'substantial dip' is correct and usable in written English.
You can use it to refer to a large decrease in something. For example, "The stock market experienced a substantial dip in prices last week."
In the days leading up to the release, analysts pointed to the company's stagnant smartphone selection as a big reason for RIM's substantial dip.
The Wall Street Journal reported earlier this month that iOS's market share in China has taken a pretty substantial dip as potential consumers wait for a new iPhone to be announced.
"We also saw a substantial dip of 33% in the amount of sewage-related debris on our beaches - that's the stuff people put down their loos but shouldn't, like cotton buds, condoms, sanitary towels and tampon applicators," she said.
When fed a low-fiber diet (about 30% less fiber than the control high-fiber chow), the animals experienced a substantial dip in gut microbial diversity (with about 60% of the microbes losing at least half their populations).
Unlike the DHS results that showed a gradual, but steady, increase in the use of modern contraceptives from Q1 to Q5, the PSI data showed a substantial dip from Q1 to Q2, followed by an increase to the wealthiest strata (Q4 and Q5).
A more global view of patches is available from TEP experiments, in which both transmitter and receiver are located at substantial distances from the dip equator (e.g., see review of TEP by Nielson and Crochet 1974).
If the significant displacement at Oshika is caused by dislocation around the hypocenter, or in the up-dip region, substantial eastward displacement at Kamaishi must be recorded with several seconds delay.
Petroleum exploitation has fallen since 1986, but this is still a substantial sector such that dips in prices have a strong effect on the economy.
This proposed transaction enables Kodak to repay a substantial amount of our initial DIP loan, satisfy a key condition for our new financing facility, and position our Commercial Imaging business for further growth and success".
Marc B. Spector, a principal in the Spector Group, an architecture firm based in Woodbury, said that the prices of concrete, lumber and copper had dipped, resulting in substantial savings.
He believes that some councils could still be delivering services much more efficiently, and if necessary dipping into their substantial reserves to meet their current challenges.
