To whom it may interest I have been reading the comments on gravitation with great interest and it started me wondering about plumb lines and if at certain locations (assuming no interference/vibration) they hang truly vertical all the time - i.e. pointing directly through the centre of the earth, and even change due the core rotating at a different speed to the crust. Likewise if a skyscraper is truly vertical at one moment, could it not be vertical at another time as compared with a plumb line... I know the amount by which it would be out of plumb would be so insignificant as not to matter in the real world. I enjoy reading the discussions that the group put up and one day, GV, I hope to take a more active role in the earth sciences ------------ Ted Rogers.
---------- > From: Tom Schmitt > To: psn-l@.............. > Subject: Re: Article on gravimeters > Date: Monday, May 01, 2000 4:40 AM > > John > > About fifteen years ago I went to a non-classified briefing about a land > version of a gravity gradiometer. The Defense Mapping Agency had it. > > It was in a Winabago of some sort and had three disks that rotated at a > fairly low rpm, a few Hz at most. The disks were less than a meter in > diameter. The disks had two accelerometers on them. The acceleration as a > function of position of the two accelerometers was reccorded and some sort > of auto-correlation or FFT was done to get the direction and magnitude of > the gradient. > > It was cute. > > > Tom Schmitt > > tschmitt@.............. > > ----- Original Message ----- > From: John Hernlund > To: > Sent: Saturday, April 29, 2000 10:58 PM > Subject: Re: Article on gravimeters > > > George Bush > > > > George, > > There was a horribly written article about an incredibly interesting > > subject a few years back in Scientific American. Even though the article > did > > not belong in the magazine, you might be interested in reading it. It was > > about a gravity gradiometer that had been designed to help submarines > "see" > > the topography on the ocean floor and avoid collision with sea mounts. > They > > needed a passive method to see the ocean floor so that they would not be > > detected. Supposedly this thing worked great (too bad all the data they > > collected is classified). Anyways, the company that made it was allowed > to > > contract its use out to some geophysical concerns, such as finding oil and > > natural gas resevoirs. The company takes a huge amount of money, throws > this > > thing in a boat and drives around for a while. They take the data, reduce > it, > > and then sell portions of it for commercial use. Nobody is allowed to buy > > one of these instruments, or use one for themselves. This is the reason > it > > did not belong in the magazine: it was really an ad for the company. The > > exact technology is still classified, so it is like a magical black box. > > > > Anyways, this device produces higher resolution images for finding > density > > anomalies than seismic methods, and does not require a whole lot of fancy > > deployment except for position tracking and recording instruments, and so > is a > > lot easier to use than seismic arrays. I found this hard to believe when > I > > first heard it, but then again I was only used to absolute vertical > gravity > > measurements. So I had to find out about it a little further. > > > > The gravity gradient is the change in the gravity for each direction > (down, > > east, north) within a given distance and with respect to each direction. > So > > the downward component of gravity changes differently in the down, east > and > > west directions, and likewise for the east and north components of > gravity. > > This makes nine different combinations of gradients that can be examined. > One > > of those nine components can tell you an awful lot more than an absolute > > gravity measurement, so just imagine having nine of them. Well, it turns > out > > that some of the components are equal, and the total number of unique > > components is reduced to six. For example, the change in the down > component > > of gravity with respect to north is equal to the change in the north > component > > of gravity with respect to the down direction. In addition, the three > terms > > where the component changes in each of its directions is regulated by the > fact > > that a gravitational field is divergenceless, which means these three > > gradients must sum to 0. So that makes five unique components, from which > the > > other four can be determined. All nine components are referred to as the > > "gravity gradient tensor." > > > > This machine is supposed to have rotatiing parts and sensors inside > some > > kind of black spherical shell. It is very mysterious, and I would love to > see > > inside one some day. I am sure everyone else would be interested too... > I > > guess I would have to hijack one of their ships or something if I wanted > to > > get it, but that won't happen any time soon. Perhaps the PSN could mount > an > > expedition, and the media would try and figure out if PSN stood for some > kind > > of terrorist organization or something. > > > > Anyhow, enjoy! > > > > John Hernlund > > E-mail: hernlund@....... > > WWW: http://www.public.asu.edu/~hernlund/ > > > > > **************************************************************************** > ** > > > > __________________________________________________________ > > > > Public Seismic Network Mailing List (PSN-L) > > > > To leave this list email PSN-L-REQUEST@.............. with > > the body of the message (first line only): unsubscribe > > See http://www.seismicnet.com/maillist.html for more information. > > __________________________________________________________ > > Public Seismic Network Mailing List (PSN-L) > > To leave this list email PSN-L-REQUEST@.............. with > the body of the message (first line only): unsubscribe > See http://www.seismicnet.com/maillist.html for more information. __________________________________________________________ Public Seismic Network Mailing List (PSN-L)
Larry Cochrane <cochrane@..............>