<PAGE>   1

                                                                   EXHIBIT 10.18

                    DEPOSITORY AGREEMENT FOR PURCHASE OPTION

     THIS DEPOSITORY AGREEMENT FOR PURCHASE OPTION (this "AGREEMENT") is entered
into as of the 1st day of December, 2000, by and between Grant Geophysical,
Corp., a Texas corporation, with its mailing address at 16850 Park Row, Houston,
Texas 77084, ("GRANT"), and Elliott Associates, L.P., a Delaware limited
partnership, with its mailing address at 712 Fifth Avenue, New York, New York,
10019 ("ELLIOTT").

                                   WITNESSETH

     In consideration of the mutual covenants and agreements contained herein
and other good and valuable consideration, the receipt and sufficiency of which
are hereby acknowledged, the parties hereto agree as follows:

     1. Option to Purchase Working Interest.

        (a) Purchase Option. Grant hereby grants to Elliott the exclusive option
            (the "Option") to purchase on or before January 31, 2001 all of
            Grant's right, title and interest, consisting of, among other
            things, a 33.333333% undivided working interest, in and to the
            mineral leases constituting the LaFite Project area in Brazoria
            County Texas, together with all of Grant's right, title and interest
            in and to any geophysical and geological data related thereto (the
            "Interest").

        (b) Purchase Price. The exercise price of the Option shall be the sum of
            $1,000,000, payable in the manner set forth in Section 1(c).

        (c) Refundable Deposit.

            (i)   Concurrent with the execution hereof, Elliott shall place on
                  deposit with Grant a $1,000,000 refundable deposit (the
                  "Deposit").

            (ii)  Elliott may exercise the Option without payment of further
                  consideration by giving Grant written notice of exercise that
                  includes the date on or before January 31, 2001 upon which the
                  purchase of the Interest shall close. On the closing date (A)
                  Grant shall deliver to Elliott an assignment, bill of sale and
                  conveyance in a form acceptable to Elliott and such other
                  documents as may be necessary to convey title to the Interest
                  and (B) upon such delivery the Deposit shall become Grant's
                  property.

            (iii) Between the date hereof and January 31, 2001, if Grant elects
                  to sell the Interest to a third party before Elliott exercises
                  the Option, the Option shall be cancelled. In such event,
                  Grant shall give Elliott written notice of such election and
                  refund the Deposit,

<PAGE>   2

                  together with a termination fee of $333.33 for each day
                  beginning on the date hereof and ending on the day the refund
                  and termination fee are paid.

            (iv)  If (A) Elliott gives Grant written notice that it elects not
                  to exercise the Option, or (B) the Option is not exercised on
                  or before January 31, 2001, Grant shall refund the Deposit to
                  Elliott no later than two business days after the earlier of
                  receipt of such written notice from Elliott or January 31,
                  2001.

        (d) Title Warranty. Grant represents and warrants to Elliott that the
            Interest, when conveyed upon exercise of the Option by Elliott,
            shall be free and clear of any mortgages, deeds of trust, voluntary
            or contractual or statutory liens, pledges, security interests,
            charges, conditional sales or other title retention documents. Grant
            hereby covenants to bind itself, its successors and assigns to
            warrant and forever defend the title to the Interest granted,
            conveyed, assigned, and transferred unto Elliott, its successors and
            assigns, against the lawful claims and demands of every Person
            whomsoever claiming or to claim the same or any part thereof, by,
            through or under Grant.

     2. General Provisions.

        (a) Notices. All notices and communications required or permitted to be
            given hereunder shall be deemed to be properly delivered the earlier
            of the date when actually delivered or three days after being
            deposited in the U.S. mail as certified mail, return receipt
            requested, with adequate prepaid postage affixed thereto at the
            addresses provided above or such other address as a party hereto
            shall designate by giving the other party at lease fifteen days
            written notice thereof.

        (b) Successors and Assigns. All agreements and conditions between the
            parties hereto shall extend to and be binding upon their respective
            heirs, permitted successors and permitted assigns.

        (c) Headings. The headings of the articles and sections of this
            Agreement are for convenience of reference only and shall not limit
            or otherwise affect any of the terms or provisions of this
            Agreement.

        (d) Unenforceable or Inapplicable Provisions. If any provision of this
            Agreement is invalid or unenforceable in any jurisdiction, the other
            provisions hereof shall remain in full

                                       2

<PAGE>   3


            force and effect in such jurisdiction and the remaining provisions
            shall be liberally construed in order to carry out the intent of
            this Agreement. The invalidity of any provision of this Agreement in
            any jurisdiction shall not affect the validity and enforceability of
            such provision in any other jurisdiction.

        (e) Counterparts. This Agreement may be executed in several original
            counterparts. Each counterpart shall be deemed to be an original for
            all purposes, and all counterparts shall together constitute but one
            and the same instrument.

        (f) References. References made in this Agreement, including use of a
            pronoun, shall be deemed to include where applicable, masculine,
            feminine, singular or plural, individuals, and other persons. As
            used in this Agreement, "Person" shall mean an individual,
            corporation, partnership, limited liability company, association,
            joint-stock company, trust or trustee thereof, estate of executor
            thereof, unincorporated organization, joint venture, court,
            government unit or any agency or subdivision thereof, or any other
            legally recognizable entity.

        (g) Choice of Law. This Agreement shall be construed under and governed
            by the laws of the State of New York without regard to the
            principles of conflicts of law.

        (h) Modifications. No modifications of this Agreement shall be effective
            unless reduced to writing and duly executed by the parties hereto.
            No course of dealing between the parties prior or subsequent to the
            date of this Agreement shall be construed to change, modify, amend,
            alter or waive the terms hereof.

                                       3

<PAGE>   4


     IN WITNESS WHEREOF, the parties hereto have executed this Agreement as of
the date first written above.


WITNESSES:                             GRANT GEOPHYSICAL, CORP.


                                       By:
-----------------------------              --------------------------------
                                       Name:
                                             ------------------------------
                                       Title:
                                              -----------------------------
-----------------------------



                                       ELLIOTT ASSOCIATES, L.P.
WITNESSES:


                                       By:
-----------------------------              --------------------------------
                                                     Paul Singer
                                                   General Partner


-----------------------------

                                       4