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                                                                   EXHIBIT 10.21

                     THIRD AMENDMENT TO DEPOSITORY AGREEMENT
                               FOR PURCHASE OPTION

     THIS THIRD AMENDMENT TO DEPOSITORY AGREEMENT FOR PURCHASE OPTION (this
"Third Amendment") is entered into as of February 22, 2001 by and between Grant
Geophysical Corp., a Texas corporation ("Grant"), and Elliott Associates, L.P.,
a Delaware limited partnership ("Elliott").

                                   WITNESSETH:

     WHEREAS, Grant and Elliott are the parties to that certain Depository
Agreement for Purchase Option dated as of December 1, 2000 (the "Original
Agreement"), as amended by that certain Amendment to Depository Agreement for
Purchase Option dated January 25, 2001 (the "First Amendment") and that certain
Second Amendment to Depository Agreement for Purchase Option dated February 20,
2001 (the "Second Amendment", the Original Agreement as amended by the First
Amendment and the Second Amendment is the "Depository Agreement"); and

     WHEREAS, Grant and Elliott deem it in their best interests to amend the
Depository Agreement to provide for a voluntary reduction in the amount of the
exercise price of the Option and the Amended Deposit at Grant's discretion.

     NOW, THEREFORE, the parties agree as follows:

     1.   Section 1(c) of the Depository Agreement is amended to add subsection
          (C) to Section 1(c)(iv) as follows:

          "(C) In the event Grant elects to voluntarily reduce the amount of the
               Amended Deposit with funds received from a source other than
               sales of licenses to the Survey it may do so by delivering a cash
               payment to Elliott. In that event, the cash payment will be
               applied in the same manner as a Cash Payment under Section
               1(c)(iv)(A) above."

     2. Except as specifically amended by this Third Amendment, all of the terms
and provisions of the Depository Agreement shall remain in full force and
effect.

     3. All capitalized terms used herein but not defined herein shall have the
meanings given to them in the Depository Agreement.

     4. This Third Amendment shall be governed by and construed in accordance
with the internal laws of the State of New York.

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

WITNESSES:                             GRANT GEOPHYSICAL CORP.

                                       By:
-----------------------------              --------------------------------
                                           Thomas Easley
                                           Executive Vice President-
-----------------------------              Finance and Administration

WITNESSES:                             ELLIOTT ASSOCIATES, L.P.

                                       By:
-----------------------------              --------------------------------
                                           Paul E. Singer
                                           General Partner
-----------------------------

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                                                                  EXHIBIT 10.22

                             AMENDMENT NUMBER FOUR
                         TO LOAN AND SECURITY AGREEMENT

     THIS AMENDMENT NUMBER FOUR TO LOAN AND SECURITY AGREEMENT (this
"Amendment"), dated effective as of February __, 2001, is entered into by and
among Grant Geophysical, Inc., a Delaware corporation ("Borrower"), Foothill
Capital Corporation, a California corporation ("Foothill"), and Elliott
Associates, L.P., a Delaware limited partnership ("EALP"), in light of the
following:

     WHEREAS, Borrower, EALP and Foothill are parties to that certain Loan and
Security Agreement (including any and all amendments, the "Loan and Security
Agreement"), dated as of May 11, 1999, as amended by Amendment Number One to
Loan and Security Agreement, dated to be effective as of August 13, 1999, by and
among Borrower, Foothill and EALP, Amendment Number Two to Loan and Security
Agreement, dated to be effective as of September 23, 1999, by and among
Borrower, Foothill and EALP, and Amendment Number Three to Loan and Security
Agreement, dated to be effective as of February 14, 2000, by and among Borrower,
Foothill and EALP; and

     WHEREAS, Borrower has requested that certain provisions of the Loan and
Security Agreement be amended, so as to provide for the following:

          (a) an increase in the concentration limit of the Designated Account
     Debtors in the aggregate from 35% to 75% of all Eligible Accounts; and

          (b) an increase in the concentration limit for each individual
     Designated Account Debtor from 20% to 25% of all Eligible Accounts.

     WHEREAS, subject to the conditions set forth in this Amendment, Borrower,
Foothill, and EALP have agreed to amend the Loan and Security Agreement as set
forth below.

     NOW, THEREFORE, in consideration of the foregoing and the mutual covenants,
conditions, and provisions as hereinafter set forth, the parties hereto agree as
follows:

     1.    DEFINITIONS. Initially capitalized terms used herein have the
meanings defined in the Loan and Security Agreement unless otherwise defined
herein.

     2.    AMENDMENT.

     2.01  AMENDMENT OF EXISTING DEFINITIONS CONTAINED IN SECTION 1.1 OF THE
LOAN AND SECURITY AGREEMENT. Subsections (i) and (j) of the definition of
"Eligible Domestic Accounts" are hereby amended and restated to read in their
entirety as follows:

          "(i) Accounts with respect to which either of the Designated Account
     Debtors is the Account Debtor, to the extent that the Accounts

AMENDMENT NUMBER FOUR TO
LOAN AND SECURITY AGREEMENT - Page 1
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     owed by either of such Designated Account Debtors exceed twenty-five
     percent (25%) of all Eligible Accounts;

          (j)  Accounts with respect to which either of the Designated Account
     Debtors is the Account Debtor, to the extent that the aggregate amount of
     such Accounts exceeds seventy-five (75%) of all Eligible Accounts;"

     3.    CONDITIONS TO EFFECTIVENESS. This Amendment shall become effective
upon fulfillment of the following conditions, in each case to the satisfaction
of Foothill:

          (a) a counterpart of this Amendment shall be executed by Borrower and
     delivered to Foothill;

          (b) a counterpart of this Amendment shall be executed by EALP and
     delivered to Foothill; and

          (c) Borrower shall pay all fees and expenses required to be paid by
     Borrower pursuant to Section 6.03 of this Amendment.

     4.    REPRESENTATIONS, WARRANTIES AND COVENANTS OF BORROWER.

     4.01  REPRESENTATIONS AND WARRANTIES OF BORROWER. Borrower hereby
represents and warrants to Foothill as follows:

          (a) the execution, delivery and performance by Borrower of this
     Amendment have been duly authorized by all necessary corporate action of
     Borrower and do not and will not require any registration with, consent or
     approval of, notice to or action by, any Person in order to be effective
     and enforceable;

          (b) the execution, delivery and performance by Borrower of this
     Amendment will not violate the articles of incorporation, bylaws or any
     other agreement to which Borrower is a party or by which the property of
     Borrower may be bound;

          (c) the Loan and Security Agreement, as amended by this Amendment,
     constitutes the legal, valid and binding obligation of Borrower,
     enforceable against Borrower in accordance with its terms, without defense,
     counterclaim or offset;

          (d) the representations and warranties contained in the Loan and
     Security Agreement (as amended by this Amendment) and each other Loan
     Document are true and correct on and as of the date hereof as though made
     on and as of the date hereof, except to the extent such representations and
     warranties relate to only a prior specified date;

          (e) Borrower is in full compliance with all covenants and agreements
     contained in the Loan and Security Agreement, as amended by this Amendment,
     and all such covenants and agreements are, and shall remain, in full force
     and effect; and

AMENDMENT NUMBER FOUR TO
LOAN AND SECURITY AGREEMENT - Page 2
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                  (f)  no Default of Event of Default is continuing as of the
         date hereof, nor shall any Default or Event of Default occur as a
         result of the execution and delivery hereof, or the Borrower's
         performance of the obligations herein or under the Loan and
         Security Agreement, as amended hereby.

         5.0   RATIFICATIONS.

         5.01  AGREEMENT OF DESIGNATED SUBSIDIARIES.  The Designated
Subsidiaries hereby join in this Amendment for the purpose of consenting to
the terms hereof. The Designated Subsidiaries hereby agree that all terms,
covenants and provision of the Loan and Security Agreement and the other Loan
Documents are, and shall remain, in full force and effect, including (without
limitation) the Designated Subsidiaries guaranty of the Obligations of Borrower
pursuant to the Subsidiary Guaranties, which Subsidiary Guaranties are hereby
acknowledged and reaffirmed with respect to all Obligations of Borrower arising
pursuant to the Loan and Security Agreement and other Loan Documents, as
amended by this Amendment.

         5.02  AGREEMENT OF EALP. EALP hereby joins in this Amendment for the
purpose of consenting to the terms hereof. EALP hereby agrees that all terms,
covenants and provisions of the Loan and Security Agreement and the other Loan
Documents are, and shall remain, in full force and effect, including (without
limitation) the subordination provisions set forth at Section 17.16 of the Loan
and Security Agreement and EALP's guaranty of the Obligations of Borrower
(other than the EALP Term Loan) pursuant to the EALP Guaranty, which EALP
Guaranty is hereby acknowledged and reaffirmed with respect to all Obligations
of Borrower (other than the EALP Term Loan) arising pursuant to the Loan and
Security Agreement and other Loan Documents, as amended by this Amendment.

         6.    MISCELLANEOUS.

         6.01  SURVIVAL OF REPRESENTATIONS AND WARRANTIES.  All representations
and warranties made herein and in the Loan and Security Agreement shall survive
the execution and delivery of this Amendment, and no investigation by Foothill
or any closing shall affect the representations and warranties or the right of
Foothill to rely on them.

         6.02  REFERENCE TO LOAN AGREEMENT.  The Loan and Security Agreement,
as amended hereby, and all other Loan Documents, whether now or hereafter
executed and delivered, are hereby amended so that any reference to the Loan
and Security Agreement shall mean a reference to the Loan and Security
Agreement, as amended by this Amendment.

         6.03  EXPENSES OF FOOTHILL AND AMENDMENT FEE.  In consideration of
Foothill's execution and delivery of this Amendment, Borrower shall pay to
Foothill an amendment fee in the amount of $10,000, which fee shall be earned
by Foothill and shall be due and payable upon the execution by Foothill of a
counterpart of this Amendment. In addition to such amendment fee and as
provided in the Loan and Security Agreement, Borrower agrees to pay on demand
all costs and expenses incurred by Foothill in connection with the preparation,
negotiation and execution of this Amendment, including, without limitation, the
costs and fees of Foothill's legal

AMENDMENT NUMBER FOUR TO
LOAN AND SECURITY AGREEMENT - Page 3
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counsel and appraiser, and all costs and expenses incurred by Foothill in
connection with the enforcement or preservation of any rights under the Loan
and Security Agreement, as amended hereby, or any other Loan Document.

     6.04 SEVERABILITY. Any provision of this Amendment held by a court of
competent jurisdiction to be invalid or unenforceable shall not impair or
invalidate the remainder of this Amendment and the effect thereof shall be
confined to the provision so held to be invalid or unenforceable.

     6.05 SUCCESSORS AND ASSIGNS. This Amendment is binding upon and shall inure
to the benefit of Foothill and Borrower and their respective successors and
assigns, except Borrower may not assign or transfer any of its rights or
obligations hereunder without the prior written consent of Foothill.

     6.06 COUNTERPARTS. This Amendment may be executed in one or more
counterparts, each of which when so executed shall be deemed to be an original,
but all of which when taken together shall constitute one and the same
instrument.

     6.07 FACSIMILE TRANSMISSION OF SIGNATURES. Any party to this Amendment may
indicate its intention to be bound by its execution and delivery of this
Amendment by its signature to the signature page hereof and the delivery of the
signature page hereof, to the other party or its representatives by facsimile
transmission or telecopy. The delivery of a party's signature on the signature
page by facsimile transmission or telecopy shall have the same force and effect
as if such party signed and delivered this Amendment in person.

     6.08 HEADINGS. The headings, captions, and arrangements used in this
Amendment are for convenience only and shall not affect the interpretation of
this Amendment.

     6.09 APPLICABLE LAW. THIS AMENDMENT AND ALL OTHER LOAN DOCUMENTS EXECUTED
PURSUANT HERETO SHALL BE DEEMED TO HAVE BEEN MADE AND TO BE PERFORMABLE IN AND
SHALL BE GOVERNED BY AND CONSTRUED IN ACCORDANCE WITH THE LAWS OF THE
COMMONWEALTH OF MASSACHUSETTS.

     6.10 FINAL AGREEMENT. THE LOAN AND SECURITY AGREEMENT, AS AMENDED HEREBY,
AND THE OTHER LOAN DOCUMENTS REPRESENT THE ENTIRE EXPRESSION OF THE PARTIES
WITH RESPECT TO THE SUBJECT MATTER HEREOF AND THEREOF ON THE DATE THIS AMENDMENT
IS EXECUTED. THE LOAN AND SECURITY AGREEMENT, AS AMENDED HEREBY, MAY NOT BE
CONTRADICTED BY EVIDENCE OF PRIOR, CONTEMPORANEOUS OR SUBSEQUENT ORAL AGREEMENTS
OF THE PARTIES. THERE ARE NO UNWRITTEN ORAL AGREEMENTS BETWEEN THE PARTIES. NO
MODIFICATION, RESCISSION, WAIVER, RELEASE OR AMENDMENT OF ANY PROVISION OF THIS
AMENDMENT SHALL BE MADE, EXCEPT BY A WRITTEN AGREEMENT SIGNED BY BORROWER AND
FOOTHILL.

AMENDMENT NUMBER FOUR TO
LOAN AND SECURITY AGREEMENT - Page 4
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     6.11 Release. BORROWER HEREBY ACKNOWLEDGES THAT IT HAS NO DEFENSE,
COUNTERCLAIM, OFFSET, CROSS-COMPLAINT, CLAIM OR DEMAND OF ANY KIND OR NATURE
WHATSOEVER THAT CAN BE ASSERTED TO REDUCE OR ELIMINATE ALL OR ANY PART OF ITS
LIABILITY TO REPAY THE OBLIGATIONS (AS DEFINED IN THE LOAN AND SECURITY
AGREEMENT) OR TO SEEK AFFIRMATIVE RELIEF OR DAMAGES OF ANY KIND OR NATURE FROM
FOOTHILL. BORROWER HEREBY VOLUNTARILY AND KNOWINGLY RELEASES AND FOREVER
DISCHARGES FOOTHILL, ITS PREDECESSORS, AGENTS, EMPLOYEES, SUCCESSORS AND
ASSIGNS, FROM ALL POSSIBLE CLAIMS, DEMANDS, ACTIONS, CAUSES OF ACTION, DAMAGES,
COSTS, EXPENSES, AND LIABILITIES WHATSOEVER, KNOWN OR UNKNOWN, ANTICIPATED OR
UNANTICIPATED, SUSPECTED OR UNSUSPECTED, FIXED, CONTINGENT, OR CONDITIONAL, AT
LAW OR IN EQUITY, ORIGINATING IN WHOLE OR IN PART ON OR BEFORE THE DATE THIS
AMENDMENT IS EXECUTED, WHICH THE BORROWER MAY NOW OR HEREAFTER HAVE AGAINST
FOOTHILL, ITS PREDECESSORS, AGENTS, EMPLOYEES, SUCCESSOR AND ASSIGNS, IF ANY,
AND IRRESPECTIVE OF WHETHER ANY SUCH CLAIMS ARISE OUT OF CONTRACT, TORT,
VIOLATION OF LAW OR REGULATIONS, OR OTHERWISE, AND ARISING FROM ANY OBLIGATIONS
(AS DEFINED IN THE LOAN AGREEMENT), INCLUDING, WITHOUT LIMITATION, ANY
CONTRACTING FOR, CHARGING, TAKING, RESERVING, COLLECTING OR RECEIVING INTEREST
IN EXCESS OF THE MAXIMUM RATE, THE EXERCISE OF ANY RIGHTS AND REMEDIES UNDER
THE LOAN AND SECURITY AGREEMENT OR ANY AGREEMENT, DOCUMENT OR INSTRUMENT
ENTERED INTO IN CONNECTION THEREWITH.

                  [REMAINDER OF PAGE INTENTIONALLY LEFT BLANK]

AMENDMENT NUMBER FOUR TO
LOAN AND SECURITY AGREEMENT - Page 5
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         IN WITNESS HEREOF, this Amendment has been executed and delivered as
of the date first set forth above.

                                         GRANT GEOPHYSICAL, INC.,
                                         a Delaware corporation

                                         By: __________________________________
                                             Executive Vice President
                                             Finance & Administration

                                         FOOTHILL CAPITAL CORPORATION
                                         a California corporation, as Agent and
                                         as a Lender

                                         By: ___________________________________
                                         Name: _________________________________
                                         Title: ________________________________

                                         ELLIOTT ASSOCIATES, L.P.
                                         a Delaware limited partnership

                                         By: ___________________________________
                                             Paul E. Singer
                                             General Partner

AMENDMENT NUMBER FOUR TO
LOAN AND SECURITY AGREEMENT - Page 6
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                                         ADVANCED SEISMIC TECHNOLOGY, INC.
                                         a Texas corporation

                                         By: __________________________________
                                         Name: Thomas L. Easley
                                         Title: Executive Vice President -
                                                Finance and Administration

                                         GRANT GEOPHYSICAL DO BROSIL LTDA,
                                         a corporation organized under the laws
                                         of the Republic of Brazil, South
                                         America

                                         By: __________________________________
                                         Name: Thomas L. Easley
                                         Title: Executive Vice President -
                                                Finance and Administration

                                         GRANT GEOPHYSICAL CORP.,
                                         a Texas corporation

                                         By: __________________________________
                                         Name: Thomas L. Easley
                                         Title: Executive Vice President -
                                                Finance and Administration

                                         GRANT GEOPHYSICAL (INT'L) INC.,
                                         a Texas corporation

                                         By: __________________________________
                                         Name: Thomas L. Easley
                                         Title: Executive Vice President -
                                                Finance and Administration

AMENDMENT NUMBER FOUR TO
LOAN AND SECURITY AGREEMENT - Page 7

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