CREATE TABLE table_6977 ("Date" TEXT, "Tournament" TEXT, "Surface" TEXT, "Partnering" TEXT, "Opponents in the final" TEXT, "Score" TEXT)

What type of surface was used for the game played on September 20, 1993?
SELECT "Surface" FROM table_6977 WHERE "Date" = 'september 20, 1993'
SELECT "Surface" FROM "table_6977" WHERE "Date" = 'september 20, 1993'