Caves are a protected scientific site of incalculable geological, paleological and anthropoligical interest. They contain the remains of an early human settlement; scientific exploration must be carried out with painstaking caution to prevent any valuable finds being destroyed. To this end, the caves have been sealed and the Committee is now in the third decade of planning its approach.

"Rumors that the caves are inhabited by ghosts, aliens or goblins are of course ridiculous"

"Thanks, Prof, here's the gold like we agreed"

Answer 3

Not really hide, but they could set up, around the city, a suitable meta-structure to diffract seismic waves around the city, instead of having them bounce off its interfaces.

The same way a stealth airplane scatters radar waves away from the direction where the radar is firing them, the underground city might use a structure with proper refraction index modulation to achieve the same result.